How to calculate roof sheathing for corrugated sheeting: a complete guide

The construction of a reliable roof begins long before laying the finishing coating, and the key stage here is the competent design of the load-bearing base. Lathing for corrugated sheets performs the function of distributing the load from snow, wind and the weight of the material itself, transferring force to the rafter system. Errors at this stage can lead to deformation of sheets, leaks and even collapse of the structure, so calculations must be carried out taking into account all standards and characteristics of the selected profile.

The process of calculating the amount of material and determining the pitch of the slats is based on several variables: the angle of inclination of the slope, the snow load in the region and the geometry of the profiled sheet. It is important to understand that there is no universal solution, and what works for your neighbor may be fatal for your roof. In this article, we will analyze all the nuances, from choosing the cross-section of timber to complex junction units, so that you can confidently begin purchasing and installation.

The influence of the angle of inclination and snow load on the pitch of the racks

The first and most critical parameter that determines the density of the sheathing is the angle of inclination of the slope. The higher the roof, the more snow is retained on it, and the higher the requirements for the load-bearing capacity of the base. For roofs with an angle of less than 15 degrees, it is often necessary continuous lathing or a minimum gap between the elements, since corrugated sheets on gentle slopes experience enormous pressure.

When the angle of inclination increases above 30-45 degrees, the snow load decreases and the pitch of the slats can be increased. However, here the wind load comes into force and tries to tear off the coating. The critical threshold is considered to be an angle of 12 degrees, below which the use of corrugated sheeting without additional waterproofing measures and reinforced sheathing is not recommended by manufacturers. In this case, water can flow into the joints under the influence of capillary effect.

⚠️ Attention: Ignoring regional standards for snow load can lead to sagging of the corrugated sheet between the slats and depressurization of the roof already in the first winter.

The distance between the centers of the slats, known as sheathing pitch, directly depends on the combination of these factors. If you live in a northern region with heavy snowfalls, you absolutely cannot skimp on the frequency of installation of bars. On the contrary, for southern regions with steep slopes, a sparse scheme can be used, which will significantly reduce the cost of lumber.

Choice of material: board, timber or moisture-resistant plywood

The traditional and most common material for creating a base for corrugated boards is edged boards. The most commonly used wood is coniferous wood - pine or spruce, as they have good strength and are affordable. The standard board section varies from 25x100 mm to 32x100 mm, however, for heavy profiles or large rafter steps, a more powerful one may be required timber 50x50 mm.

The most important requirement for lumber is its moisture content. It should not exceed 20%, otherwise during the drying process the boards will begin to warp, twist and change geometry, which will disrupt the flatness of the roof. Before installation, all wooden elements must be treated flame retardants and antiseptics to protect the structure from rotting and fire. This will extend the life of the roof by decades.

In rare cases, when absolute rigidity is required or the roof has a complex broken shape, moisture-resistant plywood or OSB boards. However, for corrugated sheeting this is redundant and not economically feasible, since this material works perfectly in bending on its own with the correct support spacing. The use of sheet materials is only justified in valleys or around chimneys.

Dependence of the sheathing pitch on the grade of corrugated sheeting

Corrugated sheeting can be different, and its load-bearing capacity is determined by the height of the corrugation and the thickness of the metal. For roofing work, brands from S21 up to NS44. The higher the corrugation and the thicker the metal, the less often the support rails can be placed. The use of thin wall corrugated sheets (for example, C8 or C10) on the roof requires either continuous sheathing or very small pitches, which negates the savings.

Below is a table that helps determine the optimal pitch depending on the grade of material and the angle of inclination. These data are averages and must be adjusted according to the design of a particular building.

Brand of corrugated sheet	Metal thickness, mm	Angle up to 15°	Angle 15-30°	Angle more than 30°
S21	0.5 - 0.7	Solid	300 - 400 mm	400 - 500 mm
NS35	0.5 - 0.8	300 mm	400 - 600 mm	600 - 800 mm
H60	0.7 - 0.9	200 - 300 mm	400 - 500 mm	up to 1000 mm
H75	0.8 - 1.0	200 mm	300 - 400 mm	up to 1200 mm

When choosing a material, pay attention to the markings: the letter “N” means load-bearing, “C” means wall, and “NS” means universal. For the roof, it is better to take options with the index “H” or “NS”, since they have additional stiffening ribs. Load capacity Such profiles allow you to make the lathing pitch much wider, saving the budget on wood.

What to do if the selected brand of corrugated sheeting does not suit the pitch?

If you have already bought corrugated sheeting with low load-bearing capacity, but want to make a rare lathing, you can use a trick. Install the main slats according to the calculated step, and between them put counter slats of a smaller cross-section, creating a double mesh. This will strengthen the structure without replacing the base material.

Algorithm for calculating the amount of materials

To calculate the exact number of boards, you need to know the area of the slope and the selected pitch. The formula is simple: the length of the slope is divided by the pitch of the sheathing, obtaining the number of rows, and then multiplied by the width of the slope. However, in reality everything is more complicated, since you need to take into account eaves overhangs, end strips and overlaps.

The first row at the eaves is always made more durable, since it bears the main load from flowing water and snow, and also holds the drain hooks. Often a board with an increased cross-section or paired boards are used here. Further installation proceeds with the calculated step to the ridge. Ridge area also requires reinforcement or installation of an additional rail for attaching the ridge strip.

Don't forget to add 5-10% to the resulting volume for scraps and scraps. When working with a 6 meter long board on a complex shaped roof with many skylights, the waste percentage can increase to 15%. It is better to purchase material with a small reserve than to then look for exactly the same board at the height of the season.

Installation technology: from cornice to ridge

Installation begins with installation cornice boards, which should be strictly parallel to the wall of the house. The evenness of the entire coating depends on the correct alignment of the first batten. To control, pull the string or use a laser level. The distance from the first batten to the second (the pitch of the first cell) is often made less than the main pitch by 30-50 mm in order to compensate for the overhang of the sheet on the cornice.

The slats are fastened to the rafters with nails or self-tapping screws. The length of the nail should be equal to twice the thickness of the board to ensure reliable fixation. When using self-tapping screws, it is important not to overtighten them so as not to split the wood. Each attachment point must be able to withstand pull-out force, since the wind creates significant lifting pressure.

While working, constantly monitor the horizontality of the rows. If the board has a curvature ("screw" or "hump"), it is better to use it for scraps or use it in less critical places. Counter-lattice, if it is provided for by the design to create a ventilation gap, it is mounted along the rafters before the transverse slats are filled.

⚠️ Attention: Never join the sheathing boards in one place across the entire width of the slope. The joints must be staggered and must rest on the rafter leg.

Strengthening nodes and working with security elements

A roof is a system where weak points must be strengthened. Around chimneys, attic windows and in valleys (inner corners of the junction of slopes), the sheathing is made continuous. This is necessary to organize a reliable connection between waterproofing carpets and metal aprons. In valleys, the pitch can be reduced to 10 cm or the flooring can be made of OSB.

Also, provide in advance places for attaching safety elements: snow guards, walkways and ladders. It is necessary to install additional bars or embedded boards under them, since the standard pitch may not coincide with the mounting points of the brackets. Snow guards create a huge pull-out load, and they cannot simply be attached to corrugated sheets.

Special drains may be required for chimney maintenance or antenna installation. Platforms made of reinforced lathing are also prepared for them. All these details must be taken into account at the drawing stage, so that later you do not have to disassemble the finished roof.

Common mistakes when constructing a foundation

One of the most common mistakes is to use untreated wood or boards with natural moisture content. Over time, such sheathing dries out, the gaps increase, and the fasteners weaken. This leads to squeaks and leakage of connections. Always ask the seller for a certificate for lumber or dry the board yourself under a canopy.

Another mistake is saving on fasteners. Using rusty nails or self-tapping screws without zinc coating will cause the wood around the head to corrode and rot. For roofing work, only galvanized fasteners are suitable, often with a rubber washer if the head is on the outside. Internal connections can be made with black self-tapping screws if they are protected from moisture.

Ignoring the ventilation of the under-roof space is the path to rapid destruction of the rafter system. If you are making an insulated roof, there must be a gap between the waterproofing and the sheathing (or counter-lattice). Without air movement, condensation that forms on the back of the metal will moisten the wood all year round.

Is it possible to use used boards for sheathing?

You can use a dismantled board, but only after a thorough inspection. It is necessary to remove all old nails and check the wood for rot, mold and bark beetles. If the board is intact, it needs to be planed or sanded to level the plane, and be sure to be treated with new protective compounds. Savings in this case may be questionable due to high labor costs.

What lathing pitch should I choose for C21 corrugated sheeting?

For brand C21, with a standard tilt angle of 20-30 degrees, a pitch of 400-500 mm is considered optimal. If the angle is less than 15 degrees, the step is reduced to 300 mm or a continuous flooring is made. For heavy snow loads, it is also recommended to reduce the pitch to 300 mm, regardless of the angle, to avoid profile deflection.

Do you need waterproofing under corrugated sheeting?

Yes, a waterproofing membrane is required for any modern metal roof. It protects the insulation and wooden structures from condensation that forms on the underside of the corrugated sheet due to temperature changes, as well as from accidental leaks through the mounting holes. The membrane is laid on top of the rafters with a sag of about 20 mm between them.