Step of crates under the flooring on the roof: how to calculate and not make a mistake

The crate for flooring is the basis of a reliable roof, on which not only the durability of the coating depends, but also the safety of the whole house. Many owners of garages, cottages and private houses make a critical mistake: mount the crate “by eye”, not taking into account the mark, roof-inclination and climate stress. The result is sagging sheets, leaks, and in the worst case, a collapse of the roof under the snow.

In this article, we will discuss how to calculate correctly. floor-stepWhat are the rules governing installation, and why there is no universal solution for all roofs? You will learn how to avoid typical mistakes, save on materials without losing quality, and what nuances professional roofers take into account.

Let us note right away: the step of the crate is not just the distance between the boards. This is an engineering calculation that depends on profleet-carrying, slope And even the construction area. For example, for flooring C21 on the roof with a slope of 15° in the Moscow region step will be one, and H75 On the same roof in Sochi, it was completely different. Ignoring these parameters leads to the fact that the roof “plays” in the wind or bends under the snow.

We will not rewrite the SNiP - instead we will give practical recommendations with examples of calculations, tables and step-by-step instructions. At the end of the article, you will find an FAQ with answers to the most common questions, including controversial points (for example, can you use a CAP crate instead of a board).

1. What is a crate and why its step is critical for the flooring

The crate is a frame of wooden bars or boards, which is mounted on top of the rafters and serves as the basis for the installation of a profile sheet. Its main functions are:

• 🔹 Load distribution - evenly transfers the weight of snow, wind and the flooring itself to the truss system.
• 🔹 Ventilation creates a gap between the proflist and waterproofing, preventing condensation and rotting.
• 🔹 Roofing fixation - provides a rigid base for screws, excluding the "walking" of sheets.

The step of the crate is the distance between centre Two of the two sides of the slab (not between the edges!). It depends on:

• 📏 Roof strength Too much will lead to a deflection of the profile sheet under load.
• 💰 Cost of materials Excessive crates will increase the budget by 15-30%.
• ⚡ Term of service The wrong step accelerates corrosion in the places of fastenings.

Flooring is not a monolithic sheet, but a corrugated material with rigid ribs. Its bearing capacity depends on height (markings) C8, C21, NC35, H60 etc.). For example, profilel C21 can withstand a load of up to 200 kg / m2 with the correct pitch of the crate, and H75 - up to 600 kg/m2. If you ignore this characteristic, even expensive flooring will sag in a year or two.

2. Standards and Standards: What SNiP Says

The main document regulating the step of the crate under the flooring - SNIP II-26-76 "Roofs" (updated version of SP 17.13330.2017). According to the standards, the step of the crate is determined based on:

• 📐 Roof angle (The smaller the slope, the more frequent the crate).
• ❄️ Snow load in the region (calculated by the map of snow regions of the Russian Federation).
• 🌬️ Wind loads (Depends on the height of the building and the openness of the area).
• 🏗️ Marks are proflame (his carrying capacity).

Here are the key requirements from SNiP for profnatstiel:

Mark proflist	Roof slope	Maximum step of the crate (mm)	Note
C8, C10	15°	300–400	Only for outbuildings
C21, C44	15°–30°	400–600	A solid crate on the skate and endowments
NC35, NC44	30°	600–900	Sparse crate is allowed
H60, H75	45°	1000–1500	For load-bearing structures only

⚠️ Attention: If your region is related to III-IV snow area (for example, Siberia, the Urals), the crate pitch should be reduced by 20-30% from the table values. For flooring C21 at a slope of 20° in Novosibirsk, the optimal pitch is 350-400 mm, not 500 mm.

The SNIP also prescribes:

• 🔸 Use boards thicker at least 25 mm (for a step of up to 500 mm) or 32 mm (for a step greater than 500 mm).
• 🔸 The width of the board. 100–150 mm (Narrow boards are prone to deformation).
• 🔸 The humidity of the wood. less than 18% (Otherwise, the slate will be “leaded” after installation).

3. How to calculate the step of the crate: step-by-step instructions

Calculation of the step of the crate consists of 5 stages. Let's go through each one with examples.

Stage 1. We're slanting the roof.

Slope is measured in degrees or percentages. To find him:

1. Measure the height of the skate (H) and the length of the projection of the stingray (L).
2. We use the formula: Slope (%) = (H/L) × 100.
3. We translate the percentages into degrees on the table (for example, 30% ≈ 16.7 °).

Stage 2. Consider the snow load.

The map of snow regions of the Russian Federation divides the territories into 8 zones (from I to VIII). For example:

• Moscow III. (180 kg/m2).
• Yekaterinburg IV. (240 kg/m2).
• Sochi-- I. (50 kg/m2).

Data can be taken from SP 20.13330.2016 Or the Hydrometeorological site.

Stage 3. We're choosing a brand of profnastiel

For roofs, a profile sheet with a wave height of 20 mm is used. Popular brands:

• 🏗️ C21 - universal (suitable for slopes from 12°).
• 🏠 NC35 - optimal for residential buildings (gradient from 15°).
• 🏭 H60/H75 - for industrial buildings (gradient from 8°).

Stage 4. We determine the step of the crate on the table

We use the data from section 2 to adjust them for snow loads. Example:

Dan: roof in Kazan (III snow area), slope 20°, flooring NC35.

According to the table for NC35 at a slope of 15°-30° step - 400-600 mm. But Kazan is in the III district, so we reduce by 20%:

⚠️ Attention: The best step is 320–480 mm. For reliability, choose 400 mm.

Stage 5. Checking for wind loads

If the house is in an open area (steppe, coast), the step of the crate is reduced by 10-15%. For coastal regions (for example, Kaliningrad) - by 20%.

Check the moisture of the boards (max). 18%) | Calculate snow load according to JV 20.13330.2016 | Account for the wind area (SP 20.13330.2016, approx. F) | Pick a step on the table taking into account the mark of the profile list |Add a solid crate on the skate, endowments, adjoinments-->

4. Typical errors in the installation of the crate

Even experienced builders make mistakes that shorten the life of the roof. Here are the most common:

❌ Mistake 1. Use of unscrewed board

Untreated wood has irregularities that lead to:

• 🔹 Uneven fit of the proflist (waves appearing)
• 🔹 Damage to waterproofing (Sharp bitches break through the membrane).

Solution: use planer (grade not lower than 2nd).

❌ Mistake 2. Savings on the crate step

Many increase the step to save on boards. Consequences:

• 🔹 The swelling between the legs (especially the C8, C10).
• 🔹 Cracks in the places of fastenings under snow load.

Example: flooring C21 On a roof with a slope of 10° in St. Petersburg (IV snow area), a step of 600 mm instead of the recommended 300-350 mm will lead to deformation after 1-2 winters.

❌ Mistake 3. Lack of ventilation clearance

If the crate is mounted close to waterproofing, a condensate is formed, which:

• 🔹 Rotting wood (the service life of the crate is reduced by 2-3 times).
• 🔹 Corrodates the flooring from the inside (rust appears even on galvanized sheets).

Solution: use counter-recreation (bars 30×50 mm), creating a gap of 3-5 cm.

❌ Mistake 4. Fixing the crate "tap"

If the boards are joined at one point without displacement, a weak spot is formed:

• 🔹 Here the profile can crack under load.
• 🔹 Water seeps through the joints of the boards.

Solution: dock the boards offset (sniffing) and sealing the joints with sealant.

❌ Mistake 5. Ignoring deformation seams

Wood expands/compresses when the humidity changes. If you do not leave gaps between the boards (2-3 mm), the crate will "swell" in a year.

Solution: fix the boards from gap 2-3 mm And use galvanized nails (not screws!), which do not tear the wood when shrinking.

What happens if the crate rots?

Rotting of the crate leads to:

(1) Loss of roof stiffness - the profiler starts to "play" in the wind.

(2) Leaks - water leaks through the cracks in the rotten boards.

3) Collapse - in critical cases (e.g. snow load of 300+ kg/m2), the roof may collapse.

For repairs, you will have to dismantle the flooring, replace the crate and waterproofing. The cost of such repairs is from 50% of the cost of a new roof.

5. Total vs. Sparse crate: what to choose

The choice between solid and sparse crate depends on roof-slop and marker. Let’s look at the pros and cons of each option.

🔨 It's a crate.

Used for:

• 🔹 Roof with slope less than 15 degrees.
• 🔹 Fresh-flooring C8, C10, C21 (low load-bearing capacity).
• 🔹 regions with high snow load (IV-VIII regions).

Pros:

• ✅ Maximum roof rigidity.
• ✅ Suitable for soft roofing (if you later want to change the profile sheet to bitumen shingles).

Cons:

• ❌ Increase in cost by 25-40% (the cost of boards is higher).
• ❌ Risk of condensation accumulation (mandatory ventilation is required).

🔧 Sparse crate

Used for:

• 🔹 Roof with slope 20°.
• 🔹 Fresh-flooring NC35, H60, H75 (high load-bearing capacity).
• 🔹 Low snow load regions (I-III regions).

Pros:

• ✅ Saving materials (consumption of boards is less by 30–50%).
• ✅ The best ventilation of the under-roof space.

Cons:

• ❌ Not suitable for flat roofs (slop < 15°).
• ❌ It requires an accurate calculation of the step (an error will lead to a deflection of the profile list).

📌 When a solid crate is required:

• 🔹 Nana rendo (inside corners of the roof).
• 🔹 Around. chimneys and ventilation pipes.
• 🔹 Nana skate (top edge of the roof).

6. Materials for crate: what is better to choose

For the crate under the flooring, use:

1. Wooden boards - Classical version (pine, spruce).
2. SWAps (OSB) - solid crate plates.
3. FSF phanera - waterproof material.
4. Metal profile - for industrial buildings.

🪵 Wooden boards

The best choice for private homes. Requirements:

• 🔹 Grade: 1st or 2nd (no knots or cracks).
• 🔹 Humidity: less than 18% (Otherwise, it will be after the installation).
• 🔹 Dimensions:
  • Thickness: 25 mm (step up to 500 mm), 32 mm (step 500-1000 mm).
  • Width: 100–150 mm (Narrow boards are prone to deformation).
• 🔹 Breed: pine-tree (The squirrel is more expensive but longer lasting).

Processing: mandatory impregnation antiseptic (e.g., Senege Ultraand flame-protection (e.g., pyrilax).

📦 SWAps (OSB-3)

Suitable for solid-crate floor-board C8-C21. Requirements:

• 🔹 Thickness: 12–18 mm (for rafters up to 600 mm in pitch).
• 🔹 Class: OSB-3 (moisture resistant).
• 🔹 Fixing: self-tapping through each 15–20 cm edges and 30cm in the middle of the sheet.

Pros: flat surface, no risk of deformation.

Cons: more expensive boards by 30-50%, requires mandatory ventilation (OSP does not "breath").

⚠️ Attention: Don't use it. chipboard or FDP These materials swell from moisture and lose strength after 1-2 seasons.

🛠️ Metal profile

It is used for industrial buildings (hangars, warehouses). Advantages:

• 🔹 The service life 50+ years (No rotting, no deforming).
• 🔹 Suitable for step-to-step 1.5 m m m m m (when using proflame flooring) H75).

Disadvantages:

• ❌ High cost (2-3 times more expensive than wooden crate).
• ❌ Difficulty of installation (welding or special brackets are required).

7. Step by step installation of crates under the flooring

Consider the installation on the example of a wooden crate for a roof with a slope of 25 ° and profnastil NC35.

📌 Preparation of materials and tools

You'll need:

• 🔹 Board 25×100 mm (length by size of the ramp).
• 🔹 30x50mm bars for counter-rack.
• 🔹 Hydroinsulating membrane (e.g., Tyvek or isospan AM).
• 🔹 Galvanized nails 70-90 mm or screws on wood.
• 🔹 Antiseptic for wood.
• 🔹 Tools: hacksaw, screwdriver, level, roulette, hammer.

📌 Stage 1. Waterproofing installation

1. Rolling the membrane horizontally (cornice to skate) with a flyover 10–15 cm.
2. We are putting the slack on the slacks (not stretching!) should be sagging 1-2 cm.
3. Joints are glued butyl-rope.

📌 Stage 2. Installation of counter-recs

1. We fill bars 30x50 mm rafter on top of the waterproofing.
2. The step of the bars rafter-step (usually 600-1000 mm).
3. Counter-break creates a ventilation gap 3-5 cm.

📌 Stage 3. Crate fastening

1. Starting editing. bottom-up (cornice to skate).
2. First board is fastened. cornice-line.
3. Step between the boards. 400 mm (for our example).
4. Fix the boards. nail-galvanized (not self-tapping!) to the counter-race.
5. We have board joints. flinging (not in one place).

📌 Stage 4. Strengthening of problem areas

• 🔹 Endows: mount a solid crate with a width 60-80 cm both sides of the axis.
• 🔹 Horse: We are installing additional boards with step 100–150 mm.
• 🔹 Pipes and skylights: the crate is solid at a distance 30-50 cm from the edge.

📌 Stage 5. Quality check

• 🔹 Check it out. level evenness of the crate (tolerance - no more than 2 mm per 1 m).
• 🔹 Make sure that no-deflection When you press the boards.
• 🔹 Check the mounts - the nails should be drowned into the wood, but not to tear it.

8. Frequent Questions (FAQ)

❓ Can I use a slat instead of a board?

Yes, but only for solid-crate under the flooring with a wave height of up to 21 mm (for example, C8, C10, C21). Requirements:

• 🔹 The thickness of the SWA 12–18 mm.
• 🔹 Class-- OSB-3 (moisture resistant).
• 🔹 The rafters' step is nothing more. 600 mm.

For the flooring NC35 and above the OSW is not recommended - it does not withstand point loads from screws.

❓ What step of crate is needed for the C21 flooring on the roof of the garage (sloping 10°, Moscow)?

For flooring C21 slant-sided 10° into Moscow (Snow region III):

• 🔹 The best step is 300–350 mm.
• 🔹 Material - board 25×100 mm (humidity up to 18%).
• 🔹 It's a whole crate on skating.

If you take a step of 400-500 mm, the profile will sag under the snow in winter.

❓ Do I need to treat the crate with antiseptic if it is under profnatile?

✅ I will! Even under the flooring, wood is subject to:

• 🔹 Condensate (due to temperature differences).
• 🔹 Fungus (if the humidity of the wood exceeds 20%).
• 🔹 Tree beetles (in untreated pine).

Recommended formulations:

• 🔹 Senege Ultra (Protection for 10-15 years).
• 🔹 Pinotex Classic (Coled antiseptic)
• 🔹 Tikkurila Valtti Poise (for wet conditions)