Incorrectly calculated lathing pitch under corrugated sheeting causes deflections of the metal sheet, wind breakage and roof leaks at the first serious load. An error in determining the distance between the beams directly affects the load-bearing capacity of the entire structure, since it is the frame that takes the weight of snow and gusts of wind, transferring them to the rafter system. If the distance between the elements is too large, the thin corrugated sheet will simply crumple under the weight of the snow cap, and an excessively dense grid will lead to unjustified waste of lumber and make the roof heavier.
To properly plan the structure, it is necessary to take into account not only the brand of profiled sheet used, but also the climatic region, the angle of inclination of the slope and the purpose of the building. Regulatory documents such as SNiP II-26-76, dictate strict requirements for the minimum cross-section of timber and the maximum span that can withstand a specific type of coating. Ignoring these parameters when building a garage or residential building can lead to the need for expensive repairs in the first year of operation.
Dependence of the pitch on the grade of corrugated sheetingCorrugated sheeting is a profiled sheet, the rigidity of which is determined by the height and shape of the corrugation. The higher the wave, the greater the load the sheet can carry per span, which allows you to increase the sheathing pitch without loss of strength. The brands most often used for roofing work are: NS-35, NS-44 or S-21, each of which requires an individual approach to the installation of the supporting frame.
⚠️ Attention: The use of wall corrugated sheeting (grades C-8, C-10) for roofing with large lathing pitches is strictly prohibited, since it does not have sufficient load-bearing capacity and will collapse under its own weight or snow.
⚠️ Attention: The use of wall corrugated sheeting (grades C-8, C-10) for roofing with large lathing pitches is strictly prohibited, since it does not have sufficient load-bearing capacity and will collapse under its own weight or snow.
When choosing the distance between the bars, they rely on the passport data of the material, where the manufacturer indicates the maximum loads. For sheets with a wave height of up to 21 mm, the step is usually made continuous or minimal, while for load-bearing grades with a corrugation height of more than 44 mm, the distance can reach 3-4 meters, provided the metal thickness is appropriate. It is important to understand that bearing capacity is a complex indicator depending on the thickness of the steel, zinc coating and profile geometry.
| Brand of corrugated sheet | Corrugation height, mm | Metal thickness, mm | Recommended lathing pitch |
|---|---|---|---|
| S-8, S-10 | 8-10 | 0.4-0.5 | Continuous sheathing |
| S-20, S-21 | 18-21 | 0.45-0.6 | Up to 300 mm (or solid) |
| NS-35 | 35 | 0.5-0.7 | Up to 1000 mm |
| N-60, N-75 | 60-75 | 0.7-1.0 | Up to 3000 mm |
The influence of the slope angle on the structureThe angle of the roof is a critical parameter that determines not only the waterproofing properties, but also the arrangement of the bars. On gentle slopes with an angle of less than 15 degrees, water and snow melt less well, so the requirements for tightness and rigidity of the base increase. In such cases it is often used continuous lathing from edged boards or moisture-resistant plywood to eliminate any risk of profile deformation in the intercostal space.
If the slope angle is from 15 to 30 degrees, a sparse step is allowed, but it must be strictly calculated according to the snow load of a particular region. With steep roofs (more than 30 degrees), the snow cover is retained less, which makes it possible to increase the distance between the frame elements, but the wind load here, on the contrary, increases. It is necessary to ensure that each sheet is securely fastened so that gusts of wind do not tear off the coating.
Features of installation on complex units
In places where chimneys, ventilation ducts and attic windows pass, the sheathing is always made continuous. This is necessary for high-quality waterproofing of junctions and installation of metal aprons.
To ensure proper drainage, the bottom row of the sheathing (eaves assembly) is always made more massive or continuous. This is due to the fact that the cornice bears the maximum snow load due to the possible formation of ice, and drainage hooks are also attached here. Often, a larger cross-section is chosen for the first beam than for the remaining rows, for example, 50x100 mm instead of 40x50 mm.
Selection of lumber for the frameThe main material for creating a load-bearing frame for corrugated sheets is traditionally coniferous wood. Edged board or timber must be dry (humidity no more than 20%) so that the structure does not deform during operation. Using raw material will cause the nails to become loose when drying, and the bars may become twisted, which will disrupt the flatness of the roof.
The optimal cross-section for bars is considered 50x50 mm or 40x60 mm, however, for heavy structures and large spans, timber may be required 50x100 mm. The board is usually used with a thickness of 25-32 mm and a width of 100-150 mm. All wooden elements must be processed before installation flame retardants (fire protection) and antiseptics (protection from rotting and insects).
To increase the service life of wooden sheathing, use “3 in 1” combined impregnations, which simultaneously protect against fire, moisture and biological damage. Apply the composition in two layers with intermediate drying.
The quality of lumber directly affects the durability of the roof. The boards should not have through knots, rot or traces of bark beetle activity. When purchasing, pay attention to the geometry: curved boards are difficult to fasten evenly, which will lead to waves on the finished roof. For critical objects, it is recommended to use chamber-drying material that is already stabilized and will not change its size.
Calculation of snow and wind loadsDetermining the exact spacing of the sheathing is impossible without taking into account the climatic characteristics of the construction region. Snow load is a dominant factor for low-slope roofs, where the weight of snow cover can reach hundreds of kilograms per square meter. Wind loads are critical for tall buildings and high-angle roofs where significant lift forces are present.
To make the calculation, you need to know your snow region (from I to VIII according to the SNiP map) and the wind region. The calculation formula takes into account the coefficient of transition from the weight of snow on the ground to the load on the roof, which depends on the angle of inclination. If the angle is less than 25 degrees, the coefficient is 1, if more than 60 degrees - 0 (snow does not linger).
⚠️ Attention: When calculating the sheathing pitch, always take the maximum load values with a margin of 10-15%, since climate standards may be revised, and abnormal snowfalls occur regularly.
Specialized calculators or SNiP tables allow you to determine the permissible span for a specific brand of corrugated sheeting. For example, for an NS-35 sheet with a thickness of 0.5 mm in the third snow region, the maximum pitch can be 1.2 meters, while for the fifth region it will have to be reduced to 0.8 meters or a more powerful profile must be chosen. Ignoring these calculations leads to emergency situations.
Do-it-yourself lathing installation technologyInstallation begins with laying a waterproofing film over the rafters, which protects the insulation and wood from condensation. The film is laid with a 1-2 cm slack between the rafters to drain water and secured with a stapler. On top of it, counter-lattice bars are packed along the rafters, which create the necessary ventilation gap to remove moisture from the under-roof space.
The main step bars are mounted directly onto the counter-lattice. The first (eaves) beam must be higher than the others by the height of the corrugated sheet profile so that the sheet lies horizontally. Fastening is done with galvanized nails or self-tapping screws, the length of which is equal to three thicknesses of the bar (so that they protrude 25-30 mm from it).
☑️ Checklist before installation
It is important to maintain the evenness of the plane. To do this, use a stretched string or a laser level. If the pitch of the sheathing under the corrugated sheeting is broken, the sheets may “go” in a wave, which will visually spoil the appearance of the roof and disrupt the joining of the locks. The fastening should be rigid, but without tightening, so as not to damage the structure of the wood.
Features of lathing for fences and wallsWhen erecting fences or covering walls with corrugated sheets, the principle of calculating the step remains the same, but the emphasis changes. There is no snow load here, but there is a high wind load, acting on the plane of the fence like a sail. For fences up to 2 meters high, 2 or 3 horizontal logs (purlins) from a profile pipe are usually used, which is analogous to the sheathing pitch.
For wall corrugated sheets (S-8, S-10, S-20), the vertical lathing pitch when facing facades is often 500-600 mm. This provides rigidity to the wall and prevents the metal from flapping in the wind. Unlike roofing, a metal profile is more often used here, since it is more durable and does not require treatment to prevent rotting.
The main conclusion: the sheathing pitch is not an arbitrary value, but a calculated parameter that depends on the brand of corrugated sheeting, the angle of the roof and the climate of the region. Saving on material is unacceptable here.
⚠️ Attention: When installing a fence made of corrugated board, the distance between the logs should be less when using thin metal. For a sheet of 0.4 mm, three logs are recommended; for 0.5 mm and above, two are sufficient for a span height of up to 2.5 meters.
Common errors and ways to resolve themOne of the most common mistakes is the use of untreated wood, which quickly loses its properties under conditions of temperature and humidity changes. Rotting of the supporting fastening points leads to weakening of the fixation of the sheets and the appearance of corrosion at the points of contact with the wood. Also, a violation of the step geometry often occurs when a master nails boards “by eye,” which leads to skewed rows of tiles or corrugated sheets.
Another problem is saving on fasteners. Using ordinary black self-tapping screws instead of galvanized or roofing screws with EPDM washers leads to rapid rusting of the fastening points. Water flowing into a rusty hole destroys both metal and wood. It is important to use only specialized fasteners selected by color and size.
FAQ: Frequently asked questions
Is it possible to make lathing from a metal profile?
Yes, galvanized profiles are often used for flat roofs, industrial facilities or wet rooms. It is lighter, does not burn or rot, but requires special self-tapping screws and is more expensive than wood.
What is the minimum sheathing pitch for S-8 corrugated sheeting?
For grade S-8 with a thickness of 0.5 mm and an angle of inclination of less than 15 degrees, a continuous lathing made of boards 25-32 mm thick is recommended. A sparse pitch is not used for this brand due to its low load-bearing capacity.
Do you need a vapor barrier under the sheathing?
If the roof is insulated, then a vapor barrier film is required on the side of the room (under the insulation), and a hydro-windproof membrane is required above the insulation, under the sheathing. For a cold roof, one waterproofing is enough.
How to correct an error if a step has already been done incorrectly?
If the step is too large, you will have to dismantle part of the covering and add additional bars. If the step is too frequent, this is not a big deal, but it leads to waste of material. There is no need to correct “too often”, this will only increase reliability.