Of all the nodes and elements of any building, the main part is the roof. And its base is a rafter structure, which takes on all the loads, including the loads of the elements. Therefore, the calculation of the truss frame must be carried out with all care, taking into account the quality of the materials. and fortress of all catch connections. Usually, for the frame base of the truss system, a tree is taken after a long exposure of several years, so that the material is well dried.
Types of rafters
Today there is almost no standard construction. All buildings in the private sector are different and differ in the design of the facade and roof structure, on which the durability of the house depends. The roof system consists of several main structures. This is the base, the Mauerlat, to which the roof is attached, the truss frame, which bears the entire weight of the roof and the loads of all the elements.
Of all the existing truss systems in the construction of private houses, the most common are layered and hanging rafters. To create a quality yet economical roof with the leastfinancial costs, it is necessary to calculate the truss system. Just with the choice of the type of house and roof, the design begins. Knowing all the advantages and disadvantages of a particular design, you can make the right choice.
Rafter design
The hanging rafter system is different in that the rafters have two points of support, such as two walls. Such a rafter system transfers all roof loads to the rafter legs, while the bursting force is transferred to the walls. Therefore, for these structures, additional fastening of the rafters is required. And this is new materials and new costs.
The layered system is arranged in a completely different way, when each rafter has not two, but three support points. Basically, such systems are arranged on houses with attic rooms. With a foot, the rafter beam rests on the Mauerlat and rests on the strapping beam of the ceiling of the attic. The third point of support is the skate. In the absence of an attic room, the third point of support may be an internal wall, against which oblique or vertical posts rest, creating a point of support.
Factors affecting the calculation of the roof
Any calculations in the construction of a house begin, of course, with determining the factors affecting the house and the roof. In our case, we will consider only what has a direct impact on the roof and on the truss system.
First of all, these are the climatic conditions that are taken into account in the calculation:
- Wind pressure must be taken into account when calculating the heightroofs and forms. If, for example, the building is in strong wind currents, then of course it is better to create a layered system that has great resistance against this element.
- Rain influences the choice of not only the design, but also the shape of the roof. It is necessary to protect the house in the best possible way from above, and especially from the side of the walls. This requires the correct calculation of overhangs.
- Snow. This factor has a direct influence on the calculation and choice of material in order to create the greatest stability during heavy snowfalls, when a thick layer of snow can create pressure on the roof of hundreds of kilograms per square meter.
Calculate area
This unit of account is needed, like all other calculations. Knowing the area of the roof plane, we can determine the amount of roofing material and not only. By area, you can calculate the number of rafter legs, fixtures and beams to strengthen the truss frame.
The area of any plane is calculated using the well-known mathematical formula:
S=A x B.
S - Area.
A - The length of the plane.
B - Width.
In the event that the roof has a complex structure, the total roof area in the calculation of the truss system is calculated by summing the areas of all slopes.
Rafter step and number of rafters
Knowing the roof area, you can calculate how many rafter legs will be needed for a particular truss structure. The calculation of the roof truss system directly depends on the quantity and quality of the material and also on the design. The weight of the structure, the main quantity responsible for strength and resistance against loads, depends on this.
The angle of inclination and the type of rafters also matter, but more on that later. First, let's decide how you can calculate the number of rafters, knowing the area and the step between the rafters.
Here is an approximate data of the rafter step depending on the length of the rafter leg and the cross section of the timber.
| Rafter length, m | Gap between rafters, cm | Bar section, mm |
| to 3 | 120 | 80 x 100 |
| to 3 | 180 | 90 x 100 |
| to 4 | 100 | 80 x 160 |
| to 4 | 140 | 80 x 180 |
| to 4 | 180 | 90 x 180 |
| to 6 | 100 | 80 x 200 |
| to 6 | 140 | 100 x 200 |
This is an approximate standard for SNiP rafter pitch, but this is not a standard.
Rafter pitch dependency
Here's what affects this parameter:
- Weight of the materials of the truss system.
- Weight of roofing material.
- Waterproofing weight.
- Insulation weight.
- Vapour barrier weight.
- Weight of attic cladding.
- Loads on the roof of natural factors - snow and wind.
For the correct calculation of the roof rafter system, rafter step and the number of rafters, it is necessary to perform the following calculations:
- Take the length of the roof.
- Dividing the length of the roof by the optimal rafter pitch.
- One is added to the received amount and the result is rounded.
- The length is divided by the rounded result.
Practical rafter spacing solution
It must be borne in mind that the rafter pitch depends on the size and quality of the roofing material, and on the size of the insulation. So, for example, when calculating the rafter pitch for slate, it must be taken into account that slate is a heavy material and requires a strong structure. The rafters must be taken with a section of 150 x 30. And the rafter pitch is set within 700-800 mm. You also need to know that slate has certain restrictions on the angle of inclination. In many cases, for the selected roof configuration, the calculation with the dimensions of the rafter system is carried out using programs and calculators for calculating the rafter pitch, which is determined not only from the table, but in accordance with the materials used. Heavy roofing sheets require a reduction in rafter pitch. In some cases, in order not to change the rafter step and not to increase the number of rafters, they fit the entire system under the roof with a stronger crate.
A reliable frame for metal tiles must be created with beams with a section of 50 x 150 mm. The standard truss pitch for this material is calculated within 600 x 900 m, but sometimes, in the case of high-quality insulation, it is required to strengthen the frame by increasing the cross section of the beam up to 200 x 50. In some cases, the rafter step is installedregarding the size of the insulation bags.
Under other roofing sheets, both the section of the rafters and the rafter pitch also change:
- Profiling - 600-900 mm.
- Soft roof 600-1500 mm.
- Ondulin 600-1000.
- 600-800mm polycarbonate sheets.
Program for calculating the truss system
It is very difficult to do calculations for complex roofs that have many slopes and other porticoes. For such a case, today there are different programs for calculating the truss system. One of these programs is an online calculator for calculating the truss system, the angle of inclination and the crate. To calculate these parameters, you need to enter the dimensional data of the roof.
- This is the roofing material used for covering.
- Laying width - A 1 - the distance from the base to the rack.
- Laying width - A 2 - the width of the attic floor.
- Rise height - B 1 - Attic floor height.
- Rise height - B 2 - Distance from base to ridge.
- Base length - D - Roof base length without overhangs.
- Overhang length - C - recommended length 50 cm.
Additional program options
For the mansard roof truss system, the calculation is made according to these parameters, the program will give all the main dimensions of the roof. In addition to the already indicated calculations, the mansard roof calculator will provide additional roof parameters:
- Angle of the side rafters.
- Slope skatingrafters.
- Roof area.
- Number of roofing sheets.
- Attic area.
- Length of side rafters.
- Length of ridge rafters.
- Suggested section of rafter legs.
- Number of crate rows.
- Distance between battens.
- Number and volume of boards for crates.
Sloping roof system
In all calculations of a broken roof, you need to take into account that such a design is assumed for houses with attics, when the roof partially or completely acts as the walls of the attic room. Based on this, you need to use high quality materials. The calculation of the sloping roof truss system has some general principles that can be applied to all types of sloping roofs for attic spaces.
The design and size of the truss system is calculated relative to the size of the house and the attic. This is the height of the ridge, and the width of the gable walls without overhangs, and the size of the overhangs. Everything needs to be considered here.
The calculation of the design of the rafters of a house with an attic is made in relation to the size of the attic room, its width and height. The angle of inclination of the slopes of the sloping roof and the length of the rafters depend on these values.
Understanding that the loads on different parts of the truss system differ from each other depending on the season, you need to create a structure with a safety margin of 10-15% of the design. You can calculate the parameters of the truss system both manually and according to the program. In every truss calculationsystems, two load parameters must be taken into account, these are strength and deformation.
Rafter length
If you do the calculation of the truss system of a gable or other design, then you need to know all the dimensions of the attic room, its width, height, length. For example, to calculate the length of the rafters for the lower slope, the triangle formula is taken. For measurements, a triangle is taken, formed by a vertical rack of the attic room of one slope and a horizontal beam from the wall riser to the end of the overhang. In this triangle, the rafter acts as the hypotenuse. In accordance with the formula of the Pythagorean theorem, the hypotenuse of a right triangle is equal to the square root of the sum of the squares of the legs.
Tilt angle
Today, there are many programs on the Internet for calculating the parameters of the truss frame. It is enough to enter the required dimensions, and in a matter of seconds the program will give the required parameters. And if someone wants to try to calculate the parameters of the truss system manually, then there is also a formula for this. Here, for example, you can make a calculation of the truss system of a gable roof or another design of the angle of inclination of each plane of a complex system. The angle of inclination is calculated using the formula for calculating the tangent of an angle:
tg A=H / L.
The tangent of an angle is calculated by dividing the height by half the width of the base. From the table of tangents we find out the angle. If, for example, the height is 4 m, the width of half the base is 4.75 m. Then when dividing, we get 0.84. This will be the tangent of the angle. In the table of tangentswe find that the tangent of the angle 0.84 is 40. The number 40 becomes the angle in degrees.
Hip design
The hip system is a four-slope structure in which there are two pairs of slopes. Two pediment pairs in the form of trapezoids and two corner pairs are triangular. The calculation of the hip roof truss system is to choose the right angles of inclination. There are several types of such structures in which the slope of all planes is the same. In some cases, a half-hip roof is created, in which the corner slopes are much higher from the eaves of the gable roofs.
There are also separate hip roof options. Each type of such a design has its own calculation rules. The main thing in the calculation of the rafter system is the determination of the slope of the slopes and the height of the ridge. In the previous section of this article, it was already said how to calculate the slope, surface area and length of the rafters. There are mathematical formulas for this. Well, if you do not want to pore over these calculations, you can find programs on the Internet that calculate these parameters.
