A shed roof for a garage is a classic and most economical solution, chosen by owners of car boxes due to its simplicity of design and speed of installation. However, the durability of the entire structure directly depends on the quality of the base on which the finishing coating is laid. Exactly sheathing takes on wind and snow loads, evenly distributing them over the rafter system. If you plan to use corrugated sheet, which is a fairly rigid but lightweight material, correct calculation of the rack spacing becomes a critical stage of construction.
Many beginners make the mistake of believing that for a garage it is enough to make a continuous deck “just in case” or, conversely, a too sparse lattice to save lumber. Both approaches can lead to problems: in the first case, to unnecessary costs and heavier construction, in the second, to metal deformation and leaks. In this article, we will look in detail at how to calculate the optimal distance between frame elements, what materials are best to use, and how to avoid common mistakes when installing it yourself.
It is important to understand that sheathing pitch - this is not a fixed value, but a variable, depending on the angle of inclination of the slope, the brand of profiled sheet used and the climatic conditions of your region. An incorrectly designed structure can bend under the weight of snow in winter or begin to “walk” in strong winds. Therefore, before purchasing materials, it is necessary to make accurate calculations and select the appropriate installation scheme.
Choice of material for the frame: board or timber?
The first step in creating a secure foundation is choosing quality lumber. For sheathing under corrugated sheets in a garage, edged boards or rectangular bars are traditionally used. The most common and economical option is considered to be a board with a section 25x100 mm or 32x100 mm. This size provides sufficient rigidity with minimal weight, which is especially important for lightweight single-pitch structures where people are not expected to walk on the roof during operation.
If you plan to install corrugated sheeting with a high load-bearing capacity or if your region receives heavy snowfall, it is worth considering the option of using timber cross section 40x40 mm or 50x50 mm. This material ensures that the ramp will withstand critical loads, but it is significantly more expensive and heavier than a standard board. It is also important that the moisture content of the wood does not exceed 20-22%, otherwise, after drying, the geometry of the frame may change, which will lead to deformation of the metal sheets.
⚠️ Attention: Never use slabs, unedged boards with bark, or material with deep cracks and roll-out knots for lathing. Under the influence of moisture and temperature changes, such wood will quickly lose its strength, which can lead to the collapse of the garage roof.
Before installation, all wooden elements must be treated with fire-retardant compounds. This will extend the life of the structure, protecting it from rot, mold and insects. Pay special attention to the ends of the boards, since it is through them that moisture is absorbed most intensively.
Buy a board with a margin of 10-15% in case of defects or the need to strengthen the nodes. It’s better to have some material left over than to have to stop work because a couple of slats are missing.
Dependence of the sheathing pitch on the slope angle
The key factor determining the distance between the slats is the angle of the pitched roof. The speed of water and snow melting, as well as the windage of the structure, depend on this parameter. For garages, the slope angle usually varies from 5 to 30 degrees, and each range has its own requirements for the pitch of the sheathing.
At low slopes (up to 15 degrees), the risk of water stagnation and accumulation of snow caps is maximum. In this case, it is recommended to do continuous sheathing or the minimum gap between the boards is no more than 10-20 mm. This will prevent the corrugated sheeting between the slats from bending under the weight of wet snow, which can lie on a flat roof for weeks.
If the angle of inclination is from 15 to 30 degrees, it is allowed to make a sparse sheathing. The standard pitch in this case is from 300 to 400 mm. With steep slopes (more than 30 degrees), the snow load becomes minimal, since the snow rolls off on its own, and the pitch can be increased to 500-600 mm, but here the wind load comes to the fore, requiring reliable fastening of each element.
- 📉 Small slope (5-10°): Continuous decking or up to 100mm pitch to prevent sagging.
- 📐 Average slope (15-25°): The optimal pitch is 300-400 mm, suitable for most brands of corrugated sheets.
- 📈 Steep slope (30°+): Sparse pitch up to 500-600 mm, emphasis on fastening from the wind.
The influence of the grade of corrugated sheets on the calculation of distances
No less important parameter than the slope angle is the brand of profiled sheet used. Corrugated sheeting is produced with different wave heights and metal thicknesses, which directly affects its load-bearing capacity. The brands most often used for garage roofing are: NS35, NS44, N57 or H60, where the number indicates the wave height in millimeters.
The higher the profile and the thicker the metal, the larger the sheathing pitch is allowed. For example, for corrugated sheets S21 (wall-mounted, but used on low-slope roofs) will require an almost solid base or a very thick lattice. At the same time, carrying corrugated sheet H60 capable of covering spans of up to 3 meters without loss of rigidity, which makes it possible to make very sparse lathing, saving money.
When choosing a material, be sure to pay attention to the manufacturer’s recommendation specified in the product technical data sheet. There are always tables of maximum spans for different loads. Ignoring this data can lead to the fact that the thin metal between the sparse slats begins to flap in the wind or succumbs under the weight of a person when servicing the roof.
| Brand of corrugated sheet | Wave height (mm) | Metal thickness (mm) | Recommended lathing pitch (mm) |
|---|---|---|---|
| S21 | 21 | 0.5 - 0.7 | Solid or up to 300 |
| NS35 | 35 | 0.5 - 0.8 | Up to 500 - 600 |
| H60 | 60 | 0.7 - 0.9 | Up to 1000 - 1200 |
| H75 | 75 | 0.8 - 1.0 | Up to 2000 - 3000 |
Installation technology: from cornice to ridge
Installation of the sheathing begins only after the rafter system has been installed and leveled and the waterproofing film has been laid. The film is attached with a sag of about 20 mm between the rafters to ensure ventilation and condensate removal. It is important not to mix up the sides of the vapor barrier if a diffusion membrane is used, and to ensure that the sheets overlap by at least 15 cm.
The first board (cornice) is always mounted thicker than the others or expanded with padding, since the drain hook will be attached to it and it experiences increased loads. The distance from the edge of the overhang to the first batten is usually 200-300 mm, but the exact value depends on the length of the corrugated sheet overhang. Next, the remaining elements are attached with the selected pitch, which must be maintained with high precision along the entire plane of the slope.
☑️ Check before installing sheets
The slats are fastened with nails or self-tapping screws to the rafter legs. The length of the fasteners must be at least twice the thickness of the sheathing to ensure reliable fixation. When joining boards along the length (if the length of the slope is greater than the length of the lumber), the joint should fall strictly on the rafter leg, and not hang in the air.
⚠️ Attention: When installing the sheathing, be sure to leave a ventilation gap between the waterproofing and the board. To do this, sometimes they use a counter-lattice - 20x50 mm bars, stuffed along the rafters on top of the film, and then across them - the main lathing. This is critical for a pitched garage roof to prevent the metal from rusting underneath.
Strengthening the structure at nodes and junctions
Particular attention is required where the roof meets the walls, ventilation outlets and areas around the chimney if it passes through the roof of the garage. In these areas, it is recommended to reduce the sheathing pitch or make the flooring continuous at a distance of 40-60 cm from the obstacle. This is necessary for reliable fastening of additional elements: abutment strips, aprons and ebbs.
If you plan to install antennas, solar panels or snow guards on the roof of the garage, their mounting points should also be reinforced. It is best to place additional crossbars at these points in advance or use a board of increased thickness. Trying to secure heavy equipment simply into a thin rail will result in loosening and destruction of the fastening unit.
For single-pitch roofs, the top connection point to the wall (if the garage is attached) or the ridge point (if it is free-standing) is critical. This often requires the installation of an additional support board on which the top roofing strip will rest. This ensures tightness and prevents water from flowing under the corrugated sheet.
Do you need a counter-lattice for your garage?
Counter-lattice is highly recommended for any residential or non-residential building with a metal roof. It creates the necessary ventilation channel, removing moisture from the under-roof space. Without it, the service life of the rafter system and the corrugated sheet itself is reduced significantly due to condensation.
Common mistakes and how to avoid them
One of the most common mistakes is saving on fasteners. Using regular nails without heads or too short self-tapping screws can lead to the corrugated sheets being torn off along with the sheathing in a strong wind. Always use galvanized fasteners with a rubber washer designed for roofing, even when attaching the board itself.
Another mistake is ignoring geometry. If you “by eye” nailed a couple of boards with a large pitch, and then realized it, it will be difficult to correct it. Use a template (a strip of the required length) to mark each subsequent board. This will take more time at the start, but will save nerves when installing the sheets.
Also worth mentioning is the wood moisture issue. If you bought a board with natural humidity and immediately covered it with corrugated sheets, after a year you risk getting a “wavy” roof. As the wood dries, it will shrink in volume, the nails will weaken, and the structure will lose its rigidity. Let the material sit under a cover or use a dry chamber dryer.
The main secret of a long-lasting garage roof is not so much the brand of corrugated sheeting, but a well-installed, dry and correctly pitched sheathing, protected from moisture.
FAQ: Frequently asked questions
Is it possible to use plywood instead of boards for lathing under corrugated sheeting?
Technically, it is possible if you use moisture-resistant plywood (FK or FSF) with a thickness of at least 18-20 mm. However, it is significantly more expensive and heavier than a board. For a garage, this is justified only in the case of a very small roof slope (less than 8 degrees), where a perfectly flat, solid base is required.
What lathing pitch should I choose if I don’t know the exact brand of corrugated sheeting?
If the brand is unknown, it is better to play it safe and take a step of 300-350 mm. This is the “golden mean”, which is suitable for most common roofing materials (HC35, C44) and will withstand standard snow loads in the middle zone.
Do I need to paint the sheathing before installation?
Painting is not necessary if you have used high-quality antiseptics with a tinting effect. Regular paint can create a film that will burst when the board dries. The main thing is deep impregnation with protective compounds, and not a decorative coating.
What to do if the sheathing board is crooked?
Curved boards (“saber-shaped” or “screw”) cannot be used, especially for the lower side. They will create bumps on the roof, which will lead to disruption of water flow and damage to the appearance. Reject material at the purchasing or cutting stage.
Is it possible to attach corrugated sheets directly to the rafters without sheathing?
Absolutely not. Without sheathing, ventilation of the under-roof space will not be ensured, which will lead to rotting of the rafters. In addition, the pitch of the rafters (usually 600-1000 mm) is too large for the corrugated sheeting; it will bend between them.