A shed roof made of corrugated sheets is a practical and economical solution that is often chosen for garages, sheds, outbuildings and even modern high-tech residential buildings. However, the durability of such a structure directly depends on the quality of the base on which the roofing material is laid. Errors in calculating the distance between frame elements can lead to deformation of the coating, leaks, or even collapse under the weight of snow.
The key parameter here is sheathing pitch - the distance between the centers of the beams or boards forming the supporting mesh. For single-pitch structures, this parameter is critical, since the angle of inclination of the slope is often small, which increases the snow load. An incorrectly selected rack frequency will not be able to compensate for pressure, and profiled sheet will begin to sag between the supports.
In this article, we will look in detail at how to calculate the optimal distance between sheathing elements, what factors influence the choice of installation scheme, and how to avoid common mistakes during construction. You will learn about SNiP standards, features of working with different grades of corrugated metal and the nuances of fastening depending on the climatic conditions of your region.
Factors influencing the choice of distance between bars
Determining the exact distance between support elements is not just arithmetic, but an engineering task that requires taking into account many variables. The main factor, of course, is the load-bearing capacity of the corrugated sheets. The higher the corrugation (wave height) and the thicker the metal, the less often the sheathing can be done, since the sheet holds its shape better.
The second critically important parameter is the slope angle. Shed roofs are often made with a slope of 5 to 30 degrees. The smaller the angle, the more snow accumulates on the surface, and the more often the crossbars should be located. It is also necessary to take into account the wind load in a particular region and the weight of the roofing cake itself, including insulation and waterproofing.
⚠️ Attention: Ignoring the snow load in your region is the most common cause of destruction of pitched roofs. If you live in northern latitudes, you absolutely cannot save on the frequency of the sheathing.
Don’t forget about the weight of the person who will be servicing the roof. Even if there is no snow, the installer or repairman must be able to move along the surface, relying only on the sheathing lines. Therefore, the minimum step is never made less than 300 mm, even for the hardest grades of metal.
SNiP standards and standard installation diagrams
During construction in Russia, it is customary to rely on building codes and regulations (SNiP), which regulate safe distances for various structures. For roofs made of metal profiles, there are well-established standards that divide the lathing into three main types: solid, sparse and variable pitch.
Continuous sheathing used at small angles of inclination (up to 10-15 degrees) or when using thin profiled sheets (grades C8, C10, C18). In this case, the gap between the boards does not exceed 10 mm, which actually creates an even plane. This provides maximum rigidity, but increases wood consumption.
Sparse lathing - the most popular option for pitched roofs with a slope of more than 15 degrees. Here the distance between the axes of the bars can be from 300 to 1000 mm or more. The choice of a specific step depends on the grade of corrugated sheeting: for load-bearing grades (H60, H75) the step can reach 3-4 meters, but in practice it is rarely made more than 1.5 meters due to the requirements for local loads.
Use a laser rangefinder or a stretched cord to mark the spacing of the sheathing - this will guarantee the ideal geometry of the slope and simplify the installation of the corrugated sheet.
There is also a scheme with variable step, when in the lower part of the slope (at the eaves) and at junctions (around pipes, roof windows) the lathing is made more frequent or even continuous. This strengthens the structure in high risk and stress areas.
Dependence of the pitch on the grade of corrugated sheet and metal thickness
The marking of corrugated sheets contains key information about its strength. The letter “C” means wall (for fences and walls), “H” means load-bearing (for roofing and ceilings), and “NS” means universal. The number indicates the wave height in millimeters. The higher the number, the stiffer the sheet and the less often supports can be mounted under it.
For light wall marks such as C8 or C10, which are sometimes used on steeply pitched roofs, require an exclusively solid base. An attempt to make a sparse sheathing under such thin metal will result in the roof “going in waves” after the first snowfall or gust of wind.
Universal brands NS35 and NS44 allow you to increase the pitch to 600-1000 mm, provided there is a sufficient slope. Support profiles H60 and H75 have high ribbing, which allows them to be mounted on thin sheathing with a pitch of up to 3000 mm, however, for single-pitched roofs of residential buildings, the pitch is usually limited to 1000-1200 mm to ensure a safety margin.
| Brand of corrugated sheet | Wave height (mm) | Metal thickness (mm) | Recommended lathing pitch (mm) | Lathing type |
|---|---|---|---|---|
| S8, S10 | 8-10 | 0.4 - 0.5 | Solid (10 mm gap) | Solid |
| S21, NS35 | 21-35 | 0.45 - 0.6 | Up to 600 | Sparse |
| H60, H75 | 60-75 | 0.7 - 0.9 | 1000 - 3000 | Sparse |
| H114 | 114 | 0.8 - 1.0 | Up to 4000 | Sparse |
Features of calculation for a single-pitch structure
A pitched roof has its own unique features that distinguish it from a gable roof. The main one is the direction of water and snow flow. The entire volume of sediment flows down one slope, creating a concentration of load in the lower part. That's why cornice unit requires special attention.
When calculating the step, it is necessary to start counting not from the ridge, but from the eaves board. The first crossbar is often made stronger or a double board is installed, since the lower edge of the sheets rests on it. The step is measured from the center of the first board to the center of the second, and so on along the entire slope.
☑️ Check before installing the sheathing
It is important to take into account the overhang of the corrugated sheeting beyond the sheathing. Usually it is 200-300 mm for pitched roofs, so that water does not flow under the structure. If the pitch is chosen incorrectly, the last sheet may end up without support in the overhang area, which will lead to its deformation.
To calculate the number of rows of sheathing, use the formula: N = (L - S) / D + 1, where N - number of boards, L - slope length, S - size of overhangs, D — selected step. The result is always rounded up to ensure reliability.
Selection of materials and preparation of the base
To create a high-quality base for corrugated sheeting, they most often use an edged board with a thickness of 25-32 mm or timber 40x40, 50x50 mm. The lumber must be dry, otherwise it will “lead” during the drying process, which will disrupt the flatness of the roof and may damage the fasteners.
Before installation, all wooden elements must be treated with fire-retardant compounds. This will extend the life of the structure for decades, protecting it from rot, mold and insects. Pay special attention to the ends of the boards, as they absorb moisture most intensively.
⚠️ Attention: Do not use slab or unedged board with bark for lathing. The bark is an ideal breeding ground for pests that will quickly destroy the roof frame from the inside.
The sheathing is fastened to the rafters using nails or self-tapping screws. The length of the fastener should be such that it enters the rafter leg at least 2-3 times the diameter of the nail. When using timber 50x50 mm and rafters 50x150 mm, nails 80-100 mm long are optimal.
Do you need waterproofing under corrugated sheeting?
In pitched roofs with an angle of less than 15 degrees, the installation of a waterproofing membrane is mandatory, since there is a risk of reverse suction of water during strong winds. At large angles and the use of high-quality seals, waterproofing is desirable, but not always required, but it significantly increases the reliability of the roofing pie.
Installation technology and quality control
Installation of the sheathing begins with the installation of the eaves board. It must be strictly parallel to the wall of the house. The parallelism check is carried out by measuring the distance from the wall to the board at several points. Any deviation will lead to the corrugated sheet lying crooked, and it will be impossible to correct this.
Next, using a template (a strip of length equal to the pitch of the sheathing), mark the installation locations for subsequent beams. The template allows you to significantly speed up the process and avoid measurement errors. Each board is attached at least at two points of intersection with the rafters.
While working, constantly monitor the level of the plane. If the rafters have a deflection, it must be compensated for by pads under the sheathing. A perfectly flat plane is the key to ensuring that the sheets of corrugated sheets will fit tightly to each other and to the base.
The quality of installation of the sheathing determines the geometry of the entire roof: it is almost impossible to correct distortions after laying the corrugated sheeting without dismantling the covering.
Frequently asked questions (FAQ)
Is it possible to use a metal profile instead of wooden sheathing?
Yes, steel profiles (rectangular pipes or C-profiles) are often used for pitched roofs. This is especially true for industrial facilities or if it is necessary to increase the fire resistance of the structure. The pitch in this case is calculated similarly, but the fastening is carried out by welding or special clamps.
What is the minimum angle of inclination that allows for sparse lathing?
According to most manufacturers' recommendations, switching to sparse lathing is possible at an angle of inclination of 15 degrees and above. At angles from 5 to 15 degrees, even for hard grades of corrugated sheeting, it is strongly recommended to make continuous decking or very frequent lathing with a pitch of no more than 200-300 mm.
Is it necessary to do lathing more often in snowy regions?
Absolutely. In regions with high snow loads, the sheathing pitch should be reduced by 10-20% of the calculated value or a thicker beam should be used. This will create additional support points and prevent critical deflection of the profiled sheet under the weight of snow.
How to properly join sheathing boards?
The joining of sheathing boards should be done exclusively on the rafters. It is not allowed for the joint of two boards to “hang” in the air between the rafter legs. It is recommended to make a gap of about 2-3 mm at the joints to compensate for the thermal expansion of the wood.