Sheathing pitch for corrugated sheets: calculation, SNiP standards and typical installation errors

Sheathing under corrugated sheets is the basis on which the strength, durability and tightness of the entire structure, be it a roof, a fence or a facade, depends. Errors in choosing the pitch of the sheathing lead to sagging sheets, leaks, squeaks in the wind, and even collapse of the coating. In this article we will look at how to correctly calculate the sheathing pitch for different types of corrugated sheeting, taking into account the angle of inclination, snow load and climatic conditions.

Many craftsmen mistakenly believe that “the more often the better,” but excessive lathing increases the cost and weight of the structure, and insufficient lathing reduces the load-bearing capacity. We will analyze SNiP and SP standards, compare the manufacturers' recommendations (Grand Line, Metrotile, Ruukki) and show you how to avoid 5 critical mistakes during installation. At the end of the article - a unique table of sheathing steps for corrugated sheets of all brands (from C8 to H114) taking into account the angle of inclination of the roof.

What is sheathing and why is the step so important?

Lathing is a frame made of wooden beams or metal profiles, which is mounted on top of the rafters (on the roof) or load-bearing posts (on fences/facades). It serves as the basis for attaching the corrugated sheet and performs three key functions:

• 🔹 Load distribution — evenly transfers the weight of the corrugated sheet, snow and wind to the rafter system.
• 🔹 Ventilation — creates a gap between the corrugated sheet and the waterproofing, preventing condensation.
• 🔹 Fastening sheets — provides rigid fixation of corrugated sheeting at joints and overlaps.

The sheathing pitch is the distance between the centers of adjacent beams (or profile axes). It depends on:

• 📉 Roof load-bearing capacity — with a large step, the profiled sheet bends under the snow.
• 💧 Tightness - sparse lathing leads to deformation of sheets and leaks at joints.
• 🎵 Noisiness — an incorrect step increases vibration and hum in rain/wind.

⚠️ Attention: Profiled sheet brand S8-S21 (wall) is not intended for roofs with a slope angle of less than 15° - even with a minimum sheathing pitch (300 mm), it will allow water to pass through the overlaps.

SNiP and SP standards: official requirements for lathing pitch

The main regulatory documents governing the pitch of lathing under corrugated sheets:

• 📜 SNiP II-26-76 (“Roofs”)—establishes minimum requirements for roofing coverings.
• 📜 SP 17.13330.2017 (updated version of SNiP) - contains calculated loads for different regions.
• 📜 GOST 24045-2016 — classifies corrugated sheets by grade and load-bearing capacity.

Key requirements from the standards:

1. For roofs with a slope of up to 15° - the pitch of the sheathing should not exceed 300–400 mm (depending on the brand of profiled sheet).
2. For slopes of 15–30° - a step up to 450–600 mm.
3. For slopes over 30° - the step can reach 1000 mm, but reinforcement is required at the joints.

Important: the standards are advisory in nature. Manufacturers of corrugated sheets (for example, Ruukki or Grand Line) often tighten the requirements in their instructions. For example, for a profiled sheet NS35 at a slope of 10° Grand Line recommends step 300 mm, while SNiP allows 400 mm.

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

Calculation of the sheathing pitch depends on 5 parameters:

1. Brand of corrugated sheet (its load-bearing capacity).
2. Roof angle (or the verticality of the fence).
3. Snow load (regional coefficient).
4. Rafter pitch (distance between sheathing supports).
5. Availability of insulation (affects the rigidity of the structure).

Calculation formula:

Lathing pitch (mm) = (Permissible load on corrugated sheet (kg/m²) × Inclination angle coefficient) / (Snow load (kg/m²) × Safety factor 1.2)

Example: for profiled sheet NS35 in Moscow (snow load 180 kg/m²) with a slope of 20°:

(300 kg/m² × 0.94) / (180 kg/m² × 1.2) ≈ 1.3 → lathing pitch no more than 400 mm.

Check the brand of corrugated sheet and its load-bearing capacity (see certificate)

Measure the angle of the roof (or the verticality of the fence)

Check the snow load for your region (SP 20.13330)

Calculate the sheathing pitch using the manufacturer’s formula or table

Prepare beams 50×50 mm (for roofs) or 40×40 mm (for fences) -->

Table of sheathing steps for corrugated sheets of all brands

Below is a universal table of sheathing steps for corrugated sheets depending on the angle of the roof. Data based on recommendations SNiP and leading manufacturers (Ruukki, Grand Line, Metrotile).

Brand of corrugated sheet	Tilt angle, °	Max. sheathing pitch, mm	Notes
S8–S10	15–30	300–400	Only for walls/fences! Not applicable on the roof.
S21	15–30	400–500	Allowed on roofs with a slope of 20°.
NS35	10–15	300–350	Continuous sheathing is required in the valleys.
NS35	15–30	450–600	Optimal for most regions of Russia.
H60–H75	5–10	400–500	Suitable for flat roofs with reinforced frames.
H114	from 3	up to 1000	Used in industrial buildings.

Explanations for the table:

• 🔸 For wall profiled sheet (S8–S21) the sheathing spacing on fences can reach 1000–1500 mm, but requires transverse joists.
• 🔸 In regions with high snow load (Ural, Siberia), the step is reduced by 20–30%.
• 🔸 For skylights and the end of the sheathing should be solid (OSB sheets or board with a gap of 10 mm).

Typical mistakes when installing sheathing

Mistake #1: Ignoring Tilt Angle.

Many craftsmen lay corrugated sheets NS35 with a pitch of 600 mm on a roof with a slope of 10°, although this angle requires 300–350 mm. The result is deflection of the sheets after the first snowfall.

Mistake #2: Saving on bars.

Using thinner timber 50×50 mm or boards less than wide 100 mm leads to deformation of the sheathing under load. Timber is acceptable for fences 40×40 mm, but with a step of no more than 800 mm.

Mistake #3: No ventilation gap.

If the sheathing is mounted close to the waterproofing, condensation accumulates under the corrugated sheet, which leads to corrosion and rotting of the rafters. Minimum clearance - 20–30 mm.

Mistake #4: Uneven surface of the sheathing.

The differences in height are more than 2 mm at 1 m lead to a loose fit of the corrugated sheet and leaks. Check the sheathing before installation usually 2 m long.

Mistake #5: Lack of antiseptic.

Untreated wood rots in 3–5 years, especially in the places where the corrugated sheet is attached (moisture accumulates there). Use fire-bioprotective impregnations (for example, Neomid 440).

⚠️ Attention: When installing corrugated sheets on the sheathing cannot be used self-tapping screws less than 28 mm - they do not provide reliable fastening. Optimal length: 35–45 mm (for roofs) and 19–25 mm (for fences).

Lathing for fences and facades: features

For vertical structures (fences, facades) other rules apply:

• 📏 Lathing step depends on fence height:
  • Up to 1.8 m - 1000–1500 mm (2 logs: top and bottom).
  • 1.8–2.5 m — 800–1000 mm (3 lags).
  • Above 2.5 m - 600 mm + transverse slopes.
• 🔩 Fastening: for fences, self-tapping screws with a press washer are used 5.5×19 mm (color to match the corrugated sheet).
• 🌬️ Ventilation: a gap is left between the ground and the corrugated sheet 50–100 mm for air circulation.

Calculation example for a fence 2 m high:

• The bottom joist is at its best 300 mm from the ground.
• The middle lag is at its best 1000 mm.
• The top joist is at its best 1900 mm.
• Step between lags: 700 mm and 900 mm.

What happens if the sheathing step is too large?

If the sheathing is sparse, the corrugated sheet on the fence will begin to “wave” in the wind, and on the roof it will begin to sag under the snow. Microcracks will appear at the attachment points, which will eventually lead to corrosion. This is especially dangerous for polymer-coated corrugated sheets (PET, Pural), since damage to the protective layer accelerates rusting by 3–5 times.

Lathing materials: what to choose

For sheathing under corrugated sheets use:

1. Wooden beam:
   • 🌲 Pine or spruce (humidity no more than 18%).
   • 📏 Section: 50×50 mm (roof), 40×40 mm (fence).
   • ✅ Pros: cheap, easy to install.
   • ❌ Cons: rots, requires treatment.
2. Metal profile:
   • 🔧 Profile U-shaped or channel (metal thickness from 1.5 mm).
   • ✅ Pros: does not rot, can withstand heavy loads.
   • ❌ Cons: more expensive than wood, requires welding.

OSB or plywood:

• 📄 Thickness no less 18 mm (for continuous sheathing).
• ✅ Pros: smooth surface, suitable for soft roofing.
• ❌ Cons: afraid of moisture, expensive.

Recommendations for selection:

• 🏠 For residential buildings timber is optimal 50×50 mm (treated with antiseptic).
• 🏭 For industrial buildings - metal profile (for example, C-shaped with a pitch of 600 mm).
• 🛠️ For temporary fences timber can be used 40×40 mm in increments of up to 1500 mm.

💡

For roofs with a slope of less than 10°, the sheathing must be continuous or with a pitch of no more than 300 mm - even for load-bearing corrugated sheets H60. This is due to the risk of snow and ice accumulation.

FAQ: Frequently asked questions about the pitch of lathing under corrugated sheets

Is it possible to use lathing in 1 meter increments for NS35 corrugated sheets on a roof with a slope of 15°?

No, this exceeds the permissible limits. For NS35 with a slope of 15° the maximum step is 450–500 mm. With a step of 1 m, the corrugated sheet will bend under the snow load, which will lead to deformation of the fastenings and leaks.

What lathing pitch is needed for a fence made of C8 corrugated sheet with a height of 1.8 m?

For a fence 1.8 m high from corrugated sheets C8 enough two logs:

• Bottom - at the top 200–300 mm from the ground.
• Top - at the height 1600–1700 mm.

If your area experiences strong winds, add a third joist in the middle.

Is it necessary to treat the sheathing under the corrugated sheet with an antiseptic if it is made of larch?

Yes, even larch requires processing. Although it is resistant to rot, in places where the corrugated sheet is attached (where condensation accumulates), the wood may darken and lose strength. Use oil antiseptics (for example, Valtti Aquacolor), which do not block the pores of the wood.

Is it possible to attach corrugated sheets directly to the rafters without sheathing?

No, this is a gross violation of technology. Lathing is needed for:

• Uniform load distribution.
• Creating a ventilation gap.
• Fastening the corrugated sheets at the correct points (not in deflections).

The exception is solid OSB sheathing, but it can also be installed perpendicular to the rafters.

What lathing pitch should I choose for H60 corrugated sheet on a roof with a 5° slope?

For profiled sheet H60 with such a small slope, the sheathing pitch should be 400 mm, and in the valleys and along the edges - solid. Also recommended:

• Strengthen the rafter system (rafter pitch no more than 600 mm).
• Use sealing tapes under self-tapping screws.
• Provide snow guards every 2 m.