Step crate under the flooring: norms, tables and installation schemes

The distance between the crate under the flooring directly determines the bearing capacity of the roof or fence and, if incorrectly calculated, leads to deformation of the sheets under the weight of snow or gusts of wind. An error in choosing the step of the beam or board causes a deflection of the metal, rupture of the fasteners and subsequent leakage of the structure, which requires a complete relay of the coating. Strict compliance with regulations SNIP II-26-76 The manufacturer’s recommendations ensure that the roofing cake will withstand the estimated snow and wind loads for the entire life of the roof without repair.

Start the design of the frame is necessary with the study of the technical passport for a specific brand profiled sheet, since it is the height of the corrugated corrugated dictates the permissible span. The higher the profile wave, the greater the stiffness on the bending it has, which allows you to increase the pitch of the supporting elements and save on lumber. Ignoring this parameter in favor of saving on the number of boards often turns into the destruction of the roof at the first serious snow load in winter.

Dependence of step on height of corrugated and thickness of metal

The key factor influencing the choice of the interval between lags is profile-geometry I'm a slacker. Low wave sheets, such as C8 or C10 grades, have minimal stiffness and require almost continuous flooring or minimal clearance between the crate elements. The use of such stamps for roofing with a rare step of the timber is unacceptable, since the metal will simply doubt the pressure of the installer's shoes or a layer of snow.

For profnats with a height of corrugated from 21 mm (marks) NC21, NC35) and above, the distance between the reference points may be increased. In this case, the sheet works as a beam, resting on several points, and its bearing capacity increases proportionally to the height of the profile. It is important to consider not only the height of the wave, but also the thickness of the steel sheet: a material with a thickness of 0.4 mm will require a more frequent crate than a sheet of 0.7 mm with the same brand.

Table of stamp compatibility

See hidden data on the brands of the flooring and the minimum wave height for different steps of the crate in the manufacturer's technical documentation.

When choosing a material for the frame, also pay attention to the quality of wood or metal. The raw board will eventually dry and deform, violating the geometry of the entire plane, which is critical for sheets with a small load-bearing capacity. Use of the calibrated Or dry beam allows you to withstand a single plane of the ramp.

Regulatory requirements of SNiP and SP for roofing systems

Building codes and regulations (SNIP) and the codes (SP) regulate not only the distance between the elements, but also the means of securing them and the timber humidity requirements. According to the current standards, the moisture content of wood for crate should not exceed 20%, which prevents rotting and shrinkage after installation of the coating. Violation of this parameter can lead to weakening of the fastener and displacement of the sheets of profnatyl.

⚠️ Attention: The use of untreated wood or materials with visible defects (knots, cracks, rot) in the bearing structures of the roof is prohibited by regulations and can cause collapse.

Normatives also dictate the need to strengthen the cornice overhangs, skate knots and places of passage of chimneys. In these zones, the pitch of the crate is often reduced to zero, creating a solid flooring, regardless of the brand of the profile sheet used. This is necessary for reliable fixation of the elements of the drain, snow delayers and fire safety.

For regions with high snow loads, regulations may require a reduction in the standard crate pitch by 10-15%. This creates an additional margin of safety, which compensates for critical loads in winter. The calculation in such cases is better to trust professionals or use specialized calculators that take into account the climatic region.

Calculation of step for different brands of profnats

Determining the exact distance between the bars requires taking into account the specific brand of material. Below is a table showing the dependence of the crate step on the type of proflist and the angle of slope of the slope. The data are advisory and the manufacturer's load-bearing tables should be consulted for final calculation.

Marka's flooring	The height of the corrugated, mm	Metal thickness, mm	Recommended step of crate (see)
C-8, C-10	8-10	0.5-0.7	It's a crate.
C-21	21	0.5-0.7	Up to 30cm
NS-35	35	0.5-0.8	Up to 60-100 cm
H-60, H-75	60-75	0.7-1.0	Up to 300 cm (3 meters)

As can be seen from the table, the transition to a higher profile brand allows you to significantly increase the step, which reduces the consumption of wood. However, for residential buildings rarely use the flooring above NS-35 Due to aesthetic considerations and excessive stiffness, the most common step is the range of 30-60 cm.

When calculating the amount of material, it is important to take into account the overlap of sheets. If the crate step is not selected correctly, the junction of the sheets can get between the boards, which will lead to a deflection at the junction and a potential leak. The junction of sheets It should always be on the backbone of the frame.

Features of installation of a crate for a fence

When erecting fences, the requirements for the distance between lags differ from roofing lags, since here the main role is played by the wind load and sailing of the structure. For fences from the flooring most often used two or three horizontal lags (run), the distance between which depends on the height of the canvas. The standard solution for a fence with a height of 2 meters is the installation of two lags with a indentation from the edges of the sheet 25-30 cm.

If the height of the fence exceeds 2 meters or the terrain is characterized by strong winds, it is recommended to install three lags. This prevents the effect of "sails" when the wind stream tears the sheets from the mount or deforms the pillars. The distance between the upper and lower lag in a three-row scheme is distributed evenly or with a shift to the bottom to strengthen the lower part of the span.

• 🔨 For fastening the flooring to the intake lags, roof screws with EPDM-laying are used, providing tightness.
• 📏 The optimal distance from the edge of the sheet to the first lag is 30 mm to avoid chipping during installation and ensure stiffness.
• 🛡️ Metal lags from a profile pipe 40x20 mm are a more durable solution compared to wooden timber, which is prone to rot.

When using long runs (more than 2.5-3 meters) between the pillars, it is recommended to weld additional vertical stiffness ribs or reduce the pitch of the pillars to avoid sagging horizontal elements under the weight of the sheets.

Solid and sparse crate: when to apply

The choice between solid decking and sparse crate depends not only on the brand of the profile sheet, but also on the angle of inclination of the slope. At small angles of inclination (less than 15 degrees), the risk of leaks through the joints increases, and even when using sealants, it is recommended to make the base more dense. A solid crate is made of trimmed board, plywood of increased moisture resistance or plates. OSB-3.

A sparse crate consisting of boards or bars laid with a certain step is used with sufficient stiffness of the profile sheet and angles of inclination above 15-20 degrees. This approach provides ventilation of the underroof space, which is critical for removing condensation and preventing rotting of the truss system. The gap between the boards in this case plays the role of a ventilation channel.

⚠️ Attention: When installing a solid crate made of plywood or OSB, be sure to leave temperature gaps between the sheets of 3-5 mm, otherwise, when expanding, the material may swell and deform the coating.

Economic feasibility also dictates the choice of the type of foundation. Continuous flooring requires much more material and labor, so its use is justified only where it requires the technical characteristics of the roofing or complex architectural forms of the roof (endows, mansard windows).

Typical errors in the construction of the base under the flooring

One of the most common mistakes is to use boards of different thicknesses or widths. The multi-size material creates an uneven plane, which causes the flooring to fall into a “wave”, which is visually noticeable when oblique lighting and violates the density of the adjoining sheets to each other. For high-quality installation, it is necessary to calibrate lumber or buy an already ready-made dry board for chamber drying.

Another common mistake is ignoring the cornice board. The first element of the crate in the cornice is often made thicker than the rest to the height of the profile of the profnatile. This is necessary so that the sheet on the overhang lies in the same plane with the rest of the roof, and was not tilted down. The absence of such a lining leads to the fact that the first row of screws twists at an angle, tearing the sealing washer.

• ❌ The absence of waterproofing film under the crate, which leads to dripping of condensate on the insulation.
• ❌ Mounting boards with nails without prior drilling, which causes wood to split, especially at the edges.
• ❌ Incorrect calculation of the step when the joint of the sheets is in the air between two lags.

In addition, often forget about the counterbar. These are bars that are attached along the rafters on top of the waterproofing, and already the crate itself is stuffed across them. The control bracket creates the necessary ventilation gap between the film and the boards, providing air current. Without this gap, moisture will not weather, which will shorten the life of the entire roofing system.

FAQ: Frequently Asked Questions

Can I use a non-circumcised board for a crate?

Using a rough board is not recommended. It has a bark, which is a habitat for pests, and uneven edges, which prevents you from creating a single plane. In addition, the uncut board is more often prone to warping. If the budget is limited, it is better to take a board of smaller section, but trimmed and dry.

What is the crate step needed under the C-21 flooring?

For the C-21 grade flooring with a roof angle of more than 15 degrees, the standard crate pitch is 300-650 mm depending on the snow load. At smaller angles of inclination, the step must be reduced, up to a solid flooring. Always look for the exact data in the load table of a particular manufacturer.

Do I need a counterbar under the flooring?

Yes, countercladding is mandatory for pitched roofs with insulation. It provides a ventilation gap between the waterproofing membrane and the main coating, allowing you to remove moisture from the insulation and prevent rotting of wooden structures.

What if the rafters’ step does not coincide with the crate’s step?

The step of the crate is set regardless of the step of the rafters. The boards of the crate cover several rafters legs. The main thing is to attach each board of crate to each rafter through which it passes, using two nails or screws at the edges of the board to prevent turning.