High-quality roof sheathing under corrugated sheets is the foundation on which the tightness, durability and wind resistance of the entire roofing system depend. It is this hidden structural element that absorbs loads from snow and wind, transferring them to the rafter frame, and ensures proper ventilation of the under-roof space. Errors in choosing the cross-section of the bars or calculating the pitch can lead to bending of the metal, tearing of sheets in strong winds and leaks at the joints.
The process of creating a load-bearing frame requires precision and understanding of the physics of the roof, since corrugated sheeting is a fairly rigid material, but sensitive to deformation of the base. Unlike soft tiles, which require a continuous flooring, or ceramic tiles with their enormous weight, a metal profiled sheet dictates its conditions in terms of the frequency of support points. A properly installed system ensures that the roof will withstand decades of use without the need for complex repairs.
Functional purpose and types of lathing
The main task of the lathing is to evenly distribute the weight load from the roofing and snow cover onto the rafters. Sparse lathing is the most popular option for corrugated sheeting, as it is economical in terms of lumber consumption and creates the necessary conditions for air circulation, removing condensation from the back side of the metal. However, in certain areas of the roof, such as the eaves, ridge and valleys, the structure is always strengthened.
There are two main types of construction: solid and sparse. Solid flooring, where the gap between the elements is minimal (up to 10 mm), is rarely used, mainly at a very small angle of inclination of the slope or at the junction of pipes and roof windows. The sparse version, where the bars are attached at a certain pitch, is the standard for most projects of country houses and industrial buildings.
It is important to understand that the corrugated sheet has a high load-bearing capacity across the wave, but requires reliable support along the eaves line. That's why first bottom row it is always made more powerful, and the pitch of the remaining bars is calculated strictly in accordance with the brand of metal used and the climatic conditions of the region.
⚠️ Attention: The use of raw wood for sheathing is unacceptable. The moisture content of the lumber should not exceed 20%, otherwise, when dry, the bars will “lead”, which will lead to deformation of the roofing and disruption of the geometry of the slope.
Choice of materials: board, timber or metal
The traditional and most common material for constructing a base for corrugated sheets remains coniferous wood - pine, spruce or larch. These rocks have sufficient strength, are resistant to rotting when properly processed, and are affordable. The standard cross-section for bars is 50x50 mm or 40x60 mm, and for boards - 32x100 mm or 50x100 mm, which allows them to withstand significant snow loads.
Recently it has been gaining popularity metal lathing, made from galvanized profile. It is perfectly smooth, not subject to rotting, burning and insects. A metal frame is especially relevant for industrial facilities, buildings with an aggressive environment, or in cases where maximum fire safety of the structure is required. However, the cost of such a solution is significantly higher than its wooden counterpart.
When choosing lumber, you need to pay attention to grade. For sheathing, grade 1-2 wood is usually used; a small number of knots, but not through cracks, are allowed. Before installation, all wooden elements must be treated with fire-retardant compounds, which will significantly extend the service life of the entire roofing system.
To save money, you can combine materials: use a more expensive edged board for the bottom row and eaves overhangs, and for the main area of the slope - an unedged board or a block of smaller cross-section, having previously cleared them of bark.
Calculation of the sheathing pitch depending on the angle of inclination
The key parameter during installation is the pitch of the sheathing, which directly depends on the angle of inclination of the roof and the load-bearing capacity of the selected brand of corrugated sheeting. The smaller the angle of the slope, the greater the snow load on it, and the more often the support bars should be located. Low-slope roofs (less than 15 degrees) often require a continuous deck or minimal clearance to prevent the metal from bending under the weight of snow.
With a tilt angle of 15 to 30 degrees, which is the most common option for private houses, the lathing pitch is usually from 300 to 650 mm. In this range, corrugated sheets cope well with loads, and material consumption remains optimal. If the slope angle exceeds 30 degrees, the snow load is reduced and the pitch can be increased to 700-1000 mm, but it is necessary to take into account the wind load, which increases on high roofs.
For an accurate calculation, you must refer to the technical specifications of a specific manufacturer of corrugated sheets, which indicate the maximum span length for each model (NS-20, S-21, NS-35 etc.). Ignoring this data may result in the metal being deformed between the bars when walking on the roof during installation or maintenance.
| Slope angle | Recommended type of sheathing | Maximum pitch (mm) | Installation features |
|---|---|---|---|
| Less than 15° | Solid or with a gap of up to 10 mm | — | Mandatory waterproofing, double cornice |
| 15° — 30° | Sparse | 300 — 650 | Reinforcement in valleys and around pipes |
| More than 30° | Sparse | 700 — 1000 | Reliable fixation from the wind is important |
Don't forget about regional characteristics. In snowy areas, the sheathing step should be taken more often, based on the lower limit of the recommended values, to ensure a margin of safety. In windy coastal areas, on the contrary, not only frequent fastening is important, but also the use of additional wind braces.
Fastening schemes and junction units
Installation of the sheathing begins with installation cornice strip and the bottom row of bars. The first element is always made thicker than the others (or an additional rail is placed on it) in order to level the roof plane taking into account the height of the profiled sheet. This is necessary so that when attaching the sheets, the screws enter the block at a right angle, and do not hang in the air, ensuring a reliable fit of the waterproofing.
In the locations of valleys (internal corners of the roof), around chimney pipes, roof windows and ventilation outlets, the sheathing is made continuous. The reinforcement zone must be at least 500 mm on each side of the obstacle. This prevents the metal from bending in the most vulnerable places, where leaks most often occur, and provides a flat surface for installing additional elements.
☑️ Checking the readiness for installation of the sheathing
The bars are fastened to the rafters using nails or self-tapping screws. The length of the fastener should be equal to two thicknesses of the sheathing bar to ensure reliable fixation. When using nails, they are driven in offset to avoid splitting the wood, especially if overdried material is used.
⚠️ Attention: It is strictly forbidden to join sheathing bars in the air between the rafter legs. The joint should always be in the middle of the rafter, and adjacent rows of sheathing should have a spacing of seams to maintain the solidity of the structure.
Installation technology: step-by-step instructions
The installation process begins after laying the waterproofing film and installing the counter-lattice (50x50 mm bars along the rafters), which creates a ventilation gap. First, the attachment points for the bottom row are marked, taking into account the overhang of the eaves overhang, which is usually 300-400 mm beyond the wall. The accuracy of the first row is critical as it sets the rhythm for all subsequent installations.
Next, moving from bottom to top, the remaining rows of bars are attached. To follow a single step, it is recommended to make a simple template (conductor) from a piece of board or timber of the required length, which will speed up the work and eliminate errors. The fastening is done in a checkerboard pattern: if in one row the block is fastened to the left of the rafter axis, then in the next row it is fastened to the right, which prevents the rafter leg from skewing.
Particular attention should be paid to the quality of fasteners. To work on the roof, it is better to use galvanized self-tapping screws with a rubber washer or special rough nails that do not become loose over time. Using ferrous metal without protection will lead to the appearance of rusty streaks after just a couple of years of use.
The secret to a perfectly even sheathing
Use a stretched cord (mooring line) along the slope to control the plane. If the pieces are of different thicknesses (as is often the case with naturally damp lumber), place small chips under the thinner pieces or use a plane to adjust the height on site.
Typical errors and ways to resolve them
One of the most common mistakes is saving on the cross-section of the bar. An attempt to use a thin 30x40 mm lath for a roof with a rafter pitch of 1 meter will result in the structure sagging under the weight of a person during installation. Load capacity must be calculated with a margin, taking into account the weight of the master himself and the tools in his hands.
Another common problem is lack of ventilation. Forgetfulness when installing the counter-lattice or the use of bars that are too narrow (less than 40 mm) leads to the fact that air does not circulate under the profiled sheet. As a result, condensation formed on cold metal drips onto the insulation, reducing its effectiveness and causing rotting of wooden structures.
Geometry violations during docking are also common. If the bars are not aligned along the top edge, the corrugated sheet will lie in waves, which will be visually noticeable and will lead to loose fit of the sheets to each other. It is almost impossible to fix this after installing the roof without completely dismantling the covering.
The quality of the sheathing determines the tightness class of the entire roof: even the most expensive corrugated sheeting will not save you from leaks if the base is installed in violation of the technology and without taking into account the snow loads of the region.
Is it possible to use unedged boards for sheathing?
You can use an unedged board, but only after thoroughly removing the bark. The bark is a breeding ground for bark beetles and other pests, and also prevents high-quality impregnation of the wood with an antiseptic. If the bark is not removed, it will fall off over time, leaving voids under the corrugated sheet, and insects can spread to the rafter system.
Do you need a vapor barrier under the sheathing?
The vapor barrier is installed on the side of the room (under the insulation), and not under the sheathing. Under the sheathing, directly on the insulation, there should be hydro-windproof membrane, which releases steam from the insulation to the outside, but does not let water inside. A ventilation gap is required between the membrane and the sheathing.
How to calculate the number of nails for sheathing?
The calculation is made based on the number of points of intersection of the sheathing bars with the rafters. For each intersection there are usually 2 nails (or 1 long screw). To the amount received, add 10-15% for defects and unforeseen expenses. A standard 100 m² house may require several kilograms of fasteners.