A pitched roof with corrugated sheeting is a popular solution for garages, sheds and outbuildings. But even such a simple design requires proper installation of sheathing so that the roof does not sag, does not leak, and lasts for decades. In this article we will look at how to correctly calculate the pitch of the sheathing for corrugated sheeting, what material to choose and how to avoid common mistakes during installation.
The secret to a long-lasting roof lies in two things: the right choice sheathing pitch (it depends on the brand of corrugated sheeting and the angle of inclination) and high-quality fastening. For example, for a profiled sheet S21 with an inclination angle of 10°, the sheathing pitch should be no more than 30 cm, and for NS35 at 15° - already 50 cm. If you ignore these nuances, the roof will begin to “play” in the wind or bend under the snow.
We will analyze each stage in detail - from choosing wood to final inspection of the structure. And at the end of the article you will find a FAQ with answers to frequently asked questions and a table of compatibility of corrugated sheeting with sheathing spacing.
1. Choosing a material for sheathing: which is better - boards or timber?
The main material for sheathing under corrugated sheets is coniferous wood (pine, spruce, larch). It should be dry (humidity no more than 18-22%) and treated with an antiseptic. But what kind of shape should you choose - board or timber?
For pitched roofs with a slight slope (up to 15°) it is optimal edged board 25-32 mm thick and 100-150 mm wide. It is lighter than timber, cheaper and easier to install. Beam (50×50 mm or 60×40 mm) is used for roofs with a large slope or with a rafter pitch of more than 1 m - it can withstand increased loads, but is more expensive and requires more fasteners.
- 🌲 Board 25×100 mm — for roofs with a slope of up to 10° and rafter spacing of up to 80 cm.
- 🪵 Beam 50×50 mm - if the slope is 15-20° or the rafters are less than 1 m apart.
- 🛠️ OSB or plywood - only for continuous sheathing under a soft roof (not suitable for corrugated sheeting!).
Important: do not use unedged boards or raw wood! Material humidity above 22% will lead to deformation of the sheathing after drying, and uneven thickness of the boards will create waves on the corrugated sheeting. Before installation, check each board for deflection - if, with a load of 50 kg, it deflects by more than 2 mm, discard it.
2. Calculation of the sheathing pitch: table for different brands of corrugated sheeting
The sheathing pitch depends on three parameters: grades of corrugated sheets, roof angle and snow load in your region. The thinner and “lower” the profiled sheet is, the more often lathing is needed. For example, for C8 (the thinnest corrugated sheeting) the pitch should not exceed 30 cm even with a slope of 20°, and for H75 you can do 1-1.5 m.
The table below shows the recommended values for popular brands of corrugated sheeting with a snow load of up to 180 kg/m² (suitable for most regions of Russia). If there is more snow in your area (for example, in Siberia or the Urals), reduce the step by 10-15%.
| Brand of corrugated sheet | Tilt angle, ° | Max. sheathing pitch, cm | Note |
|---|---|---|---|
| S8, S10 | 5-10 | 30 | Continuous sheathing for slope <5° |
| S20, S21 | 10-15 | 40-50 | With rafter pitch >1 m - 40 cm |
| NS35, NS44 | 15-20 | 60-80 | For regions with high wind load - 50 cm |
| H60, H75 | >20 | 100-150 | You can use sparse lathing |
Calculation example: you have a roof with a slope of 12° and corrugated sheets S21. According to the table, the sheathing pitch is 40-50 cm. If the rafters are 1 m apart, choose 40 cm to evenly distribute the load. If there is a lot of snow in winter (more than 200 kg/m²), reduce the pitch to 30-35 cm.
⚠️ Attention: if the roof slope is less than 8°, the sheathing must be solid (the gap between the boards is no more than 10 mm) or in increments of no more than 30 cm. Otherwise, the corrugated board will sag under the weight of the snow, and leaks will appear at the joints.
3. Tools and fasteners: what you will need for installation
To install sheathing under corrugated sheeting you will need:
- 🔨 Jigsaw or circular saw — for cutting boards/timbers.
- 📏 5m tape measure and square - for marking.
- 🔩 Screwdriver with bit for self-tapping screws - for attaching the sheathing to the rafters.
- 🪛 Wood screws 50-70 mm (galvanized, with press washer).
- 🧲 Level or laser level - to check evenness.
- 🛡️ Antiseptic and brush — for processing wood before installation.
Fastening the sheathing to the rafters is a crucial moment. Use galvanized screws with a diameter of 4-5 mm and a length of at least 50 mm (they should fit into the rafters by 30-40 mm). Mounting step:
- For boards - 2 screws at each intersection with the rafters (at the edges of the board).
- For timber - 1 self-tapping screw is enough, but the diameter is at least 5 mm.
Don't skimp on fasteners! Ordinary black self-tapping screws will rust in 2-3 years, and the sheathing will begin to “walk.” For regions with high humidity (for example, Leningrad region), it is better to take screws with EPDM gasket — they seal the hole and prevent moisture from entering.
Before installing the sheathing, treat all boards with an antiseptic on both sides and ends. This will protect the wood from rot and fungus, especially if the roof is insulated (condensation is inevitable).
4. Step-by-step installation of the sheathing: from marking to fastening
Now let's move on to practice. Installation of sheathing under corrugated sheets on a pitched roof is carried out in 5 stages:
- Marking the rafters. Mark the lathing fastening lines on the rafters, taking into account the selected step. Use a tapping cord for straight lines.
- Fastening the first board. It should be 10-15 mm thicker than the others (for example, 32 mm instead of 25 mm), since the lower edge of the corrugated sheet will rest on it. Secure it flush with the bottom edge of the rafters.
- Installation of intermediate elements. Lay the boards/beams perpendicular to the rafters, checking each element with a level. The permissible difference in height is no more than 2 mm per 1 m of length.
- Installation of counter-lattice (if necessary). If the roof is insulated, between the rafters and the sheathing you need to lay diffusion membrane and secure it with a counter-lattice (20×40 mm slats).
- Geometry check. After installation, check the roof diagonals - the difference should not exceed 2 cm. Otherwise, the corrugated sheeting will lie crooked.
Pay special attention the outer boards of the sheathing (first and last). They should be 10-15 mm thicker than the others, since they bear the main load from wind and snow. If you use timber, choose elements without knots - they can split when tightening the screws.
All sheathing boards are treated with an antiseptic | The pitch of the sheathing corresponds to the brand of corrugated sheeting and the slope of the roof | The outer boards are 10-15 mm thicker than the others | All screws are recessed into the wood (do not protrude) | The diagonals of the roof are the same (difference ≤2 cm)
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5. Common mistakes and how to avoid them
Even experienced builders sometimes make mistakes when installing sheathing under corrugated sheets. Here are the most common ones and how to prevent them:
- 🔧 Uneven surface of the sheathing. Reason: the use of boards of different thicknesses or untested rafter geometry. Solution: before installation, sort the boards by thickness (the difference is no more than 1-2 mm) and use a level when fastening.
- 🌧️ No ventilation gap. If the sheathing is mounted without a counter-lattice on an insulated roof, condensation will accumulate under the corrugated sheeting. Solution: use a diffusion membrane and counter-lattice with a thickness of 20-40 mm.
- 🌀 The sheathing pitch is too large. The corrugated sheet bends under the snow and waves appear. Solution: Always check the pitch table for your brand of corrugated sheet and angle of inclination.
- 🛠️ Fastening the sheathing with nails. Over time, the nails become loose and the boards begin to “play.” Solution: Use only galvanized self-tapping screws with a press washer.
Another common problem is discrepancy between the pitch of the sheathing and the width of the corrugated sheet. For example, if the sheets of corrugated sheets have a useful width of 1100 mm, and the sheathing is installed in increments of 50 cm, then the joints of the sheets will fall into the “emptiness”. In this case, you need to move the sheathing so that the joints are on the boards.
⚠️ Attention: if you are installing sheathing on an old roof (for example, when replacing slate with corrugated sheeting), be sure to check the condition of the rafters! Rotten or damaged elements must be replaced, and the rest must be treated with an antiseptic. Otherwise, the new sheathing will sag along with the rafters in 2-3 years.
What to do if the sheathing has already been installed incorrectly?
If the sheathing pitch turns out to be too large, you can add additional boards between the main ones (but this will increase the weight of the structure). If the plane is uneven, use roofing felt or thin plywood pads under the corrugated sheeting in problem areas. As a last resort, dismantle the sheathing and redo it correctly. This is cheaper than repairing a leaking roof after a year.
6. Additional elements: valley, ridge and cornice strips
The sheathing under the corrugated sheet is not only the main plane, but also the attachment points for additional elements. They need to be provided for in advance:
- 🏠 Cornice strip. Attaches to the first (bottom) board of the sheathing. For it, you need to provide an additional 50x50 mm block attached along the cornice.
- 🔺 Horse. If the roof is pitched, a ridge is not needed, but the end strips are attached to the outer boards of the sheathing. Reinforce them with additional screws.
- 🌊 Endova (adjacent to the wall). At the junction of the roof and the wall, a continuous sheathing 30-50 cm wide is installed to secure the valley.
- 🪜 Staircase or railing. If you plan to install a ladder for roof maintenance, secure additional sheathing bars in the places where it is attached.
To attach additional elements, use self-tapping screws with rubber gasket (for example, 4.8×28 mm). They seal the hole and prevent leaks. The mounting step for the planks is 20-30 cm.
If the roof is adjacent to the wall of the house, do continuous sheathing to a width of 30-50 cm from the joint. This is necessary for reliable fastening of the valley or sealing tape. Also don't forget to leave temperature gap 5-10 mm between the corrugated sheet and the wall - it compensates for the expansion of the metal when heated.
7. Checking and finishing before laying corrugated sheets
Before laying corrugated sheets, be sure to check the sheathing for:
- Evenness of the plane. Walk along the sheathing (if the roof allows it) or check with a level - differences should not exceed 2 mm per 1 m.
- Fastening strength. Try rocking the boards by hand - if there is any play, tighten the screws or add fasteners.
- No protruding screws. They must be recessed into the wood by 1-2 mm, otherwise they will damage the waterproofing of the corrugated board.
- Ventilation gaps. If the roof is insulated, make sure that there is a gap of at least 40 mm between the insulation and the corrugated sheeting for air circulation.
After checking, you can begin laying the waterproofing film (if it is provided for in the project). Suitable for corrugated sheets diffusion membrane (for example, Tyvek or Yutawek). It is laid with an overlap of 10-15 cm and secured to the sheathing with a construction stapler. Mounted on top of the membrane counter-lattice (slats 20x40 mm), and already on it - the main sheathing for corrugated sheets.
The main rule: the sheathing must be stronger than the corrugated sheet. If the sheets can withstand a load of 200 kg/m², the lathing must withstand at least 300 kg/m² with a margin. Otherwise, during heavy snowfall, the roof will fail.
FAQ: Answers to frequently asked questions
Is it possible to make lathing from a metal profile?
Yes, but this is not practical for pitched roofs. Metal profile (for example, CD-60) is more expensive than wood, more difficult to install and requires additional thermal insulation (otherwise there will be condensation). Wooden sheathing is cheaper, easier to install and retains heat better. Metal is used only for industrial facilities or if increased fire resistance is needed.
What lathing pitch is needed for C21 corrugated sheeting at a slope of 7°?
For S21 with a slope of 7°, the sheathing pitch should be no more than 30 cm. If the snow load in your region is high (more than 180 kg/m²), reduce the pitch to 20-25 cm or make a continuous sheathing from a 25x100 mm board with a gap of 10 mm.
Does the sheathing need to be treated with a fire retardant?
For outbuildings (shed, garage), fire protection is not required. But if the roof is over a residential building or bathhouse, it is better to use fire-bioprotective impregnations (for example, Neomid 450 or Senezh Ognebio). They protect the wood from both rotting and fire.
Is it possible to attach corrugated sheets directly to the sheathing without waterproofing?
For a cold roof (not insulated), waterproofing is not required, but is recommended. It protects the rafters and sheathing from condensation that forms on the inside of the corrugated sheet due to temperature changes. If your budget is limited, at least use glassine - it’s cheaper than a membrane, but better than nothing.
What to do if the rafters are installed unevenly?
If the difference in rafter height is no more than 10 mm, you can compensate for it with the thickness of the sheathing boards (for example, use 25 mm and 30 mm boards in different places). If the difference is greater, you will have to level the rafters using plywood pads or boards. As a last resort, dismantle and reinstall the rafter system.