A correctly calculated sheathing pitch for corrugated sheeting is a critically important parameter, on which the load-bearing capacity of the entire structure directly depends. An error in planning the distance between the bars or boards can lead to the metal sheet bending under the weight of snow or gusts of wind. As a result, the tightness of the coating is compromised, and in the worst case, the roof collapses or the fence is deformed.
Many novice builders neglect accurate calculations, relying on intuition or advice from friends, which is a gross violation of technology. A professional approach requires taking into account the grade of steel profile, regional snow and wind loads, as well as the angle of inclination of the slope. Only a comprehensive assessment of these factors makes it possible to determine what distance between frame elements will be optimal for a particular object.
In this article we will analyze in detail the calculation methodology, regulatory requirements of SNiP and practical nuances of installation. You will learn how the wave height of the corrugated sheet influences the choice of beam cross-section and why saving on sheathing material often results in expensive repairs. Proper planning at the start guarantees the durability and reliability of your structure.
Factors influencing the choice of spacing between bars
Determining what the sheathing pitch for corrugated sheeting should be is based on several key variables that cannot be ignored. The grade of steel sheet used plays a primary role, since different profiles have different load-bearing capacities. For example, a profiled wall sheet with a low wave height will require a more frequent arrangement of supporting elements compared to the load-bearing option.
The second critical factor is the angle of inclination of the slope, which directly affects the speed at which snow and rainwater melt. The lower the roof, the more snow is retained on it, creating significant static pressure. In such conditions, the pitch of the sheathing under the corrugated sheet must be reduced in order to distribute the load and avoid metal deformation.
It is also necessary to take into account the climatic features of the construction region. In areas with heavy snowfall and strong winds, the requirements for frame rigidity are much higher. Regulatory documentation recommends the use of increasing factors in calculations to ensure a margin of structural strength in case of extreme weather events.
Don’t forget about the weight of the roofing pie itself. If you plan to lay heavy waterproofing, insulation and use massive sheets of corrugated sheets, the overall load on the rafter system increases. In this case, the distance between the sheathing elements should be minimal to prevent the covering from sagging between the supports.
⚠️ Attention: The use of wall corrugated sheets (marking C) for roofing with large lathing pitches is strictly prohibited. Such material does not have sufficient rigidity and can fold under load, leading to leaks and destruction of the roof.
The choice of material for the sheathing itself also dictates its own conditions. A wooden beam, edged board or metal profile behave differently under load. Wood can dry out and become deformed, so the fastening step in this case is often made more frequent to compensate for possible changes in the geometry of the frame.
Regulatory requirements and SNiP standards
Construction in Russia is regulated by sets of rules and regulations that establish minimum requirements for the safety and reliability of structures. The main document to refer to is SNiP II-26-76 “Roofs” and updated version SP 17.13330.2011. These documents contain tables and formulas for calculating the lathing pitch depending on the type of coating.
According to the standards, for different brands of corrugated sheeting, limit values for the distance between supports are established. For roofing materials with a wave height of less than 20 mm, the sheathing pitch should usually not exceed 300 mm for a continuous deck or be calculated individually. For more powerful load-bearing profiles, the distance can increase to 1000 mm or more, but only with an appropriate angle of inclination.
It is important to understand that the standards set the minimum acceptable parameters. The recommended pitch can be exceeded only after engineering calculations have been carried out confirming that a particular sheet will withstand the load without deflection. In private housing construction, simplified schemes are often used, but they must remain within a reasonable safety margin.
Particular attention in the standards is paid to node connections. Cornice and end overhangs, as well as areas around chimneys and roof windows require reinforcement. Here, the sheathing step under the corrugated sheeting is often made continuous or reduced to a minimum to ensure reliable fastening of the elements and tightness of the junctions.
| Brand of corrugated sheet | Wave height (mm) | Sheet thickness (mm) | Recommended pitch (mm) | Tilt angle |
|---|---|---|---|---|
| NS-35 | 35 | 0.5 - 0.7 | 1000 - 1200 | > 15° |
| N-60 | 60 | 0.7 - 0.9 | 2000 - 3000 | > 8° |
| N-75 | 75 | 0.7 - 1.0 | 3000 - 4000 | > 6° |
| S-21 | 21 | 0.5 - 0.6 | 300 - 650 | > 15° |
Compliance with standards not only guarantees safety, but also simplifies the process of approval of construction by supervisory authorities. In addition, the use of proven schemes allows you to avoid problems with the warranty from the material manufacturer, which often require compliance with installation technology.
Calculation of sheathing pitch for roofing
The process of calculating the optimal distance between the bars begins with determining the type of corrugated board. If you are using generic brands such as NS-35 or NS-44, they have sufficient rigidity for most pitched roofs. For such sheets, the pitch can vary widely, but the optimal value is considered to be from 800 to 1000 mm at an angle of inclination of more than 15 degrees.
For thinner and less rigid sheets, e.g. S-21, the requirements are stricter. Here the distance between the supports should not exceed 650 mm, and with gentle slopes (less than 15 degrees) it is reduced to 300-400 mm. This is necessary to prevent the effect of “flapping” of the metal in the wind and to eliminate deflections under the snow cap.
When calculating, it is important to take into account the overlap of sheets. The corrugated sheeting is laid with an overlap in one or two waves, and the joint line should fall strictly on the axis of the sheathing bar. Therefore, the pitch is often adjusted so that the joints of the sheets coincide with the support, which ensures the tightness and strength of the connection.
- 📏 Measure the length of the slope and divide it by the number of sheets to understand the number of rows.
- 📐 Determine the angle of the roof using a protractor or building level.
- ❄️ Find out the snow load for your region using SNiP maps.
- 🔨 Choose a brand of corrugated board with a safety margin.
There is a formula for approximate calculation, which is used by experienced craftsmen: the pitch is equal to the length of the sheet minus 100 mm for the overlap, but not more than the standard value for a given brand. However, for complex roofs it is better to use special calculators or contact an engineer who will take into account all the nuances.
⚠️ Attention: When calculating the step, be sure to take into account the actual width of the board or beam. If you plan to use material of a non-standard cross-section, the step must be recalculated, since the load-bearing capacity of the support will change.
A special case is a roof with a rafter pitch that does not correspond to the optimal sheathing pitch. In such cases, a counter-lattice is installed, which allows you to set the desired rhythm of the bars, regardless of the location of the rafter legs. This also creates a ventilation gap necessary to remove moisture from the under-roof space.
Specifics of installing sheathing under a fence
Unlike the roof, where the main load is snow and wind, other forces act on the fence. Here the windage of the structure and the risk of mechanical damage are critical. The lathing pitch for a fence made of corrugated board is usually fixed and depends on the height of the fence. For standard fences up to 2 meters high, a distance of 2 or 3 transverse joists is considered optimal.
If the height of the fence exceeds 2 meters, the number of horizontal logs is increased to three or four. The step between them in this case is approximately 600-800 mm. This allows you to evenly distribute the wind load and prevents the corrugated sheet from bending between the supports.
For gates and wickets the requirements are even stricter. Since these elements are mobile and experience constant vibration, the sheathing step here is kept to a minimum, and a solid frame or a fine mesh of profile is often used. This ensures rigidity of the sash and prevents the hinges from loosening.
Use a profiled rectangular pipe (for example, 40x20 mm) instead of timber for the fence. The metal does not rot, does not dry out and provides an ideal geometry for fastening the sheets.
Attaching corrugated sheeting to joists also has its own characteristics. Self-tapping screws are screwed into each wave on the outer sheets and through the wave in the central span. If the lag pitch is too large, the sheet may begin to “play” during gusts of wind, which will lead to the fasteners being pulled out and corrosion occurring in the holes.
It is also important to consider the type of fence foundation. If the pillars are installed with a large pitch (3-4 meters), then the logs must have increased rigidity, or additional vertical ribs must be welded between them. Otherwise, the long span of corrugated sheeting between the pillars will deform under its own weight and wind.
Materials for sheathing and their preparation
The traditional material for lathing is softwood lumber: pine, spruce, larch. The most popular edged board is 32 mm thick and 100 mm wide. A block with a cross section of 50x50 mm is also often used. The main requirement is that the moisture content of the wood should not exceed 20%, otherwise the structure may fail when it dries.
An alternative to wood is a metal profile, especially for corrugated roofs. Steel purlins made from profiled pipes or channels have a stable geometry and are not subject to rotting. The pitch of the metal sheathing can be greater than that of wood due to the high bending strength of the metal.
Before installation, the wooden sheathing must be treated with antiseptics and fire retardants. This will protect the structure from mold, insects and fire. Impregnation is carried out carefully, paying attention to the ends of the boards, where moisture is absorbed most intensively.
- 🌲 Choose wood without knots and cracks for maximum strength.
- 🛡️ Treat the material with fire bioprotection in two layers.
- 📏 Calibrate the boards according to thickness so that the lathing plane is even.
- 🔩 Use galvanized fasteners to prevent rust.
When using metal, it is important to remember thermal expansion. Metal sheathing can change its size due to temperature changes, so during installation you need to leave small gaps in the joints. In addition, contact of steel with other metals must be excluded or isolated to avoid electrochemical corrosion.
Can I use unedged boards?
The use of unedged boards is allowed only for outbuildings. The edges of such a board must be cleared of bark, since pests infest under it and rot quickly develops. This material is not recommended for residential buildings.
Installation technology and common mistakes
Installation of the sheathing begins with laying the waterproofing film on the rafters and installing the counter-lattice. This creates the necessary ventilation gap. Then the first (eaves) beam is attached, which usually has a cross-section larger than the others, since it bears the load of the first row of sheets and the drain.
Next, using a template or tape measure, mark the position of the remaining bars. The pitch is measured from center to center of the elements. It is important to ensure that the rows are horizontal, checking them with a stretched thread or a laser level. Any distortion will cause the sheets of corrugated sheets to fall in a wave.
The bars are fastened to the rafters with nails or self-tapping screws. The length of the fastener must be such that it enters the body of the rafter by at least 30-40 mm. When using metal sheathing, welded joints or bolted fasteners with lock washers are used.
⚠️ Attention: It is strictly forbidden to use foam or glue to attach the sheathing to the rafters. Only mechanical fasteners (nails, screws, staples) can withstand wind and snow loads.
One of the common mistakes is saving on the amount of fasteners. Each beam must be secured at at least two points of intersection with the rafters. Also considered a mistake is the lack of reinforcement at the exit points of ventilation pipes and chimneys, where the sheathing should be continuous.
☑️ Checklist before installing corrugated sheets
After completing the installation of the sheathing, it is recommended to check the plane of the slope again. If significant differences in heights are detected, they must be eliminated before laying the corrugated sheeting, otherwise it will be impossible to correct this later without dismantling the roof.
Final recommendations and conclusions
The correctly selected sheathing pitch for corrugated sheeting is a balance between saving materials and reliability of the structure. You should not try to make the step as rare as possible in pursuit of savings, since roof repairs will cost much more. Compliance with SNiP standards and recommendations of corrugated sheet manufacturers is a guarantee of durability.
When choosing between wood and metal, consider operating conditions and budget. Wood is cheaper and easier to process, but requires protection. Metal is more durable, but requires welding and corrosion protection. In both cases, the quality of installation plays a decisive role.
Remember that each property is unique and there are no universal solutions. It is critical to take into account the snow load specifically for your region, since it is this that often becomes the decisive factor when choosing a step. Always include a margin of safety, especially if the construction is carried out on your own without the involvement of professional designers.
The optimal lathing pitch is a compromise between the load-bearing capacity of the corrugated sheet and climatic loads, as specified in SNiP.
Attentive attention to detail at the stage of preparing the frame will save you time, nerves and money in the future. High-quality sheathing is the hidden, but most important element of the roofing system, which supports the entire weight of the covering and protects your home from the elements.
Frequently asked questions (FAQ)
Is it possible to make the sheathing step larger than recommended?
The pitch can be increased only after an engineering calculation confirming that the corrugated sheet will not bend under snow load. Without calculation, there is a risk of roof deformation and leakage.
What lathing pitch is needed for C-8 corrugated sheeting?
For grade S-8, which has a low wave, it is recommended to make the sheathing solid (from plywood or boards with a minimum gap) or with a pitch of no more than 300 mm, especially on gentle slopes.
Is a counter-lattice needed for corrugated sheeting?
Yes, if the roof is insulated. The counter-lattice creates a ventilation gap to remove moisture from the insulation. For cold roofs without insulation, it is not necessary, but is desirable for ventilation.
How to replace wooden sheathing?
An excellent replacement is a metal profile (rectangular pipe, channel). It is stronger, more durable and does not require treatment to prevent rotting, but it is more expensive and more difficult to install.
How to calculate the pitch for a complex roof?
For roofs of complex shape (hip, multi-gable), the pitch is calculated individually for each slope, taking into account the angle of inclination. It is better to use specialized programs or contact a designer.