The distance between the axes of the bars or boards in the supporting frame directly determines the ability of the roof to withstand snow and wind loads without deflection or destruction. An error in calculations of even 5-10 centimeters when using thin metal or in regions with heavy snowfalls leads to deformation of waves, tearing of sheets and leaks of the insulating cake. That is why the choice of a specific installation interval for supporting elements is carried out strictly according to SNiP and the manufacturer’s recommendations profiled sheet, and not “by eye”.
An incorrectly selected interval between slats causes local deflection of the metal under the weight of a person during maintenance or accumulation of snow, which breaks the tightness of the joints and accelerates corrosion. In some cases, too frequent filling leads to excessive consumption of lumber and weighting of the entire structure, which requires strengthening the rafter system and foundation. Precise adherence to laying technology allows you to distribute the weight evenly and ensure a long service life of the roofing covering.
There are three main types of sheathing: solid, sparse with a standard pitch and reinforced, each of which is used depending on the angle of inclination of the slope and the brand of profile used. Sloping roofs with an angle of less than 15 degrees often require a continuous deck of OSB boards or plywood to prevent sagging. On steep slopes, it is allowed to increase the distance between elements, which saves the construction budget without losing strength.
Factors influencing the choice of spacing between bars
The main criterion dictating the installation scheme is the load-bearing capacity of the profiled sheet itself, which depends on the height of the corrugation and the thickness of the steel base. The higher the profile, the greater the load it can take on itself, allowing you to increase the step between the supports. The brands most often used for roofing work are: S21, NS35, N57 and higher, where the number indicates the wave height in millimeters.
The second critically important parameter is the climatic region of construction, which determines the estimated snow and wind load. In the northern regions, where the weight of snow cover can reach hundreds of kilograms per square meter, the pitch of the lathing under the corrugated sheeting must be reduced or switched to a more powerful profile. Ignoring this factor leads to the fact that in winter the roof simply cannot withstand the pressure.
The third factor is the angle of inclination of the slope, which determines the speed of snow and water melting. On flat roofs, snow lies longer and creates a greater static load, requiring more frequent installation of bars or boards. On steep slopes, snow rolls off faster, which makes the structure more sparse.
⚠️ Attention: Using timber with natural humidity without pre-treatment with antiseptics will lead to rotting of the frame and corrosion of the metal from the inside within 3-5 years.
It is also worth considering the weight of the roofing cake itself, including insulation, vapor barrier and the weight of the people who will service the chimney or antenna. For heavy structures such as sandwich panels or thick corrugated sheets, the pitch can be increased, but only after checking the load-bearing capacity of the rafters.
Types of lathing and their areas of application
In construction practice, there are three main methods of forming a load-bearing base for a corrugated sheet, and the choice of a specific method depends on the technical requirements of the project. Solid sheathing is a flooring made of planks with minimal gaps or sheet materials such as plywood. This type is required for slope angles of less than 15 degrees and where the roof meets pipes or roof windows.
Sparse lathing is the most common option for slopes with an angle of more than 15-20 degrees. In this case, the bars or boards are mounted parallel to the cornice at a certain interval, which is calculated based on the wavelength of the profile. This allows you to save up to 40% of lumber compared to the solid option.
- 🏗️ Solid flooring is used for soft roofing, bitumen shingles and corrugated sheeting with low wave height on flat slopes.
- 📐 The sparse standard scheme is used for most residential buildings with a slope angle from 20 to 45 degrees.
- 🔨 An increased step is required when installing heavy materials or in regions with extreme snow loads.
The third type is combined, where a continuous flooring is made in the area of the cornice, ridge and around the attic windows, and the main field of the slope is sewn up sparsely. This approach provides additional rigidity in the most vulnerable places and prevents the roof from being undermined by the wind. To implement the combined method, 50 mm thick boards are often used in critical areas.
To ensure ventilation of the under-roof space, the first element at the eaves is always made higher than the others by a height equal to the thickness of the counter-lattice.
Standard step sizes for different brands of corrugated sheets
Rolled metal manufacturers provide tables of load-bearing capacity for each specific profile grade, and this data is primary in the design. For popular corrugated sheets S21, which is often used for the roofs of private houses, a pitch of 500-600 mm with a metal thickness of 0.5 mm is considered optimal. If the metal thickness is less, the distance must be reduced.
More powerful brands such as NS35 and H57, have high rigidity due to the deep trapezoidal profile. This allows you to increase the distance between supports to 1-1.5 meters, which significantly speeds up the installation process. However, even when using a powerful profile, it is not recommended to exceed a step of 1 meter without engineering calculation.
Below is a reference table showing the dependence of the sheathing pitch on the grade of corrugated sheet and the angle of inclination of the slope. The data is relevant for standard snow loads (region 2-3).
| Brand of corrugated sheet | Tilt angle up to 15° | Tilt angle 15-30° | Tilt angle more than 30° |
|---|---|---|---|
| C21 (thickness 0.5 mm) | Solid | 300-400 mm | 500-600 mm |
| NS35 (thickness 0.5 mm) | Solid | 500 mm | 800-1000 mm |
| H57 (thickness 0.7 mm) | 300 mm | 1000 mm | 1500 mm |
| H75 (thickness 0.8 mm) | 300 mm | 1500 mm | 2000 mm |
It is important to understand that the table values are averages. In reality, it is necessary to take into account the specific span length and the quality of the wood. If a board with knots or a low grade is used, it is better to reduce the pitch by 10-15% to create a safety margin.
Effect of metal thickness on pitch
Thin metal (0.35-0.4 mm) is categorically not recommended for roofs with sparse sheathing, as it is easily deformed when walking. For such brands, continuous flooring is required.
Materials for making the frame
The quality of lumber plays no less a role than the correctness of calculations. For lathing under corrugated boards, edged softwood boards with a thickness of 32 or 50 mm are traditionally used. The 32mm board is suitable for standard spacing up to 600mm, while the 50mm provides high strength for larger spans.
Timber with a cross section of 50x50 mm or 40x60 mm is used less frequently, mainly to create reinforced structures or when installing very heavy roofs. The main requirement for the material is humidity not more than 20%. When dry, raw wood will begin to warp, which can lead to screws being unscrewed and the seal breaking.
- 🌲 Pine and spruce are the most accessible and common materials that require mandatory fire-bioprotection treatment.
- 🌳 Larch is an expensive but durable option that is resistant to rotting without additional chemicals.
- 🏗️ Metal profile - used in industrial construction, does not rot, but requires special fasteners.
Before installation, all wooden elements must be treated with deep penetration antiseptic compounds. This protects the frame from mold, mildew and insects, which can destroy the structure from the inside over several seasons. Particular attention is paid to the ends of the boards, where moisture absorption occurs more intensely.
⚠️ Attention: Joining of sheathing boards must be done strictly on supporting elements (rafters or crossbars); hanging joints are unacceptable.
Installation technology and fasteners
The assembly process begins with the installation of the eaves board, which should be thicker than the other elements of the sheathing, as it supports the weight of the drainage system and the first row of roofing. Next, moving from bottom to top, install the remaining slats, observing the calculated step. To control the distance, it is convenient to use a template made from a strip of the required length.
Fastening the boards to the rafters is done with nails or self-tapping screws. The length of the nail should be equal to twice the thickness of the sheathing board to ensure reliable fixation. When using self-tapping screws on wood, the risk of wood splitting is reduced, which is especially important for the edge sections of boards.
☑️ Check before installing sheets
In places where chimney pipes, ventilation ducts and roof windows pass, a continuous flooring is made. This is necessary to securely fasten the connecting elements and prevent leaks. A fire-prevention gap is left around the pipes, which is filled with non-flammable material.
To ensure the evenness of the slope, pull the string along the cornice and along the ridge. This allows you to visually control the horizontality of the rows and adjust the position of the boards in time. Distortions of the sheathing will cause the corrugated sheets to fall in a wave, which will spoil the appearance of the roof.
The first and last rows of sheathing are always made solid or reinforced for reliable fastening of additional elements.
Common mistakes and their consequences
One of the most common mistakes is saving on the cross-section of a board or timber. An attempt to use a “inch” (25 mm) for a pitch of more than 400 mm often leads to the roof sagging under the snow. Corrugated sheeting is a rigid material, but it works in conjunction with the base, and weak support negates its advantages.
Ignoring the counter-lattice is the second critical mistake. Without a ventilation gap between the waterproofing and the sheathing, condensation formed on the inside of the metal will flow onto the insulation, causing it to get wet and rot the rafter system. Ventilation is an essential element of a modern roof.
An uneven pitch, when the distance between the boards “floats”, leads to an uneven fit of the sheets. In places where it sags, the metal will flap in the wind, and in places where it stops, it will experience excess stress. This is especially noticeable on long slopes, where the errors add up.
- 📉 The use of uncalibrated material leads to a difference in thickness and skew of the slope plane.
- 🔩 Fastening to one point at the junction of the board causes it to rotate and lose its load-bearing capacity.
- 🌧️ The lack of waterproofing under the sheathing allows moisture to enter wooden structures with small leaks.
They also often forget that the pitch of the sheathing must be a multiple of the wavelength of the corrugated sheeting, so that fastening with self-tapping screws occurs at the crest or bottom of the wave, and not at a random point. This ensures aesthetics and proper functioning of the fastener.
How to calculate the sheathing pitch yourself?
To make your own calculations, you need to know the snow load in your region (kg/m²), the brand of corrugated sheeting and its load-bearing capacity. The formula looks like checking the condition: the load on the sheet should not exceed its load-bearing capacity for a given span. The easiest way is to use online calculators or ready-made tables from manufacturers for a specific brand.
Is it possible to use old timber for sheathing?
Timber from previous collapsible structures can only be used after a thorough inspection. There should be no traces of rot, blue stains, deep cracks or a large number of knots. It is necessary to remove the old paintwork and re-treat it with an antiseptic. However, for critical roof structures it is better to use new material.
Do you need lathing for a corrugated fence?
For fences, a horizontal frame made of a profile pipe is used, which is also called sheathing. The lag pitch for a fence up to 2 meters high is usually 2-2.5 meters. If the fence is higher or the windage is large, the step is reduced to 1.5 meters or a third lag is added in the middle.