When planning a roof from corrugated sheets, most amateur builders make the same mistake: they choose the pitch of the sheathing by eye or according to the principle “so as not to fall.” Distance between bars - this is not just an arbitrary value, but a critically important parameter on which the load-bearing capacity of the entire roof depends. If you make a sparse grille under a thin sheet, the metal will bend under the weight of snow, and the tightness of the coating will be compromised.
Too frequent lathing is also not a panacea. This is an unjustified consumption of lumber, an extra load on the rafter system and an increase in the weight of the entire structure, which is especially dangerous when building on weak foundations. Corrugated sheet works as a single membrane only when properly attached to the supports, so adherence to technology is more important here than economy.
In this article we will look at how to calculate the optimal distance, what standards exist SNiP for different brands of corrugated sheets and why the angle of inclination of the slope dictates its strict conditions. You will learn how to avoid sagging and rain noise by choosing the right spacing for laying the boards.
Factors influencing the sheathing pitch
Before picking up a tape measure, it is necessary to analyze three key parameters that directly dictate the installation scheme. The first and most important is load-bearing capacity of corrugated sheets, which depends on the height of the corrugation and the thickness of the metal. Sheets with a wave height of 8 mm (C8) require almost continuous sheathing, while H-shaped profiles with a wave height of 60 mm and above allow significant gaps between supports.
The second factor is the climate load in your region. Snow load in Siberia and the Krasnodar Territory differs significantly. If there is a lot of snow in your area in winter, the pitch of the sheathing will have to be reduced so that the sheet does not push through. Wind loads also play a role, especially for tall buildings where more frequent windage support is required.
⚠️ Attention: Never use the “maximum” sheathing step if your house is located in a snowy region. Saving two boards can lead to the fact that in the spring the roof will have to be completely covered due to deformation of the profiled sheet locks.
The third factor is the slope angle. The lower the roof, the more snow falls on it and the worse water rolls off it. Therefore, for flat slopes (less than 15 degrees), the requirements for the frequency of supports are the most stringent. For steep roofs (more than 30-40 degrees), the snow load is reduced, and the pitch can be increased, relying only on the load-bearing capacity of the sheet itself.
Regulatory requirements SNiP and GOST
Construction in Russia is regulated by a set of rules SP 17.13330.2011 (updated edition of SNiP II-26-76 “Roofs”). According to these documents, the sheathing pitch is not a fixed value for all cases, but sets the minimum requirements for reliability. The standards state that corrugated sheeting must be fastened into every wave around the perimeter and into every second recess in the center of the sheet, which is impossible without a properly mounted base.
Particular attention in the standards is paid to overhangs and ridges. In these areas, as well as around chimneys and roof windows, it is always installed continuous lathing or the step decreases significantly. This is necessary to ensure rigidity in areas of high stress and for high-quality waterproofing of junctions. Ignoring these requirements leads to leaks in the most vulnerable places.
There are recommended maximum spans for different brands of profiled sheets. For example, for the popular sheet S21 at an angle of inclination over 15 degrees, the standard pitch is about 650 mm, but only in the absence of extreme snow loads. Engineering calculations always take precedence over tabular data, especially for industrial facilities or large hangars.
What does GOST say about wood moisture content?
GOST requires that the moisture content of lumber for sheathing should not exceed 20%. Using a damp board will lead to the fact that when it dries, it will change its geometry, the nails will become loose, and the corrugated sheets will begin to “walk” and creak in the wind.
Dependence of the pitch on the grade of corrugated sheeting
The choice of distance between the bars directly correlates with the profile height. The logic is simple: the higher the “stiffening rib”, the further the supports can be spaced apart. For roofing work, grades C21, C35, C44, NS35 and H60 are most often used. Each of them has its own “comfort zone” in terms of installation steps.
Low wave sheets such as C8 and C10, are practically not intended for roofing without a continuous base. Their rigidity is insufficient for spans of even 40-50 cm under load. If you nevertheless choose such a material to save money, get ready to install plywood or OSB on top of a rare board, which will increase the cost of the structure.
More powerful profiles such as NS35 or H60, allow you to take steps of 1 meter or more. This speeds up installation and reduces wood consumption. However, it is important to take into account the thickness of the metal: a 0.4 mm thick H60 sheet will behave differently than a 0.7 mm thick sheet, even with the same wave height.
When purchasing corrugated sheets, pay attention to the actual thickness of the metal. Often the marking “0.5 mm” actually means 0.45 mm or even 0.4 mm. For such sheets, it is better to reduce the sheathing pitch by 10-15% of the calculated one.
The influence of the slope angle on calculations
The angle of inclination is a variable that can change a design dramatically. Snow does not stay on steep slopes; it rolls down under the influence of gravity. Here the main enemy is the wind, which tries to tear off the leaf. Therefore, for steep roofs, it is not so much the frequency of the bars for support from snow that is important, but the reliability of fastening the corrugated sheeting itself to these bars.
For flat roofs (less than 15 degrees) the situation is reversed. Snow lies on the roof for months, creating a static load. Water also drains slowly, and the risk of leakage through micro-cracks in joints is higher. Therefore sheathing pitch for flat roofs it is always done minimally, often turning into a continuous flooring.
There is a direct relationship: reducing the angle of inclination requires increasing the number of support points. If for 30 degrees a step of 100 cm is acceptable, then for 10 degrees it can be reduced to 30-40 cm. Neglecting this rule guarantees that water will accumulate between the corrugations, which, when frozen, will expand the gaps and break the tightness.
| Brand of corrugated sheet | Slope angle up to 15° | Slope angle 15-30° | Slope angle more than 30° |
|---|---|---|---|
| S21 | Solid | up to 400 mm | up to 650 mm |
| S35 | up to 300 mm | up to 500 mm | up to 800 mm |
| NS44 | up to 400 mm | up to 600 mm | up to 1000 mm |
| H60 | up to 500 mm | up to 800 mm | up to 1200 mm |
Lathing materials: board or timber?
The classic option for corrugated roofing is an edged board with a thickness of 25, 30 or 40 mm and a width of 100-150 mm. The use of a 25 mm board is permissible only with a small pitch (up to 40-50 cm) and the absence of heavy snow loads. The optimal choice is considered to be a board thickness 30-32 mm, which provides sufficient rigidity and does not require an excessive number of supports.
Beams of 50x50 mm or 40x60 mm are used less frequently, mainly to create a counter-lattice or in cases where the pitch between the rafters is very large and a reinforced structure is required. The downside of timber is its high cost and greater likelihood of deformation (twisting) when drying compared to a wide board.
The quality of the wood is critical. The board must be dry (humidity no more than 20%), without rot, through knots and wane (bark around the edges). Unplaned board is acceptable, but it creates a rough surface, which may interfere with the tight fit of some types of waterproofing, although this is not a critical factor for corrugated sheeting.
⚠️ Attention: Be sure to treat all lumber with a fire-retardant compound before installation. Access to the sheathing after laying the corrugated sheeting will be closed forever, and a rotten or fungus-affected frame will be impossible to replace without completely dismantling the roof.
Installation technology and pitch check
Installation begins with the installation of the cornice board. It is always placed first and is often thicker than ordinary elements in order to level the plane of the slope taking into account the gutter. The evenness of the entire subsequent row depends on the correct installation of the first board. The step is measured from the center of the cornice board to the center of the first row.
For marking, it is convenient to use a template - a piece of a block or strip with a length equal to the required pitch. This eliminates the need to constantly work with a tape measure and reduces the risk of error. The boards are attached to the rafters with nails (3-4 times the thickness of the board) or self-tapping screws. Nails are driven into two edges of the board to prevent it from twisting.
☑️ Checklist before laying corrugated sheets
It is important to maintain flatness. If the board has a curvature (“helicopter” or arc bend), it must either be turned over or replaced. The bulges will prevent the corrugated sheet from fitting tightly, creating stress points where the metal can burst under foot traffic or snow loads. Counter-lattice (bars along the rafters) is required if a waterproofing membrane is used, it ensures condensation weathering.
The main secret to a smooth roof is to stretch the string (fishing line) along the lower and upper edges of the sheathing before final fastening. This will allow you to visually control the plane of the slope and avoid “waves” on the finished roof.
Common mistakes when constructing a foundation
One of the most common mistakes is saving on fasteners. Using rusty nails or screws that are too short will cause the sheathing to begin to creak over time. When there are gusts of wind, the corrugated sheet rubs against the caps, making an unpleasant sound. Fasteners must be galvanized and match the thickness of the wood.
The second mistake is ignoring reinforcement in places where elements pass through. Around pipes, installation of snow guards or antennas, the pitch of the sheathing should be reduced or made continuous. Craftsmen often forget about this, and after a year or two, heavy snow retainers tear out a piece of the roof along with part of the sheathing.
The third mistake is the lack of calibration of the board thickness. If you use an inch board (25 mm), it can have a thickness range from 22 to 27 mm. Installing such a board without sorting will lead to steps on the slope. On corrugated sheeting this may not be as noticeable as on metal tiles, but on a long slope the “step” can disrupt the flow of water.
Is it possible to use unedged boards for sheathing?
It is highly not recommended to use unedged boards. The bark left on the edges is an ideal breeding ground for bark beetles and fungus. In addition, the edges of such a board are uneven, which makes it difficult to maintain precise pitch and flatness. If your budget is limited, it is better to take a smaller grade of board, but edged.
Is continuous lathing required for C21 corrugated sheeting?
For C21 corrugated sheeting (wave height 21 mm), continuous sheathing is required only at the corners of slopes, overhangs and around chimneys. On the main plane of the slope, at an angle of more than 15 degrees, a low pitch (300-400 mm) is acceptable. However, if the slope angle is less than 15 degrees, manufacturers often recommend making the base solid or with a minimum gap (up to 10 mm) to prevent sagging.
What is the lathing pitch for N75 corrugated sheeting?
N75 corrugated sheeting is a load-bearing profile with a high wave and additional stiffening ribs. It is capable of covering spans of up to 3-4 meters without support, but as a roof it is usually placed at intervals of 2-3 meters for industrial facilities. For a private house, the sheathing pitch for H75 is usually 1000-1500 mm, which makes it one of the most economical options in terms of wood consumption.
What to do if the pitch of the rafters does not match the pitch of the sheathing?
The pitch of the rafters and the pitch of the sheathing are independent quantities. The sheathing is attached across the rafters. If the rafters are sparse (for example, 1.2 m), and the sheathing board is thin (25 mm), it may sag between the rafters. In this case, you need to either reduce the pitch of the rafters (install additional ones), or use a thicker board (40-50 mm) or timber for the sheathing.