Incorrectly chosen rafter spacing on the pitched roof of the garage under corrugated sheet leads to sagging of sheets, squeaks under wind load or collapse of the structure in winter. If you are laying corrugated sheets S21 or NS35 for a garage of 3x6 m, and the rafters are installed in increments of 1.5 m “by eye”, after a year waves will appear at the joints, and the fastenings will begin to rust from micro-movements. The distance between the rafters depends on three critical parameters: the thickness of the corrugated sheet, the angle of the roof and the snow load in the region - ignoring at least one of them will result in repairs in 2-3 seasons.

In 90% of cases, shed roofs with a slope of 5–15° are used for garages, where the rafters rest on walls of different heights. At this angle maximum pitch of rafters under corrugated sheets should not exceed 1.2 m - even if the sheet manufacturer indicates the permissible 1.5 m for the brand H60. The fact is that garage roofs rarely have ideal geometry, and additional loads (snow, icing, the weight of a person during repairs) add up. For example, in the Moscow region with a slope of 10° and corrugated sheet C44 a step of 1.3 m already requires reinforcement with 40x40 mm lathing every 50 cm.

1. SNiP and SP standards for calculating the pitch of rafters for corrugated sheets

The basic requirements for rafter spacing for shed roofs of garages are regulated SP 20.13330.2016 (updated version of SNiP 2.01.07-85*) and SP 17.13330.2017. These documents take into account:

  • 📏 Snow load (calculated according to the map of zones of the Russian Federation, for example, for St. Petersburg - 180 kg/m², for Sochi - 50 kg/m²).
  • 💨 Wind load (depending on the height of the garage and the wind rose of the region).
  • ⚖️ Corrugated sheet weight (from 4.5 kg/m² for C8 up to 10 kg/m² for H75).
  • 📐 Roof angle (for a slope of less than 8°, the pitch of the rafters is reduced by 20%).

For garages in central Russia (III snow zone) minimum rafter spacing for corrugated sheets S21–NS35 amounts to 0.8–1.0 m at a slope of 10–15°. If the slope is less than 8°, the pitch is reduced to 0.6–0.8 m or the sheathing is strengthened. For example, for a profiled sheet H57 with a load-bearing capacity of 1.5 kN/m² in the Leningrad region with a slope of 7°, the pitch of the rafters should not exceed 0.7 m, otherwise a continuous OSB sheathing will be required.

⚠️ Attention: If the garage is adjacent to a house or fence, the wind load on the windward side increases by 40%. In this case, the rafter pitch is reduced by 10–15% of the calculated one.

2. Table of rafter pitch depending on the grade of corrugated sheet and the angle of inclination

Brand of corrugated sheet Thickness, mm Tilt angle, ° Max. rafter spacing, m (III snow zone) Max. rafter spacing, m (I snow zone)
C8 0.4–0.5 5–10 0.6 0.8
S21 0.45–0.7 10–15 0.8–1.0 1.0–1.2
NS35 0.6–0.8 8–12 1.0–1.2 1.2–1.5
H57 0.7–0.9 5–10 0.7–0.9 1.0–1.2
H75 0.8–1.0 10–15 1.2–1.5 1.5–1.8

The data in the table is given for standard conditions: span up to 6 m, no skylights or additional loads (for example, solar panels). If the length of the garage exceeds 6 m, the rafter pitch is reduced by 10–20% or used double rafters with a crossbar in the middle. For example, for a garage 4x8 m under corrugated sheeting NS44 the optimal step is 0.9 m, even if the table allows 1.2 m.

📊 What corrugated sheeting do you plan to use for the garage roof?
S8–S21 (wall)
NS35–N57 (load-bearing wall)
H60 and above (load-bearing)
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3. Calculation of rafter pitch: step-by-step instructions

To independently calculate the distance between the rafters, follow these steps:

  1. Determine the snow load on the map SP 20.13330.2016 (for example, for Nizhny Novgorod - 240 kg/m²).
  2. Multiply the load by the roof slope factor (at 10° - 1.0, at 5° - 1.2).
  3. Add corrugated sheet weight (for example, NS35 thickness 0.7 mm - 7.4 kg/m²).
  4. Divide the total load by the permissible load for the selected rafter section (for example, a 50x150 mm board can withstand 200 kg/m over a span of 3 m).

Calculation example for a 3x6 m garage Kazan (snow load 240 kg/m², slope 10°, corrugated NS35 0.7mm):

  • Snow load adjusted for slope: 240 × 1.0 = 240 kg/m².
  • Corrugated sheet weight: 7.4 kg/m².
  • Total load: 240 + 7.4 = 247.4 kg/m².
  • Permissible load for a board 50×150 mm: 200 kg/m (at a step of 1 m).
  • Required rafter pitch: 200 / 247.4 ≈ 0.81 m (rounded to 0.8 m).

☑️ Checklist before installing rafters

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4. Typical mistakes when choosing rafter spacing

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

  • 🔧 Ignoring rafter deflection. A 50x100 mm board with a pitch of 1.2 m and a load of 200 kg/m bends by 2–3 cm, which leads to deformation of the corrugated board.
  • 🌨️ Failure to account for local snow bags. If the garage is located in a low area or next to tall trees, the snow load increases by 30–50%.
  • 📉 Saving on sheathing. With a rafter pitch of 1.2 m, the sheathing under S21 need to be mounted after 30 cm, not 50 cm.
  • 🔩 Incorrect fastening of corrugated sheets. Self-tapping screws screwed into the lower wave weaken the sheet by 15–20%.

One of the most dangerous mistakes is use of raw or unplaned boards for rafters. Wet wood (more than 20%) shrinks up to 5% after a year, which leads to weakening of the fastenings and warping of the roof. For example, if you installed rafters made of freshly sawn pine with a pitch of 1 m, after a season the distance between them may decrease to 0.95 m, and the corrugated flooring will begin to “play” in the wind.

⚠️ Attention: If you are using metal rafters (for example, from a channel 100x50 mm), the pitch can be increased by 10–15% compared to wooden ones. However, in this case, be sure to use anti-condensation film, since the metal freezes and condensation forms on the inside of the corrugated sheet.

5. How to strengthen the rafter system with a large step

If the project requires a rafter spacing of more than 1.2 m (for example, for a 4x9 m garage), use one of the reinforcement methods:

Methods for strengthening rafters with a span >6 m

🔹 Double rafters (50x150 mm boards sewn with nails or bolts) increase the load-bearing capacity by 80%.

🔹 Struts and crossbars divide the span into two segments, reducing the load by 40%.

🔹 Glulam beam (for example, LVL beam) allows you to increase the pitch to 1.8 m without deflections.

🔹 Continuous sheathing made of OSB-3 with a thickness of 18 mm distributes the load evenly, but makes the structure heavier by 10–15 kg/m².

🔹 Corners and punched tapes for attaching rafters to the mauerlat (for example, MZP or LK) increase rigidity by 25%.

For garages with flat roof (slope up to 5°) the only reliable option is the pitch of the rafters 0.5–0.6 m with mandatory continuous sheathing. In this case, even corrugated H60 may sag under the weight of wet snow. An alternative is to use profiled sheets with trapezoidal profile (for example, T-35 or T-55), which can withstand loads of up to 700 kg/m² with a support spacing of 1 m.

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For garages in northern regions (IV–V snow zones), use rafters made of boards 50×200 mm in increments of 0.7–0.8 m. This will avoid the installation of snow guards, which create additional load in winter.

6. The influence of rafter material on the pitch

The pitch of the rafters depends not only on the loads, but also on the material:

  • 🌲 Wood (pine, spruce):
    • Board 50×150 mm - step up to 1.0 m (with a slope of 10°).
    • Beam 100×100 mm - step up to 1.5 m (for corrugated sheets H75).
  • 🏗️ Metal (channel, I-beam):
    • Channel 100×50 mm - pitch up to 1.8 m (with reinforcement with ties).
    • I-beam 120 mm - pitch up to 2.5 m (for industrial garages).
  • 🏗️ Laminated timber (LVL):
    • Section 60×180 mm - step up to 1.5 m (without deflections).

Critical for metal rafters quality of welds. For example, if you use a channel 10P in increments of 1.5 m, but the seams are made intermittently, the load-bearing capacity is reduced by 30%. In this case, it is better to reduce the pitch to 1.2 m or strengthen the structure braces from a corner 50×50 mm.

7. Practical installation tips

When installing rafters under corrugated sheets, follow these recommendations:

  • 🔨 Use a template for marking the rafters - this will eliminate distortions. For example, for a step of 0.9 m, make a rail 90 cm long and transfer the marks along the mauerlat.
  • 📏 Control the diagonals after installing the first three rafters. A difference of more than 2 cm will lead to waviness of the corrugated board.
  • 🌡️ Treat the wood antiseptic (eg Senezh Ultra) and fire retardant (for example, Neomid 450). This will extend the life of the rafters by 10–15 years.
  • 🔩 Fasten the corrugated sheet self-tapping screws with a rubber washer (for example, 4.8×35 mm) into the upper wave. The fastening step is one wave horizontally.

If you are installing a roof in winter, leave expansion gaps 2–3 mm between the ends of the rafters and the walls of the garage. Wood expands when it thaws, and without gaps the structure can “sag.” For metal rafters, gaps are not needed, but be sure to use thermal break gaskets (for example from paronitis) at the points of attachment to the Mauerlat.

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When the rafter spacing is more than 1.2 m, be sure to install wind connections (braces from boards 25x100 mm) every 3-4 rafters. This will prevent the roof from “sailing” in heavy winds.

FAQ: Frequently asked questions about garage rafter spacing

Is it possible to make a rafter pitch of 1.5 m for corrugated sheeting? H60 in Krasnodar?

B I snow zone (Krasnodar region) for corrugated sheeting H60 with a thickness of 0.7 mm and a slope of 10°, a step of 1.5 m is acceptable, but only under the condition:

  • Using rafters made from 50×200 mm boards or laminated veneer lumber.
  • Install sheathing 30x50 mm every 40 cm.
  • No additional loads (for example, solar panels).

If the garage is located in an open area, it is better to reduce the step to 1.2 m.

What rafter spacing is needed for a 6x4 m garage with a 5° slope in the Moscow region?

With a slope of 5° and III snow zone (Moscow region) minimum rafter spacing:

  • For corrugated sheets S21: 0.6 m.
  • For corrugated sheets NS35: 0.7–0.8 m.
  • For corrugated sheets H57: 0.8–0.9 m.

We recommend continuous lathing made of OSB or boards 25×100 mm in increments of 10 cm.

Is it possible to use rafters from 50x100 mm boards for corrugated sheeting?

Board 50×100 mm is only suitable for:

  • Garages up to 4 m wide.
  • Corrugated sheets S8–S21 when stepping rafters 0.6–0.7 m.
  • Regions with snow load up to 100 kg/m² (I–II zone).

Most regions of Russia require a board 50×150 mm or timber 100×100 mm.

Is it necessary to insulate the garage roof if the rafter pitch is 1 m?

Insulation does not affect the pitch of the rafters, but:

  • If you are laying mineral wool (for example, Rockwool Light Butts), the pitch of the rafters should be 1–2 cm less than the width of the insulation boards (usually 60 cm).
  • For expanded polystyrene (for example, Penoplex) the pitch of the rafters is not critical, since the slabs are cut to size.
  • When insulating, be sure to use vapor barrier (for example, Izospan B) and a ventilation gap of 3–5 cm.
How to check the correct pitch of the rafters after installation?

After installing the rafters:

  1. Measure the distance between the axes of the rafters in 3-4 places along the length of the roof. Permissible deviation is ±5 mm.
  2. Check the flatness of the plane with a cord or laser level. A deflection of more than 1 cm per 3 m requires adjustment.
  3. Lay 2-3 sheets of corrugated board and walk on them (weight ~80 kg). If the sheets sag or “play,” the rafter pitch is too large.