The garage rafter system is not just a frame for the roof, but the basis on which the strength of the entire structure depends. If you are planning to roof professional sheet (and this is one of the most popular materials for garages due to the combination of price and durability), then the distance between the rafters becomes a critical parameter. A step that is too large will lead to the sheets sagging under snow or wind, while a step that is too small will result in excess wood consumption and weighting of the structure.

Many garage owners make the mistake of copying the pitch of rafters from neighboring buildings or “by eye.” However profiled sheet - the material is rigid, but not capable of compensating for the deformation of the sheathing. For example, sheets of stamp S21 or NS35 have different load-bearing capacities, which means the requirements for rafter spacing will be different. In this article we will look at how to calculate the optimal distance between the rafters for a gable roof of a garage, taking into account climatic loads, type of corrugated sheet and section of timber.

We will pay special attention regional standards: in snowy regions (for example, Siberia or the Urals), the pitch of the rafters under the corrugated sheet is reduced by 10–15% compared to central Russia. And in windy areas (coastal cities), it is important to take into account the windage of the roof. If you are building a garage in Moscow region, Leningradskaya or Krasnodar region, the standards will be different - we’ll also talk about this below.

1. Norms and standards: what SNiP says about the spacing of rafters for corrugated sheets

The main document regulating the pitch of rafters in Russia is SNiP II-26-76* "Roofs" (updated edition of SP 17.13330.2017). According to him, for profiled sheet The minimum distance between rafters depends on:

  • 📏 Thicknesses and grades of corrugated sheets (for example, C8 withstands less load than H75).
  • ❄️ Snow load (calculated using a map of snowy regions of the Russian Federation).
  • 🌬️ Wind load (depending on the height of the building and the region).
  • 🪵 Sections of rafter beams (standard - 50×150 mm or 100×150 mm).

For garages with a gable roof under corrugated sheets optimal rafter pitch usually amounts to 60–90 cm (from axis to axis). However, this is an average value. For example:

  • For profiled sheet S20–S21 (thickness 0.5–0.7 mm) the pitch should not exceed 80 cm.
  • For NS35–N60 (thickness 0.7–0.9 mm) step up to 120 cm, but only with reinforced sheathing.

⚠️ Attention: If the roof slope is less than 20°, the pitch of the rafters must be reduced by 10–15%. This is due to an increase in snow load on gentle slopes.

📊 What kind of profiled sheet do you plan to use for the garage?
S8–S20 (economy)
S21–NS35 (standard)
N60–N75 (reinforced)
Haven't chosen yet

2. How to calculate rafter pitch: formulas and examples

To accurately calculate the pitch of rafters for corrugated sheets, use the formula: 

Rafter pitch (m) = (Ramp length × sin (slope angle)) / (Safety factor × (Snow load + Roof weight))

Where:

  • Reliability factor — 1.1 for residential regions, 1.2 for snowy regions.
  • Snow load taken from SP 20.13330.2016 (for example, for Moscow - 180 kg/m²).
  • Roof weight — corrugated sheet weighs 5–7 kg/m², insulation (if any) adds 10–15 kg/m².

📌 Calculation example for a 6x4 m garage in the Moscow region:

  • The length of the slope is 3.5 m, the angle of inclination is 25°.
  • Snow load - 180 kg/m², corrugated sheet weight S21 — 5.5 kg/m².
  • The final load is 185.5 kg/m² × 1.1 (coefficient) = 204 kg/m².
  • The optimal step is 75–80 cm (with timber 50×150 mm).

If you don’t want to count manually, use online calculators (for example, on the websites Roofer.ru or Stroika.online). But remember: programs do not take into account wood quality and installation accuracy — these factors can adjust the pitch by ±10%.

☑️ What to consider when calculating the pitch of the rafters

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3. Table: rafter spacing depending on the profiled sheet and region

Below is a table with recommended rafter pitch values for different types of corrugated sheets and climatic zones. The data is relevant for timber with a cross-section 50×150 mm and roof angle 20–30°.

Brand of corrugated sheet Thickness, mm Rafter pitch (cm) for regions Notes
S8–S10 0.4–0.5 60 (south) / 50 (north) Only for temporary buildings
S20–S21 0.5–0.7 80 (south) / 60–70 (north) The most popular option for garages
NS35–H44 0.7–0.8 100 (south) / 80–90 (north) Suitable for insulated roofs
H60–H75 0.9–1.0 120 (south) / 100 (north) For industrial garages

⚠️ Attention: If in your region wind load exceeds 35 kg/m² (for example, in Rostov region or Crimea), the pitch of the rafters must be reduced by 10%, regardless of the brand of corrugated sheet.

4. Typical mistakes when choosing rafter spacing

Even experienced builders sometimes make mistakes when installing a rafter system. Here are the most common mistakes and their consequences:

  1. Too big a step (more than 1 m for S21):
    • 🏚️ Deflection of the corrugated sheet under the snow (especially dangerous for garages with a flat roof).
    • 💧 Leaks at fastening points due to sheet deformation.
  2. Ignoring Tilt Angle:
    • 📉 On roofs with a slope of less than 15°, the pitch of the rafters should be 15–20% less than the standard.
    • 📈 On steep slopes (more than 45°), you can increase the pitch by 10%, but only for profiled sheets with a thickness of 0.7 mm.
  • Not taking into account the weight of insulation:
    • 🔥 Mineral wool or polystyrene foam adds 10–15 kg/m², which requires a step reduction of 5–10%.

    🔧 How to avoid mistakes?

    • 📐 Use laser level to check the evenness of the rafters - distortions of more than 5 mm per 1 m are unacceptable.
    • 🛠️ Attach the corrugated sheet to the sheathing self-tapping screws with rubber washer (fastening step - 20–30 cm).
    • 🌡️ Check the moisture content of the wood - it should not exceed 20% (use a moisture meter).
    What happens if the rafters are installed crookedly?

    Uneven distribution of load will lead to skewed roof within 1-2 winters. In places where there are deflections, the corrugated sheet will begin to “play” in the wind, which will cause leaks at the joints. This is especially critical for garages with soft roofs (for example, if roofing felt is laid on top of the profiled sheet).

    5. How to save on rafters without losing strength

    You can reduce the cost of a rafter system without sacrificing reliability. Here are some proven methods:

    • 🔄 Use a 50x150 mm board instead of a 100x150 mm timber, but reduce the step by 10%. For example, instead of 80 cm, make 70 cm.
    • ♻️ Use roof trusses — they allow you to increase the pitch to 1.2–1.5 m due to the triangular design.
    • 📉 Increase the roof angle up to 30–35° - this will reduce the snow load and allow the rafter pitch to be increased by 10–15%.
    • 🛒 Buy profiled sheets wholesale - many factories (for example, Metal Profile or Grand Line) give a discount of 5% when ordering over 50 m².

    ⚠️ Attention: Don't skimp on antiseptic for wood! Without treatment, rafters rot in 5–7 years, especially in garages with poor ventilation. Use formulations based on boric acid or Senezh.

    💡

    If the garage is adjacent to the house, the pitch of the rafters on the wall side can be increased by 10% - part of the load will be transferred to the permanent structure.

    6. Sheathing under corrugated sheets: how the pitch of the rafters affects its installation

    The pitch of the rafters directly determines the parameters of the sheathing - the second frame layer on which the corrugated sheet is attached. Here are the key rules:

    • 📌 For professional sheet S8–S20 the sheathing is being done solid (step 10–15 cm) or with a vacuum of up to 30 cm.
    • 📌 For NS35–N75 the sheathing pitch can be increased to 50–60 cm (with a rafter pitch of 80–100 cm).
    • 📌 In places sheet joints and valleys install additional bars.

    📌 Example: If the pitch of the rafters is 80 cm, and the profiled sheet is S21, then the sheathing is mounted in increments of 30–40 cm. For H60 with the same pitch of rafters, the sheathing can be done with a vacuum of 50 cm.

    ⚠️ Attention: When installing sheathing under corrugated sheets don't use nails - only self-tapping screws or rough nails (for example, 5×120 mm). Smooth nails become loose over time, causing the sheets to “float.”

    💡

    The sheathing under the corrugated sheet must be perfectly flat. Differences of more than 2 mm per 1 m lead to deformation of sheets due to temperature changes.

    7. Regional features: snow, wind and rafter pitch

    Russia is divided into 8 snow and 7 wind regions, and each has its own standards for rafter systems. Let's look at the key regions:

    • ❄️ Snow regions (I–VIII):
      • Districts I–III (Crimea, Krasnodar region): the pitch of the rafters can be increased by 10–15%.
      • Districts IV–V (Moscow, St. Petersburg): standard values (60–90 cm).
      • Districts VI–VIII (Siberia, Ural): the step is reduced by 15–20%.
    • 🌬️ Wind regions (I–VII):
      • Coastal zones (Kaliningrad, Vladivostok): the pitch of the rafters is reduced by 10%, the fastening of the Mauerlat is strengthened.
      • Steppe regions (Rostov region, Stavropol region): use corrugated sheets with trapezoidal profile (for example, NS44), which resists wind better.

    📌 Example for Siberia: B Novosibirsk region (VI snow region) for profiled sheets S21 the pitch of the rafters should not exceed 60 cm, even if the roof angle is 30°.

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

    Is it possible to make a rafter pitch of 1 meter for C21 profiled sheet?

    Yes, but only in southern regions (I–III snow regions) and provided that the roof angle is at least 25°. In central Russia, the step is 1 m for S21 will lead to sagging of sheets in winter.

    Which timber is better to use for rafters: 50×150 or 100×150?

    For a garage up to 6 m wide, timber is sufficient 50×150 mm. timber 100×150 mm needed if:

    • The span between supports is more than 6 m.
    • The roof is insulated (the weight of the structure increases).
    • Region with high snow load (VI–VIII regions).
    Do I need to treat the rafters with an antiseptic if the garage is not heated?

    ✅ Definitely! In an unheated garage, changes in temperature and humidity create ideal conditions for fungus. Use antiseptics for external works (for example, Senezh Ultra or Pinotex).

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

    ❌ No! The corrugated sheet is attached only to the sheathing, which:

    • Levels the roof plane.
    • Creates a ventilation gap (especially important for insulated roofs).
    • Prevents sheet deformation under load.
    How to check whether the rafter pitch is calculated correctly?

    After installing the rafter system:

    1. Install temporary sheathing and lay 2-3 sheets of corrugated sheets.
    2. Walk on the roof (person's weight is ~80 kg). If the sheets do not bend, the step is chosen correctly.
    3. After the first winter, inspect the roof: if there are no dents from snow, the calculation is correct.