A gable roof with a slope of 30° is one of the most popular solutions for private houses, garages and outbuildings. Corrugated sheeting as a roofing material wins here due to the combination of strength, durability and affordable price. But even the highest quality profiled sheet will not last long if the sheathing underneath is not installed correctly. Sheathing pitch under corrugated sheeting at a slope of 30° - this is a critical parameter on which the roof’s resistance to snow loads, wind and deformations depends.

In this article we will figure out how to calculate the optimal lathing pitch, taking into account grades of corrugated sheets, climatic conditions and features of the rafter system. You will learn what mistakes lead to sheet bending and leaks, how to avoid unnecessary material costs and why for corrugated sheet C21 with a slope of 30°, the lathing pitch should not exceed 300 mm in regions with high snow loads. We will also present current standards from SNiP and GOST, which are often ignored by “self-taught craftsmen”.

Why does a 30° slope require a special approach to lathing?

A roof inclination angle of 30° is considered “borderline” for corrugated sheeting. On the one hand, such a slope ensures good drainage of water and snow, but on the other hand, the snow load is distributed unevenly here. With a smaller slope (15–25°), the snow rolls off worse, and the sheathing is done more often. With a larger one (35°+), the load is reduced and the step can be increased. But 30° is the “golden mean”, where errors in calculations lead to:

  • 🏔️ Deflection of profiled sheets under the weight of wet snow (especially important for grades C8–C20).
  • 💨 The roof blown off by the wind with sparse lathing - due to the “sailing effect” of the sheets.
  • 💧 Leaks in places of overlap, if the sheathing does not withstand deformations.

In addition, at a slope of 30° slope length increases compared to flat roofs, which affects the choice of cross-section of the sheathing boards. For example, for slopes more than 6 meters long, even with an optimal pitch, it may be necessary to reinforce them with counter-lattice or use boards 32 mm thick instead of the standard 25 mm.

📊 What corrugated sheeting do you plan to use?
S8–S10 (economy)
S18–S21 (universal)
S44–N75 (load-bearing)
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Regulatory requirements: what SNiP and GOST say

Basic documents regulating the installation of sheathing under corrugated sheets:

  • 📜 SNiP II-26-76 (“Roofs”) - determines the minimum requirements for sheathing spacing depending on the slope and type of material.
  • 📏 GOST 24045-2016 — classifies corrugated sheets according to their load-bearing capacity (grades C, NS, N).
  • ❄️ SP 20.13330.2016 (“Loads and impacts”) - contains maps of snow and wind loads for regions of Russia.

According to these documents, the following rules apply for roofs with a slope of 30°:

Brand of corrugated sheet Maximum sheathing pitch (mm) Notes
S8–S10 300 Only for regions with a snow load of up to 120 kg/m² (for example, Krasnodar Territory).
S18–S21 300–400 A pitch of 400 mm is acceptable with reinforced sheathing (board 32x100 mm).
S44, NS35 400–500 For industrial facilities or roofs with high loads.
H60–H75 500–1000 It is rarely used due to its high cost, but it allows you to reduce the cost of lathing.
⚠️ Attention: In the Moscow, Leningrad regions and the Urals, the snow load exceeds 180 kg/m². Here for corrugated sheeting C21 lathing step necessarily reduced to 300 mm, even if the manufacturer allows 400 mm.
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Before purchasing corrugated sheets, check its certificate of conformity to GOST 24045-2016. Some “economy” brands (for example, C8) may have a real load-bearing capacity that is 15–20% lower than declared.

How to calculate the sheathing pitch: step-by-step instructions

Calculation of the sheathing pitch includes 4 key parameters:

  1. Brand of corrugated sheet (determines load-bearing capacity).
  2. Snow load (taken from SP 20.13330.2016 for your region).
  3. Ramp length (affects the deflection of the sheathing boards).
  4. Rafter pitch (standard - 600–900 mm).

Calculation algorithm:

  1. Determine the snow load for your region (for example, for Moscow it is 180 kg/m², for Sochi it is 120 kg/m²).
  2. Multiply the load by a factor of 0.7 (for a 30° slope). We get effective load:
    3. 180 kg/m² × 0.7 = 126 kg/m²
  3. Compare the result with the load-bearing capacity of the corrugated sheet (indicated in the product passport). For example, for C21 it is 150 kg/m².
  4. If the effective load is ≤ 80% of the bearing capacity of the corrugated sheet, the lathing pitch can be taken maximum (from the table above). If more, the step is reduced by 20–30%.

Specify snow load according to SP 20.13330.2016|

Check the brand of corrugated board and its certificate|

Calculate the effective load taking into account a slope of 30°|

Select the sheathing pitch with a margin of 20% (in case of extreme precipitation)|

Prepare the sheathing boards (humidity ≤ 18%, without knots) -->

Typical mistakes and their consequences

Even experienced roofers sometimes make mistakes when installing sheathing under corrugated sheeting at 30°. Here are the most common:

  • 🔨 Using boards of different thicknesses. For example, alternating 25 mm and 32 mm leads to unevenness, due to which the profiled sheets are deformed when fastened.
  • 📏 Wrong step at the ridge and cornice. At the ridge, the sheathing is often done less frequently “to save money,” but this leads to sagging of the top row of corrugated sheeting.
  • 🌧️ No ventilation gap between waterproofing and sheathing. With a slope of 30°, condensation drains worse, and without a gap (minimum 20 mm), wood rotting begins.
  • 🔩 Fastening the sheathing "by surprise" between the rafters. The boards must lie freely, otherwise when they dry out they will move and the corrugated sheeting will “go in waves.”
⚠️ Attention: If you are installing sheathing on a roof with skylights, the step around the window should be 1.5 times smaller than the main one! For example, with a step of 400 mm, 250–300 mm are made around the window. Otherwise, when the sash is opened, the profiled sheets will vibrate.
What happens if you exceed the pitch of the sheathing?

With a pitch of 500 mm for C21 corrugated sheeting (instead of the recommended 300–400 mm), after 2–3 winters the following will appear:

- Waves on sheets due to sagging under the snow.

- Cracks in places where screws are attached (especially at the ridge).

- Leaks at the joints of sheets, since the sealing gaskets of self-tapping screws do not compensate for deformation.

Such a roof can only be restored by completely rebuilding the sheathing and replacing damaged corrugated sheeting.

Practical advice from roofers

Experienced craftsmen share life hacks that you won’t find in SNiP:

  • 🛠️ Use softwood boards for sheathing (pine, spruce) - they are less susceptible to rotting. Optimal cross-section: 25×100 mm or 32×100 mm for long slopes.
  • 🔧 Attach the sheathing to the rafters with galvanized nails 70–90 mm (not with self-tapping screws!). The nail must enter the rafter at least 40 mm.
  • 📐 Check the evenness of the sheathing with a string. A difference in height of more than 2 mm per 1 m will lead to unevenness of the corrugated sheet.
  • 🌡️ Install the lathing at air humidity ≤ 70%. After drying, raw wood can “turn” and the sheets will lie crooked.
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The most common cause of leaks on roofs with a 30° slope is discrepancy between the pitch of the sheathing and the wave of corrugated board. For example, if the pitch of the lathing is 350 mm, and the wavelength of the sheet is 200 mm, then the screws will be attached to the “sagging” areas, which will lead to deformation.

One more nuance: if you lay insulated roof, the sheathing pitch must match the width of the insulation boards (usually 600 mm). In this case, install sparse lathing with additional bars for the joints of corrugated sheets.

Calculation example for the Moscow region

Let's consider a real example: a gable roof with a slope of 30° in the Moscow region, corrugated board S21-1000 (thickness 0.5 mm), slope length 5 m, rafter pitch 800 mm.

  1. Snow load for Moscow: 180 kg/m².
  2. Effective load taking into account the slope: 180 × 0.7 = 126 kg/m².
  3. Load-bearing capacity C21: 150 kg/m² (according to the passport).
  4. Safety margin: 150 / 126 ≈ 1.19 (19%). This is less than the recommended 20–25%, so we reduce the lathing pitch by 10% of the maximum (from 400 mm to 360 mm).
  5. Final step: 350 mm (rounded down to simplify installation).

Additionally:

- We use a 25×100 mm board (since the length of the slope is ≤ 6 m).

- At the ridge and cornice, reduce the pitch to 300 mm.

- We attach the waterproofing with a sag of 20 mm for ventilation.

FAQ: Frequently asked questions about lathing under 30° corrugated sheeting

Is it possible to use OSB instead of sheathing boards?

Yes, but only moisture-resistant OSB-3 thickness ≥ 18 mm. However, there are nuances:

- The pitch of the rafters must be ≤ 600 mm.

- OSB sheets must be installed with a gap of 3–5 mm (for thermal expansion).

- For corrugated sheets C8–C21, the step for attaching screws to OSB is reduced to 300 mm (instead of the standard 500 mm).

What fasteners should I use for the sheathing?

The best option is rough nails 3.5×70 mm or wood screws 4.2×75 mm. Important:

- The fastening must penetrate the rafter to a depth of ≥ 40 mm.

- Step for attaching the sheathing to the rafters: 500–600 mm.

- Do not use “black” screws - only yellow (galvanized) or stainless ones.

Is a vapor barrier under the sheathing necessary on a 30° slope?

Vapor barrier is required if the roof insulated. For cold roofs (for example, a garage), it does not need to be installed. With a slope of 30°, the vapor barrier is laid with an overlap of 150–200 mm and glued butyl rubber tape (not tape!).

Is it possible to make a sheathing pitch of 500 mm for C21 corrugated sheeting if the roof is well ventilated?

No, even with perfect ventilation a pitch of 500 mm for C21 with a slope of 30° is unacceptable. It's not a matter of ventilation, but distributed load: snow will bend the sheets between the sheathing boards. The maximum for C21 is 400 mm (and then with a snow load of ≤ 120 kg/m²).

How to check the quality of the sheathing before laying corrugated board?

Check:

- Flatness: Pull the string along the diagonals of the slope - the deflection should not exceed 5 mm per 3 m.

- Humidity of boards: Use a moisture meter (normal ≤ 18%).

- Fastening: try rocking the boards by hand - there is no play.

- Gaps: there should be no gaps > 5 mm between the boards (with the exception of ventilation gaps at the ridge).