What kind of lathing is needed for a corrugated sheet on the roof: a complete guide

Corrugated sheeting is one of the most popular roofing materials due to its strength, durability and affordable price. But for a corrugated sheet roof to last for decades without leaks or deformations, a key role is played by sheathing - a frame on which the sheets are attached. Errors in its construction lead to sagging of the material, squeaks in the wind, and even collapse of the roof.

In this article we will look at what kind of lathing is needed for corrugated sheets depending on the angle of the roof, the grade of corrugated sheeting and climatic conditions. You will learn how to choose a material (wood or metal), calculate the pitch between boards, avoid common mistakes during installation and save money without losing quality.

Let us note right away: there is no universal solution. Sheathing under corrugated sheets for a garage roof with a slope of 10° and for a private house with an angle of 45° will be radically different. As well as the requirements for it for load-bearing corrugated sheets NS-35 and wall S-8. Therefore, we will analyze in detail all the options - from budget to premium, taking into account SNiP and the practice of professional roofers.

If you are planning to install the roof yourself, this article will help you avoid costly alterations. For clarity, we present tables with exact lathing steps for different brands of corrugated sheets, as well as a checklist for calculating materials. Let's start with the basics - what functions does the sheathing perform and why it cannot be done “by eye”.

1. Why do you need lathing for corrugated sheets: 3 key functions

Many people believe that sheathing is simply a “substrate” for fastening sheets. In fact, it performs three critical tasks:

1. Uniform load distribution. Corrugated sheeting is a rigid material, but under point pressure (for example, from snow or the weight of a person), it can bend. The sheathing transfers the load to the rafter system, preventing deformation. This is especially true for roofs with a low slope (up to 15°), where snow lingers longer.

2. Ventilation of the under-roof space. Air must circulate between the corrugated sheet and the waterproofing, otherwise condensation will lead to rotting of the wooden elements. The correct sheathing creates a ventilation gap (usually 20–50 mm), which removes moisture outside.

3. Compensation for temperature expansions. Corrugated sheeting heats up to +80°C in summer and contracts to -30°C in winter. The sheathing fixes the sheets with a gap, preventing “waves” when the size of the metal changes.

Without properly installed sheathing, even expensive corrugated sheeting will last 2–3 times less. For example, with a step of 1 m instead of the recommended 30 cm for NS-44 the sheets will begin to “play” in the wind, and the fastenings will become loose within a year.

2. Materials for lathing: wood vs metal

The choice of material depends on the budget, climate and type of roof. Let's look at the pros and cons of each option.

Wooden sheathing - the most common choice (about 80% of roofs). Use coniferous species (pine, spruce, larch) with a moisture content of no more than 20%. Optimal board cross-section:

• 📏 25×100 mm — for a step of 30–50 cm (standard version);
• 📏 32×100 mm — for a pitch of 60–100 cm or heavy sheets (NS-57);
• 📏 50×50 mm — for counter-lattice (installed on top of waterproofing).

Pros of wood:

• ✅ Low price (from 150 rub/m²);
• ✅ Easy installation (does not require welding);
• ✅ Good thermal insulation.

Cons:

• ❌ Rots with poor ventilation;
• ❌ Requires treatment with an antiseptic (for example, Senezh Ultra);
• ❌ Deforms due to humidity changes.

Metal sheathing made of galvanized profile (thickness 0.7–1.2 mm) or aluminum. Suitable for industrial facilities or roofs with aggressive environments (for example, near the sea).

Metal advantages:

• ✅ Service life 50+ years;
• ✅ Does not rot and is not flammable;
• ✅ Withstands pitch up to 1.5 m (for profiled sheet N-114).

Cons:

• ❌ 3-5 times more expensive than wood;
• ❌ Requires welding or special fastenings;
• ❌ “Bridges of cold” - worsens thermal insulation.

For private houses, in 90% of cases, wooden sheathing is optimal. Metal is only suitable for large spans (such as hangars) or in regions with high humidity.

3. Lathing pitch for corrugated sheets: table for different brands

The sheathing pitch is the distance between the centers of adjacent boards. It depends on:

• 🔹 Profiled sheet brands (the higher the wave, the thinner the sheathing);
• 🔹 Roof angle (on flat roofs the pitch is reduced);
• 🔹 Snow and wind load (for northern regions the step is reduced by 20%).

The table below shows recommended lathing pitch for popular brands of corrugated sheeting under standard conditions (snow load up to 180 kg/m², slope 15–30°):

Brand of corrugated sheet	Wave height (mm)	Roof slope	Lathing pitch (mm)	Notes
S-8	8	15° or more	300–400	Only for walls or roofs with a slope of 45° or more
S-21	21	20° or more	300–600	For residential buildings, the pitch is no more than 400 mm
NS-35	35	10–15°	300	Continuous sheathing in valleys
NS-44	44	8–15°	500–700	For snowy regions, pitch 500 mm
N-114	114	from 6°	1000–1500	Requires reinforced rafters

For sloped roofs less than 10° the pitch of the sheathing is reduced by 30–50%, and installed at the joints of the sheets continuous sheathing (boards without gaps). This is due to an increased snow load and the risk of leaks.

Example: for profiled sheet NS-35 on a roof with a slope of 8° in the Moscow region (snow load 180 kg/m²), the sheathing pitch should be 300 mm, and not 500 mm, as for an angle of 20°.

4. Calculation of sheathing: formulas and example for a house 6x8 m

In order not to overpay for excess material and not face a shortage, the calculation of the sheathing is carried out in 3 stages:

1. Determine the roof area.

For a gable roof: Area (m²) = House length × (2 × Slope length + 2 × Overhang)

Example: house 6x8 m, projection 0.5 m, slope length 5 m: Area = 8 × (2 × 5 + 2 × 0.5) = 88 m²

2. Calculate the number of boards.

Formula: Number of boards = (Ramp length / Sheathing pitch) × Number of slopes

For the same house with a pitch of 400 mm: (5000 mm / 400 mm) × 2 = 25 boards per slope

Total: 25 × 2 = 50 boards 6 m long (across the width of the house).

3. We take into account the stock.

We add 10–15% for trimming and waste. In the example: 50 boards × 1.15 = 57–58 boards

For the counter-lattice (installed on top of the waterproofing), 50x50 mm bars are used with a pitch of 300–600 mm. Their number is calculated in the same way.

If the roof has a complex shape (hip, multi-gable), break it into simple shapes (triangles, trapezoids) and calculate the area of each separately.

Determine the slope of the roof (measure the angle or calculate using the formula)

Select the brand of corrugated sheet and specify the wave height

Calculate the snow load for your region (according to SP 20.13330.2016)

Check the moisture content of the boards (maximum 20%)

Buy fasteners (screws with rubber washer) at the rate of 8–10 pcs/m²-->

5. Installation of sheathing: step-by-step instructions with photos

The technology for installing sheathing under corrugated sheets includes 5 key stages. Let's look at them using the example of wooden sheathing for a gable roof.

Stage 1. Preparing the rafters

• 🔨 Check the evenness of the rafters with a level (tolerance - no more than 2 mm per 1 m).
• 🔨 Treat the wood with an antiseptic (for example, Pirilax) in 2 layers.
• 🔨 Install a waterproofing membrane (e.g. Tyvek) with an overlap of 10–15 cm, securing it with a construction stapler.

Stage 2. Installation of counter-lattice

• 📐 Stuff 50×50 mm bars over the waterproofing along the rafters.
• 📐 The pitch of the bars is 300–600 mm (depending on the pitch of the main sheathing).
• 📐 Secure the bars with galvanized nails or self-tapping screws.

Stage 3. Laying the main sheathing

• 📏 Start with the eaves overhang - the first board should be thicker than the rest (for example, 50x100 mm).
• 📏 Fasten the boards perpendicular to the rafters, starting from bottom to top.
• 📏 Use a template (cut board) to maintain the same step.
• 📏 Attach the boards to the counter-lattice with 50–70 mm self-tapping screws (2 pcs per rafter).

Stage 4. Strengthening problem areas

• 🏠 Make a continuous sheathing (boards without gaps) in the valleys and around the chimneys.
• 🏠 Add additional boards where snow guards are installed.
• 🏠 Strengthen the sheathing on eaves overhangs to secure the drainage system.

Stage 5. Quality check

• 🔍 Walk along the sheathing - the boards should not sag.
• 🔍 Measure the diagonals of the slope - the difference is no more than 20 mm.
• 🔍 Check the fastenings: the screws should be recessed, but not break through the board.

For metal sheathing, profiles are used instead of boards C- or Z-shape, which are attached to the rafters with metal screws or rivets. The installation step and procedure are similar to wooden sheathing.

What happens if the sheathing is installed incorrectly?

If the step is too large, the profiled sheet will bend under the snow, and the fastenings will begin to “break out”. If the boards are uneven, the sheets will lie crookedly and the joints will separate. In the absence of a ventilation gap, condensation will appear under the roof, which will destroy the rafters in 2–3 years.

6. Common mistakes and how to avoid them

Even experienced builders sometimes make mistakes when installing sheathing. Here are the most common ones and how to prevent them:

❌ Error 1: Saving on material

Using boards thinner than 25 mm or a pitch larger than recommended leads to deflection of the corrugated sheet. For example, for NS-35 with a pitch of 600 mm instead of 300 mm, the sheets “play” when walking on the roof.

⚠️ Attention: If the sheathing pitch exceeds the recommended one by more than 20%, the warranty for the corrugated sheeting (usually 10–15 years) is automatically canceled by the manufacturer.

❌ Mistake 2: Ignoring the counter-lattice

Without a counter-batten, the waterproofing is pressed against the insulation, and the ventilation gap disappears. This leads to wet insulation and rotting of the rafters. Solution: Always install 50x50 mm bars on top of the waterproofing.

❌ Mistake 3: Uneven boards

If the boards are not planed or have different thicknesses, the corrugated sheet will fall in waves. Before installation, sort the material and use boards with a thickness difference of no more than 1–2 mm.

❌ Error 4: Fastening without gaps

When joining boards, leave a gap of 2-3 mm for thermal expansion. If the boards are laid closely, they may “sway” in the summer, and the sheathing will become deformed.

❌ Error 5: Unaccounted snow load

In the northern regions (for example, Murmansk region), the sheathing pitch is reduced by 30%. Otherwise, the roof will not support the weight of the snow. Check the regulations for your region according to SP 20.13330.2016.

To avoid mistakes, before installation, draw up detailed drawing indicating the spacing of the sheathing, places of reinforcement and fastenings. Use a laser level to check for evenness.

7. Lathing under corrugated sheets for different types of roofs

The design of the sheathing depends not only on the profiled sheet, but also on the type of roof. Let's look at the features for the most common options.

🏠 Shed roof (slope 5–20°)

• Use a profiled sheet with a wave height of 35 mm (NS-35, N-60).
• Sheathing pitch: 300–400 mm (for an angle of 10° - solid).
• Reinforce the sheathing along the edges of the slope - this is where the maximum wind load is.

🏠 Gable roof (slope 20–45°)

• Optimal profiled sheet: NS-44 or MP-35.
• Lathing pitch: 500–700 mm (for NS-44 at a slope of 30°).
• Leave a 50 mm gap in the ridge for ventilation.

🏠 Hip or hip roof

• Complex geometry requires precise calculation - use 3D modeling (for example, in SketchUp).
• In the valleys, install a continuous sheathing 600–800 mm wide.
• For the joints of the slopes, use special strips (valleys).

🏭 Industrial buildings (slope 5–15°)

• Use corrugated sheet N-114 or N-75 with a sheathing pitch of 1000–1500 mm.
• Use metal lathing made of galvanized profile.
• Install snow guards every 2–3 m.

For sloped roofs less than 5° (flat roofs) corrugated sheets are rarely used - they do not provide sufficient tightness. In such cases, it is better to choose seam roofing or membrane materials.

8. Advice from professionals: how to save money without losing quality

Experienced roofers know how to reduce the cost of sheathing without compromising the longevity of the roof. Here are their recommendations:

💰 Tip 1: Optimize the sheathing pitch

For profiled sheet NS-35 with a slope of 20°, the pitch can be increased from 300 to 400 mm if:

• 🔹 The region does not belong to the high snow zone;
• 🔹 Rafters are installed in increments of no more than 600 mm;
• 🔹 Boards with a section of 32 × 100 mm are used.

💰 Tip 2: Use an unedged board

An unedged board (30–40% cheaper than a planed board) is suitable for the counter-lattice. The main thing is to clean it from the bark and treat it with an antiseptic.

💰 Tip 3: Buy profiled sheets and lathing from one supplier

Many companies give a discount of up to 10% for a complex order. For example, when purchasing a profiled sheet MP-20 and boards for sheathing in Metal Profile or Grand Line.

💰 Tip 4: Install the sheathing yourself

The cost of installing the sheathing is from 300 rubles/m². With a roof area of ​​100 m², you will save 30,000 rubles if you do it yourself (if you have the skills).

💰 Tip 5: Use used materials

For temporary buildings (for example, a garage), you can use used boards if they:

• 🔹 Have no traces of rot;
• 🔹 At least 30 mm thick;
• 🔹 Treated with antiseptic before installation.

⚠️ Attention: Don't use used boards for RVs - the risk of wood-boring beetles or mold is too high. For outbuildings, this option is acceptable if carefully processed.

Another way to save money is to order lathing from aspen instead of pine. It is 15–20% cheaper, but less resistant to rotting, so it requires mandatory processing deep penetration antiseptic (for example, Xyladecor).

FAQ: Frequently asked questions about lathing under corrugated sheets

Is it possible to make sheathing from plywood or OSB?

Yes, but only for roofs with a slope of 20° and corrugated sheets with a wave height of 35 mm or more. Use moisture-resistant plywood FSF 18–22 mm thick or OSB-3 (22 mm). The pitch of the lag under the plywood is 300–400 mm. This option is not suitable for flat roofs (up to 15°) - the plywood may swell from condensation.

Do you need a vapor barrier under the sheathing?

Vapor barrier is installed under insulation, and not under the crate. If the roof is cold (without insulation), a vapor barrier is not needed. For an insulated attic, the order of layers is as follows (from bottom to top): vapor barrier → insulation → waterproofing → counter-lattice → sheathing → corrugated sheet.

How to attach a profiled sheet to the sheathing?

Use roofing screws with rubber washer (EPDM) with a diameter of 4.8–6.3 mm. Self-tapping screw length = thickness of corrugated sheet + sheathing + 20 mm. Mounting step:

• 🔹 Along the perimeter of the leaf - through the wave;
• 🔹 In the center - in a checkerboard pattern through 2 waves;
• 🔹 In overlaps - in every wave.

Do not tighten the screws too much - the rubber washer should be compressed by 1-2 mm, but not cut through.

What to do if the sheathing is rotten?

For local damage (1–2 boards):

1. Carefully dismantle the corrugated sheet in the repair area (unscrew the screws).
2. Remove rotted boards and clean mold from rafters.
3. Install new boards treated with an antiseptic.
4. Return the corrugated sheet to its place by replacing the screws (rubber washers lose their elasticity after 5–7 years).

If a large area is damaged, you will have to completely redo the sheathing. In this case, check the condition of the waterproofing and replace it if necessary.

Is it possible to put corrugated sheets on old sheathing?

Yes, if:

• 🔹 The boards have no traces of rot or wood-boring beetles;
• 🔹 The pitch of the sheathing corresponds to the new profiled sheet;
• 🔹 The fastenings are reliable (the screws are not loose).

Before installation, remove the old waterproofing, check the ventilation gaps and, if necessary, add a counter batten.