Calculation of roof truss system online calculator. Calculator for calculating the rafters of a gable roof. Calculation of the amount of wood required for the manufacture of rafters
The online gable roof calculator will help you calculate the rafter tilt angles, the required amount of lathing, the ultimate roof load, as well as the materials required for the construction of this type of roof at the given dimensions. You can calculate the roof from such popular roofing materials as slate, ondulin, ceramic, cement-sand and bituminous tiles, metal tiles and other materials.
The calculations take into account the parameters given in TCP 45-5.05-146-2009 and SNiP "Loads and Impacts".
A gable roof (also known as a gable or gable roof) is a type of roof that has two sloping ramps that run from the ridge to the outer walls of the building. This is the most common roof type today. This is explained by its practicality, low construction costs, effective protection of premises and aesthetic appearance.
The rafters in the gable roof structure rest on each other, connecting in pairs. On the front side, the gable roofs are in the shape of a triangle, such ends are called tongs or gables. Usually, an attic is arranged under such a roof, which is illuminated using small windows on the gables (attic windows).
When entering data into the calculator, be sure to check the additional information marked with the icon.
At the bottom of this page, you can leave feedback, ask your own question to the developers, or suggest an idea for improving this calculator.
Explanation of calculation results
Roof angle
The rafters and roof slope are inclined at this angle. It is understood that a symmetrical gable roof is planned. In addition to calculating the angle, the calculator will inform you how the angle corresponds to the norms for the roofing material you have chosen. If you need to change the angle, then for this you need to change the width of the base or the height of the roof rise, or choose another (lighter) roofing material.
Roof surface area
The total area of \u200b\u200bthe roof (including overhangs of a given length). Determines the amount of roofing and insulation materials that will be needed for the job.
Approximate weight of roofing material
The total weight of the roofing material required to fully cover the roof area.
Number of overlapped rolls of insulation
The total amount of rolls required to insulate the roof. The calculations are based on rolls 15 meters long and 1 meter wide.
Maximum load on the rafter system. The calculations take into account the weight of the entire roofing system, the shape of the roof, as well as the wind and snow loads of the region you specified.
Rafter length
The full length of the rafters from the beginning of the ramp to the ridge of the roof.
Number of rafters
The total number of rafters required to build a roof at a given pitch.
Minimum cross-section of rafters, Weight and Volume of timber for rafters
The table shows the recommended cross-sectional dimensions of the rafters (in accordance with GOST 24454-80 Softwood lumber). To determine compliance, the type of roofing material, the area and shape of the roof structure, and the loads applied to the roof are taken into account. The adjacent columns show the total weight and volume of these rafters for the entire roof.
Number of rows of crate
The total number of battens for the entire roof. To determine the number of rows of crate for one slope, it is enough to divide the resulting value by two.
Uniform distance between battens
Use the value shown here to install the battens evenly and avoid unnecessary overruns.
Number of battens in standard length
To frame the entire roof, you will need the number of boards shown here. The calculation uses the standard 6 meter board length.
The volume of lathing boards
The volume of the boards in cubic meters will help you calculate the cost of the crate costs.
Approximate weight of crate boards
Estimated total weight of the sheathing boards. The calculations use the average density and moisture values \u200b\u200bfor softwood.
Rafter system. Calculation of rafters and floor beams. Before proceeding with the construction of a roof, it is of course desirable that its rafter system be designed for strength. Immediately after the publication of the last article "Do-it-yourself gable roof of a house", I began to receive questions in the mail regarding the choice of the section of rafters and floor beams. Yes, it is really not easy to understand this issue on the vastness of our beloved Internet. There is a lot of information on this topic, but, as always, it is so scattered and sometimes even contradictory that an inexperienced person, who in his life may not even have encountered such a subject as "Strength" (someone was lucky), can easily get confused in these wilds. I, in turn, will now try to draw up a step-by-step algorithm that will help you independently calculate the rafter system of your future roof and finally get rid of constant doubts - and suddenly it will not stand, but suddenly it will fall apart. I must say right away that I will not delve into terms and various formulas. Well, why? There are so many useful and interesting things in the world that you can fill your head with. We just need to build a roof and forget about it. The whole calculation will be described using the example of a gable roof, which I wrote about in the last article. So, Step # 1: Determine the snow load on the roof. For this we need a map of snow loads of the Russian Federation. To enlarge the picture, click on it with the mouse. Below I will give a link where you can download it to your computer. Using this map, we determine the number of the snow region in which we are building the house and from the following table we select the snow load corresponding to this region (S, kg / m²): If your city is on the border of the regions, choose a higher load value. It is not necessary to correct the resulting figure depending on the angle of inclination of the slopes of our roof. The program that we will use will do it itself. Let's say in our example we are building a house in the suburbs. Moscow is located in the 3rd snow region. The load for it is 180 kg / m². Step # 2: Determine the wind load on the roof. For this we need a map of the RF wind loads. It can also be downloaded from the link below. Using this map, we also select the corresponding region number and determine the value of the wind load for it (the values \u200b\u200bare shown in the lower left corner): Next, the resulting figure must be multiplied by the correction factor "k", which in turn is determined from the table: Here, column A - open coasts seas, lakes and reservoirs, deserts, steppes, forest-steppe and tundra; Column B - urban areas, woodlands, etc. terrain evenly covered with obstacles. It should be noted that in some cases the type of terrain may differ in different directions (for example, the house is located on the outskirts of a settlement). Then we select values \u200b\u200bfrom column "A". Let's go back to our example. Moscow is located in the 1st wind region. The height of our house is 6.5 meters. Suppose that it is being built in a settlement. Thus, we take the value of the correction factor k \u003d 0.65. That is, the wind load in this case will be equal to: 32x0.65 \u003d 21 kg / m². Step 3: You need to download to your computer a calculation program made in the form of an Excel table. Further we will work in it. Here is the download link: "Calculation of the rafter system". It also contains maps of snow and wind loads of the Russian Federation. So, download and unpack the archive. Open the file "Calculation of the rafter system", while we get into the first window - "Loads": Here we need to change some values \u200b\u200bin the cells filled with blue. All calculations are done automatically. Let's continue to consider our example: - in the "Initial data" plate we change the angle of inclination by 36 ° (what angle you will have, write this, well, I think everyone understands this); - we change the step of the rafters, to the one we chose. In our case, this is 0.6 meters; - Load. roofing (load from the own weight of the roofing material) - we select this value from the table: For our example, we select a metal tile with a weight of 5 kg / m². - Snow. area - here we enter the sum of the values \u200b\u200bof snow and wind loads that we received earlier, ie 180 + 21 \u003d 201 kg / m²; - Insulation (mans.) - we leave this value unchanged if we lay insulation between the rafters. If we make a cold attic without insulation, change the value to 0; - enter the required dimensions of the crate into the "Crate" plate. In our case, for metal tiles, we will change the pitch of the lathing by 0.35 m and the width by 10 cm. Leave the height unchanged. All other loads (from the own weight of the rafters and lathing) are automatically taken into account by the program. Now let's see what we've got: We see the inscription "The load-bearing capacity of the crate is provided!" We don't touch anything else in this window, there is even no need to understand what numbers are in other cells. If, for example, we choose another step of the rafters (more), it may turn out that the bearing capacity of the lathing will not be provided. Then it will be necessary to select other dimensions of the crate, for example, to increase its width, etc. n. In general, I think you will figure it out. Step 4: Click at the bottom of the working screen on the tab "Sling 1" and go to the window for calculating rafters with two points of support. Here, all the input data we entered earlier have already been substituted by the program automatically (this will be the case in all other windows). In our example from the article "Do-it-yourself gable roof of a house", the rafters have three points of support. But let's imagine that there are no intermediate racks and make a calculation: - change the length of its horizontal projection on the rafter diagram (the cell is filled with blue). In our example, it is 4.4 meters. - in the plate "Calculation of rafters" we change the value of the thickness of the rafters B (set) to the one chosen by us. We put 5 cm. This value must necessarily be greater than the value indicated in the Ttr cell (stable); - now in the line "Accept H" we need to enter the selected rafter width in centimeters. It must be greater than the values \u200b\u200bindicated in the lines "Ntr. (Strength)" and "Ntr. (Deflection)". If this condition is met, all the inscriptions at the bottom under the rafter scheme will look like "Condition met". The line "H, (by grade)" indicates the value that the program itself offers us to choose. We can take this figure, or we can take another. Usually we choose the sections available in the store. So, what we got is shown in the figure: In our example, in order to comply with all strength conditions, it is necessary to choose rafters with a section of 5x20 cm.But the roof scheme shown by me in the last article has rafters with three points of support. Therefore, to calculate it, go to the next step. Step number 5: Click at the bottom of the working screen on the tab "Line 2" or "Line. 3 ″. This opens a window for calculating rafters with 3 support points. The choice of the tab we need is made depending on the location of the middle support (rack). If it is located to the right of the middle of the rafter, i.e. L / L1<2, то пользуемся вкладкой «Строп.2″. Если стойка расположена левее середины стропила, т. е. L/L1> 2, then we use the "Line 3" tab. If the rack is exactly in the middle, you can use any tab, the results will be the same. - on the rafter diagram, we forward the dimensions in cells filled with blue (except for Ru); - according to the same principle as described above, we choose the dimensions of the rafter section. For our example, I took the dimensions 5x15 cm. Although 5x10 cm could have been. I just got used to working with such boards, and the margin of safety will be more. Now it is important: from the figure obtained during the calculation, we will need to write out the value of the vertical load acting on the rack (in our example (see the figure above) it is 343.40 kg) and the bending moment acting on the rack (Mop. \u003d 78.57 kghm). We will need these figures further when calculating the racks and floor beams. Further, if you go to the "Arch" tab, a window for calculating the rafter system will open, which is a ridge arch (two rafters and a tightening). I will not consider it, it will not fit our roof. We have too large a span between the supports and a small angle of inclination of the slopes. There you will get rafters with a section of about 10x25 cm, which is certainly unacceptable for us. For smaller spans, such a scheme can be used. I am sure whoever understood what I wrote above will figure it out on his own with this calculation. If you still have questions, write in the comments. And we move on to the next step. Step 6: Go to the "Rack" tab. Well, everything is simple here. - the previously determined values \u200b\u200bof the vertical load on the rack and the bending moment are entered in the figure, respectively, in the cells "N \u003d" and "M \u003d". We have them written in kilograms, we enter them in tons, while the values \u200b\u200bare automatically rounded; - also in the figure we change the height of the rack (in our example it is 167 cm) and set the dimensions of the section we have chosen. I chose a 5x15 cm board. Below, in the center, we see the inscription "Central provided!" and “Vnecenter. provided ”. So everything is all right. The safety factors "Kz" are very large, so you can safely reduce the cross-section of the racks. But we will leave it as it is. The result of the calculation in the picture: Step 7: Go to the "Beam" tab. Distributed and concentrated loads act on the floor beams simultaneously. We need to consider both. In our example, beams of the same section cover spans of different widths. We, of course, make calculations for a wider span: - in the “Distributed load” plate we indicate the step and span of the beams (we take 0.6 m and 4 m, respectively, from the example); - take the values \u200b\u200bof Load. (normal) \u003d 350 kg / m² and Load (calc.) \u003d 450 kg / m². The values \u200b\u200bof these loads, in accordance with SNiP, are averaged and taken with a good margin of safety. They include the load from the own weight of the floors and the operating load (furniture, people, etc.); - in the line "B, given" we enter the selected width of the section of the beams (in our example, it is 10 cm); - in the lines "H, strength" and "H, deflection" the minimum possible cross-section heights of the beams will be indicated at which it will not break and its deflection will be acceptable. We are interested in the largest of these numbers. We take the height of the beam section proceeding from it. In our example, a beam with a section of 10x20 cm is suitable: So, if we did not have racks resting on the floor beams, the calculation would be over. But there are racks in our example. They then create a concentrated load, so we continue to fill in the plates "Concentrated load" and "Distribution + focus": - in both plates we enter the dimensions of our spans (here I think everything is clear); - in the "Concentrated load" plate, change the values \u200b\u200bof Load (normal) and Load (calculated) by the figure that we received above when calculating rafters with three support points - this is the vertical load on the rack (in our example, 343.40 kg) ; - in both plates we enter the accepted width of the beam section (10 cm); - the height of the section of the beam is determined by the plate "Distribution + concent." Focusing on a higher value again. For our roof, we take 20 cm (see the picture above). This completes the calculation of the rafter system. I almost forgot to say: the calculation program we use is applicable for roof systems made of pine (except for Weymouth), spruce, European and Japanese larch. All used wood is of the 2nd grade. When using other woods, some changes will need to be made to the program. Since other types of wood are rarely used in our country, I will not describe now what needs to be changed. Read more.
A gable roof is formed on the basis of a frame that combines elementary structure and unsurpassed reliability. But the backbone of the roof in two rectangular slopes can boast of these advantages only in the case of a careful selection of rafter legs.
Parameters of the gable roof rafter system
It is worth starting the calculations if you understand that the rafter system of a gable roof is a complex of triangles, the most rigid elements of the frame. They are assembled from boards, the size of which plays a special role.
Rafter length
The formula will help determine the length of strong boards for the rafter systema² +b² \u003dc², derived by Pythagoras.
The length of the rafter can be found by knowing the width of the house and the height of the roof.
Parameter "a" stands for height and is self-selected. It depends on whether the under-roof space will be residential; it also has certain recommendations if an attic is planned.
Behind the letter "b" is the width of the building, divided in two. And "c" represents the hypotenuse of the triangle, that is, the length of the rafter legs.
Let's say that the width of half of the house is three meters, and it is decided to make the roof two meters high. In this case, the length of the rafter legs will reach 3.6 m (c \u003d √a² + b² \u003d 4 + √9 \u003d √13≈3.6).
60-70 cm should be added to the figure obtained from the Pythagorean formula. Extra centimeters will be needed to take the rafter leg over the wall and make the necessary cuts.
The six-meter rafter is the longest, therefore it is suitable as a rafter leg
The maximum length of a bar used as a rafter leg is 6 m. If a durable board of greater length is required, then they resort to the splicing technique - nailing a piece from another bar to the rafter leg.
Cross-section of rafter legs
For various elements of the rafter system, there are standard sizes:
- 10x10 or 15x15 cm - for the Mauerlat bar;
- 10x15 or 10x20 cm - for the rafter leg;
- 5x15 or 5x20 cm - for running and strut;
- 10x10 or 10x15 cm - for the rack;
- 5x10 or 5x15 cm - for the bed;
- 2x10, 2.5x15 cm - for crates.
The thickness of each part of the roof supporting structure is determined by the load that it has to experience.
A beam with a section of 10x20 cm is ideal for creating a rafter leg
The cross-section of the rafter legs of a gable roof is affected by:
The rafter pitch affects the cross-section of the rafter legs most significantly. An increase in the distance between the beams entails increased pressure on the supporting structure of the roof, and this obliges the builder to use thick rafter legs.
Table: cross-section of rafters depending on length and pitch
Variable effect on the rafter system
The pressure on the rafter legs is constant and variable.
From time to time and with varying intensity, wind, snow and atmospheric precipitation affect the supporting structure of the roof. In general, the roof slope is comparable to a sail, which can break under the pressure of natural phenomena.
The wind tends to overturn or raise the roof, so it is important to make all the calculations correctly
The variable wind load on the rafters is determined by the formula W \u003d Wo × kxc, where W is the wind load indicator, Wo is the value of the wind load characteristic for a certain part of Russia, k is a correction factor due to the height of the structure and the nature of the terrain, and c is the aerodynamic coefficient.
The aerodynamic coefficient can range from -1.8 to +0.8. A minus value is typical for a rising roof, and a plus value is for a roof on which the wind presses. In a simplified calculation with a focus on improving strength, the aerodynamic coefficient is considered equal to 0.8.
The calculation of the wind pressure on the roof is based on the location of the house
The normative value of wind pressure is recognized by map 3 of Appendix 5 in SNiP 2.01.07–85 and a special table. The coefficient taking into account the change in wind pressure with height is also standardized.
Table: guideline value of wind pressure
Table: value of the coefficient k
It is not just the terrain that affects the wind load. The area of \u200b\u200bhousing is of great importance. Behind a wall of tall buildings, the house is almost not threatened, but in open space the wind can become a serious enemy for it.
The snow load on the rafter system is calculated according to the formula S \u003d Sg × µ, that is, the weight of the snow mass per 1 m² is multiplied by a correction factor, the value of which reflects the degree of slope of the roof.
The weight of the snow layer is indicated in the SNiP "Roof systems" and is determined by the type of terrain where the building is built.
The snow load on the roof depends on where the house is located
The correction factor, if the roof slopes are inclined by less than 25 °, is equal to one. And in the case of a roof slope of 25-60 °, this figure decreases to 0.7.
When the roof is tilted more than 60 degrees, the snow load is discounted. Still, snow rolls down quickly from a steep roof, without having time to negatively affect the rafters.
Constant loads
Loads acting continuously are considered the weight of the roofing cake, including the sheathing, insulation, films and finishing materials for arranging the attic.
The roofing cake creates constant pressure on the rafters
Roof weight is the sum of the weight of all materials used in the construction of the roof. On average, it is equal to 40–45 kg / sq. M. According to the rules, 1 m² of the rafter system should not account for more than 50 kg of the weight of roofing materials.
So that there is absolutely no doubt about the strength of the rafter system, 10% should be added to the calculation of the load on the rafter legs.
Table: weight of roofing materials per 1 m²
| Type of roof topcoat | Weight in kg per 1 m2 |
| Rolled bitumen-polymer cloth | 4–8 |
| Bituminous-polymer soft tile | 7–8 |
| Ondulin | 3–4 |
| Metal roof tiles | 4–6 |
| Decking, seam roofing, galvanized metal sheets | 4–6 |
| Cement-sand tile | 40–50 |
| Ceramic tiles | 35–40 |
| Slate | 10–14 |
| Slate roof | 40–50 |
| Copper | 8 |
| Green roof | 80–150 |
| Rough flooring | 18–20 |
| Lathing | 8–10 |
| The rafter system itself | 15–20 |
Number of beams
How many rafters will be needed to equip a gable roof frame is set by dividing the width of the roof by a step between the beams and adding one to the resulting value. It denotes an additional rafter that will need to be placed on the edge of the roof.
Let's say it is decided to leave 60 cm between the rafters, and the length of the roof is 6 m (600 cm). It turns out that 11 rafters are needed (taking into account the additional beam).
The rafter system of a gable roof is a structure made of a certain number of rafters
The pitch of the beams of the roof supporting structure
To determine the distance between the beams of the roof supporting structure, you should pay close attention to such points as:
- weight of roofing materials;
- the length and thickness of the timber - the future rafter leg;
- degree of roof slope;
- level of wind and snow loads.
After 90-100 cm, rafters are usually placed in the case of choosing a light roofing material
A step of 60–120 cm is considered normal for rafter legs.The choice in favor of 60 or 80 cm is made in the case of building a roof sloped by 45˚. The same small step should be, if desired, to cover the wooden roof frame with heavy materials such as ceramic tiles, asbestos-cement slate and cement-sand tiles.
Table: rafter pitch depending on length and section
Formulas for calculating the rafter system of a gable roof
The calculation of the rafter system is reduced to establishing the pressure on each beam and determining the optimal section.
When calculating the gable roof truss system, proceed as follows:
- According to the formula Qr \u003d AxQ, they find out what the load per linear meter of each rafter leg is. Qr is the distributed load per linear meter of the rafter leg, expressed in kg / m, A is the distance between the rafters in meters, and Q is the total load in kg / m².
- Go to the definition of the minimum cross-section of the timber-rafter. To do this, study the data of the table entered in GOST 24454-80 “Lumber of coniferous species. Dimensions ".
- Based on the standard parameters, the section width is selected. And the section height is calculated using the formula H ≥ 8.6 Lmax sqrt (Qr / (B Rben)) if the roof slope α< 30°, или формулу H ≥ 9,5·Lmax·sqrt(Qr/(B·Rизг)), когда уклон крыши α > 30 °. H is the height of the section in cm, Lmax is the working section of the rafter leg of maximum length in meters, Qr is the distributed load per linear meter of the rafter leg in kg / m, B is the width of the section, cm, Rben is the resistance of wood to bending, kg / cm². If the material is made from pine or spruce, then Rben can be equal to 140 kg / cm² (1 grade of wood), 130 kg / cm 2 (2 grade) or 85 kg / cm 2 (3 grade). Sqrt is the square root.
- Check if the deflection value complies with the standards. It should not be more than the number that is obtained by dividing L by 200. L is the length of the working section. Correspondence of the deflection value to the ratio L / 200 is feasible only if the inequality is correct 3.125 · Qr · (Lmax) ³ / (B · H³) ≤ 1. Qr denotes the distributed load per linear meter of the rafter leg (kg / m), Lmax - the working area of \u200b\u200bthe rafter leg maximum length (m), B - section width (cm), and H - section height (cm).
- When the above inequality is violated, the B and H scores increase.
Table: nominal dimensions of thickness and width of sawn timber (mm)
| Board thickness - section width (B) | Board width - section height (H) | ||||||||
| 16 | 75 | 100 | 125 | 150 | - | - | - | - | - |
| 19 | 75 | 100 | 125 | 150 | 175 | - | - | - | - |
| 22 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | - | - |
| 25 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 32 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 40 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 44 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 50 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 60 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 75 | 75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 100 | - | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 275 |
| 125 | - | - | 125 | 150 | 175 | 200 | 225 | 250 | - |
| 150 | - | - | - | 150 | 175 | 200 | 225 | 250 | - |
| 175 | - | - | - | - | 175 | 200 | 225 | 250 | - |
| 200 | - | - | - | - | - | 200 | 225 | 250 | - |
| 250 | - | - | - | - | - | - | - | 250 | - |
An example of a structural analysis
Suppose that α (angle of inclination of the roof) \u003d 36 °, A (distance between the rafters) \u003d 0.8 m, and Lmax (working section of the rafter leg of maximum length) \u003d 2.8 m. Material from first grade pine is used as the beams , which means that Rben \u003d 140 kg / cm².
Cement-sand tiles have been chosen for the roofing and therefore the roof weight is 50 kg / m². The total load (Q) experienced by each square meter is 303 kg / m². And for the construction of the rafter system, beams with a thickness of 5 cm are used.
Hence the following computational steps follow:
- Qr \u003d A · Q \u003d 0.8 · 303 \u003d 242 kg / m - distributed load per running meter of rafter timber.
- H ≥ 9.5 Lmax sqrt (Qr / B Rben).
- H ≥ 9.5 2.8 sqrt (242/5 140).
- 3.125 · Qr · (Lmax) ³ / B · H³ ≤ 1.
- 3.125 · 242 · (2.8) ³ / 5 · (17.5) ³ \u003d 0.61.
- H ≥ (approximate height of the rafter section).
In the table of standard sizes, you need to find the height of the rafter section close to the indicator of 15.6 cm.A parameter equal to 17.5 cm is suitable (with a section width of 5 cm).
This value is quite consistent with the deflection index in the regulatory documents, and this is proved by the inequality 3.125 · Qr · (Lmax) ³ / B · H³ ≤ 1. Substituting the values \u200b\u200b(3.125 · 242 · (2.8) ³ / 5 · (17, 5) ³), it turns out that 0.61< 1. Можно сделать вывод: сечение пиломатериала выбрано верно.
Video: detailed calculation of the rafter system
The calculation of the gable roof rafter system is a whole complex of calculations. In order for the bars to cope with the task assigned to them, the builder needs to accurately determine the length, quantity and cross-section of the material, find out the load on it and find out what should be the step between the rafters.
The roof in a building is designed to hold external loads and redistribute them to load-bearing walls or support structures. These loads include the weight of the roofing cake, the weight of the structure itself, the weight of the snow cover, and so on.
The roof is located on the rafter system. This is the name of the frame structure on which the roof is fixed. It accepts all external loads, distributing them over the supporting structures.
The gable roof rafter system includes the following elements:
- Mauerlat;
- Braces and braces;
- Side and ridge runs;
- Rafter legs.
A roof truss is a structure that includes all of the listed elements with the exception of the Mauerlat.
Calculation of gable roof loads
Constant loads
The first type is called those loads that always act on the roof (in any season, time of day, and so on). This includes the weight of the roofing cake and the various equipment installed on the roof. For example, the weight of a satellite dish or aerator. It is necessary to calculate the weight of the entire rafter structure, together with fasteners and various elements. Professionals use computer programs and special calculators to accomplish this task.
The calculation of the gable roof is based on the calculation of the loads on the rafter legs. First of all, you need to determine the weight of the roofing cake. The task is quite simple, you just need to know the materials used, as well as the dimensions of the roof.
As an example, let's calculate the weight of a roofing cake with ondulin material. All values \u200b\u200bare taken approximately; high accuracy is not required here. Usually builders perform calculations of the weight per square meter of the roof. And then this indicator is multiplied by the total roof area.
The roofing cake consists of ondulin, a layer of waterproofing (in this case, insulation on a polymer-bitumen basis), a layer of thermal insulation (the weight of basalt wool will be calculated) and lathing (the thickness of the boards is 25 mm). Let's calculate the weight of each element separately, and then add all the values.
Calculation of the roof of a gable roof:
- A square meter of roofing material weighs 3.5 kg.
- A square meter of waterproofing layer weighs 5 kg.
- A square meter of insulation weighs 10 kg.
- A square meter of battens weighs 14 kg.
Now let's calculate the total weight:
3.5 + 5 + 10 + 14 = 32.5
The resulting value must be multiplied by the correction factor (in this case, it is 1.1).
32.5 * 1.1 \u003d 35.75 kg
It turns out that a square meter of roofing cake weighs 35.75 kg. It remains to multiply this parameter by the roof area, then it will turn out to calculate the gable roof.
Variable roof loads
Variables are loads that act on the roof not constantly, but seasonally. Snow in winter is a prime example. Snow masses settle on the roof, creating an additional impact. But in the spring they melt, respectively, the pressure decreases.
Wind also belongs to variable loads. This is also a weather phenomenon that does not always work. And there are a lot of such examples. Therefore, it is important to take into account variable loads when calculating the length of the gable roof rafters. The calculation must take into account many different factors affecting the roof of the building.
Now let's take a closer look at snow loads. When calculating this parameter, you need to use a special card. The mass of snow cover in various regions of the country is marked there.
To calculate this type of load, the following formula is used:
Where Sg is an indicator of the terrain taken from the map, and µ is a correction factor. It depends on the slope of the roof: the stronger the slope, the lower the correction factor. And here there is an important nuance - for roofs with a slope of 60 o it is not taken into account at all. After all, snow will simply roll off them, and not accumulate.
The whole country is divided into regions not only by the amount of snow, but also by the strength of the winds. There is a special map on which you can find out this indicator in a certain area.
When calculating roof rafters, wind loads are determined by the following formula:
Where x is the correction factor. It depends on the location of the building and its height. And W o is the parameter selected from the map.
Calculation of the dimensions of the rafter system
When the calculation of all types of loads is over, you can proceed to calculate the dimensions of the rafter system. The execution of the work will differ depending on what kind of roof structure is planned.
In this case, a gable is considered.
Rafter leg section
The calculation of the rafter leg is based on 3 criteria:
- Loads from the previous section;
- Remoteness of the railings;
- The length of the rafters.
There is a special table of sections of rafter legs, in which you can find out this indicator, based on the above criteria.
The length of the rafters in the gable roof
When calculating manually, you will need a basic knowledge of geometry, in particular - the Pythagorean theorem. The rafter is the hypotenuse of a right-angled triangle. You can find out its length by dividing the length of the leg by the cosine of the opposite angle.
Let's consider a concrete example:
It is required to calculate the length of the rafters of a gable roof for a house with a width of 6 m, in which the slope of the slopes is 45 o. Let L be the length of the rafters. Let's put all the data into the formula.
L \u003d 6/2 / cos 45 ≈ 6/2 / 0.707 ≈ 4.24 meters.
The length of the visor must be added to the resulting value. It is approximately equal to 0.5 m.
4.24 + 0.5 \u003d 4.74 meters.
This completes the calculation of the length of the rafters for a gable roof. It was a manual way of doing the task. There are special computer programs designed to automate this process. The easiest way is to use Arkon. This is a completely free program that even a person who is poorly versed in computers can easily figure out.
It is enough to simply specify the input parameters based on the size of the house. The program will independently perform calculations and show the required cross-section, as well as the length of the gable roof rafters.
A simple online calculator will accurately calculate the rafter length, rafter overhang length, rafter cut angle. Start calculating the rafters now!
Do-it-yourself rafter system
This calculator is indispensable for those who decided to make do it yourself rafters... The smart online calculator will accurately calculate the length to the rafter overhang, the overhang length, the cut angle and the distance from the edge of the rafter to the start of the saw. The online calculator is suitable for calculating the rafters of a gable roof and a 1-pitched roof.
The permissible range of the angle of inclination of the roof is from 20 ° to 60 °, the smaller the angle, the less lumber will be needed for the farm, but the larger the angle, the more spacious it will be under the roof of the second floor of the house. If you choose an angle of 30 °, then for a building width of 10 m, the elevation of the ridge above the upper floor will be 2.5 m.The length of the rafter legs will be 7 meters, of which 6.2 are above the house, and the rest will be out of the roof. The minimum departure size is taken to be 50 cm for safety from bad weather. Rafters with a length of 7 meters are considered the maximum permissible for an inclined design to the ridge of the roof, if the length of the rafters is more than 7 meters, additional reinforcement of the gable roof in the form of beams is required. When calculating rafters on a gable roof the step between the individual lags is 80-130 cm... The exact step size depends on the weight of the roof, the amount of rainfall and wind load in your area of \u200b\u200bconstruction. All rafter blanks must be treated with antiseptic and fire-fighting compounds.
