Calculation of what rafters are needed for a gable roof under the flooring begins with determining the snow load in your region and the length of the span, since these parameters dictate the choice of timber section. An error in choosing the thickness of the board or timber can lead to a deflection of the entire structure, deformation of the coating sheets and even collapse of the roof in winter. Flooring, being a light material, creates a minimum load on the roof itself, but it requires a perfectly flat geometry of the truss system, otherwise there will be cracks at the joints.
For a reliable frame, it is necessary to take into account not only the weight of the profile sheet itself, but also the mass of the crate, insulation, waterproofing, as well as wind forces. load-bearing capacity The truss leg directly depends on the wood, its humidity and the quality of treatment with antiseptics. In most cases, for private housing, the best choice is a trimmed board or a beam of coniferous rocks, but their parameters must strictly correspond to engineering calculations.
It is important to understand that saving on the cross section of rafters is unacceptable, since replacing the bent elements will require dismantling the entire roofing pie. A properly selected installation scheme and step of installing supports will ensure a long life of the roof without the need for complex repairs. Next, we will discuss specific types of rafting systems and methods for calculating their parameters.
Selection of material and wood species for frame
The basis of a durable roof is high-quality lumber that meets certain standards of strength. For the manufacture of truss legs, coniferous wood is most often used, since it has good resin content, which increases its resistance to rotting. The most common pine, which combines affordable cost and sufficient strength characteristics for most standard projects.
The most important parameter in the selection is timber. The use of raw material will lead to the fact that in the process of drying the rafters will begin to βleadβ, which will cause deformation of the entire geometry of the roof and violation of the tightness of the coating from the flooring. Optimal is the humidity within 15-20%, for which the boards must undergo chamber drying or prolonged natural drying under the canopy.
- π² Pine is the most popular option, affordable and easy to process, suitable for flights up to 6 meters with the right cross section.
- π² Spruce - has fewer knots than pine, but is more susceptible to rotting, requires careful antiseptic treatment.
- π² Larch - is characterized by high density and resistance to moisture, ideal for regions with high humidity, but is more expensive.
- π² Oak is rarely used due to the high price and complexity of processing, used in elite construction to create exclusive designs.
When purchasing material, it is necessary to carefully inspect each board or beam for the presence of large falling knots, cracks and traces of damage by a bark beetle. The presence of such defects sharply reduces load-bearing This makes it a weak link in the whole system. For responsible nodes, such as mauerlat or skate run, the highest grade material should be selected without visible damage.
Buy wood with a margin of length to be able to cut off damaged ends and align the geometry before installation.
Types of rafting systems for gable roof
The structural solution of the frame depends on the presence of internal load-bearing walls in the building. If the capital partition passes in the middle of the house, it is rational to use the elevation system, where the truss legs are based on the skating run and the bed. This scheme allows you to cover large spans using timber of smaller section, as the load is distributed to three points of support.
In the case where there are no internal walls or they are located far from each other, a hanging truss system is used. Here the legs rest only on the mauerlat, and in the upper part they connect to each other, forming a triangle. To compensate for the bursting force in such a design, puffs (rigels) are necessarily used, which connect the lower parts of the rafters or are located higher, depending on the layout of the attic.
β οΈ Attention: In the hanging system (it is categorically impossible) to remove puffs after installation, as this will lead to the driving of the walls under the action of a rebuff from the weight of the roof and snow.
The choice between these two types affects which ones are rigidity I'll need it. The elevation system requires the installation of racks and tarmacs, which can reduce the useful space of the attic, but benefits in reliability. The suspension system, on the other hand, frees up the interior space, but requires a more powerful cross section of the legs themselves or a frequent step of mounting them.
Calculation of cross-section and pitch of rafters
Determining what section of the board is needed for rafters is based on complex engineering formulas that take into account the bending moment and deflection. However, for private construction, there are proven tables that allow you to choose the optimal dimensions depending on the length of the span and the step of the installation. The main input data are the length of the ramp and the distance between the axes of the truss legs.
The smaller the installation step, the smaller the cross section can be used, and vice versa. The standard step for roofing under the flooring is considered a distance from 60 to 120 cm. Reducing the pitch to 60 cm is often recommended in regions with high snow load, as this allows for a thinner board while maintaining the overall bearing capacity of the structure.
| Length of span (m) | Step of the rafters (m) | Board section (mm) | Section of the bar (mm) |
|---|---|---|---|
| 3.0 | 1.2 | 50x150 | 100x100 |
| 4.0 | 1.0 | 50x200 | 100x150 |
| 5.0 | 0.8 | 50x200 | 100x200 |
| 6.0 | 0.6 | 50x250 | 150x200 |
Therefore, for long stingrays, it is better to take a higher board, even if its width remains standard. It is also worth considering that the merging of boards along the length is permissible only in places of minimum loads and should be performed with the observance of technology.
Formula for calculating snow load
Total snow load S = Sg * ΞΌ, where Sg is the weight of snow cover per 1 m2 of the horizontal surface of the ground (taken from the SNiP for your region), and ΞΌ is the coefficient depending on the angle of inclination of the roof. For angles up to 25 degrees ΞΌ = 1, from 25 to 60 degrees ΞΌ = 0.7, snow loads above 60 degrees are not taken into account.
Mauerlat installation and mounting of rafters' legs
The foundation for the entire truss system is mauerlat - a support timber laid along the perimeter of the walls. For its manufacture, it is usually used a timber cross section 150Γ150 mm or 200Γ200 mm, which is securely attached to the armopositor or upper crown of the log. The quality of mauerlat fastening directly affects the roofβs resistance to wind loads, which can tear the roof with a weak connection.
Mounting the truss legs themselves to the mauerlat can be performed in two main ways: rigid and sliding. Hard fastening excludes any movement and is used in elevation systems or houses that are not subject to significant shrinkage. Sliding mounting, implemented with the help of special metal corners with oval holes or "sleds", is necessary for wooden houses to compensate for the shrinkage of the walls without destroying the roof.
- π¨ Hard knot - is performed using a support bar and a nail joint or a screw, completely fixing the leg.
- π¨ Sliding knot - allows the rafter to move along the axis when changing the geometry of the walls, preventing cracks.
- π¨ Bolt connection β used to strengthen the nodes, requires accurate drilling and the use of large diameter washers.
To ensure tightness in the places of passage of rafters through the roof cake, it is important to properly organize the adjoining. Use of the vapor-insulating and waterproofing membranes should be made with overlap and gluing of joints to exclude condensation inside the structure.
βοΈ Installation readiness check
Features of the crate under the flooring
After installing the rafters, the stage of installation of the crate comes, which takes on the load from the profnastile and transfers it to the frame. For metal shingles and proflist most often used sparse crate of boards 25-32 mm thick and 100 mm wide. The step of the crate depends on the brand of profnatsil: the smaller the wave height and the thinner the metal, the more often the boards should lie.
The most important element is the cornice board, which is installed first and has an increased cross section, as it holds the weight of the drain system and the edges of the roof. Also, special attention is paid to the places of passage of chimneys, installation of attic windows and endov, where the crate is made continuous to provide reliable support for waterproofing carpets.
β οΈ Attention: All wooden elements, including the crate, must be treated with a flame-retardant composition of at least two layers before the installation of the finishing coating.
Mounting the boards of the crate to the rafters is made by nails longer than double the thickness of the board, or screws. It is important to observe flatness, since the flooring, being a sheet material, will instantly show all the irregularities of the base. Using a string-piercing during the installation of each row will help to avoid waves on the finished roof.
The ideal plane of the crate is the guarantee that the flooring will lie flat and will not collect water in micro-deepenings, which will prevent corrosion.
Protection of wood and extension of service life
The durability of the truss system directly depends on the quality of protection from biological and chemical influences. Even the strongest wood will begin to break down with constant contact with moist attic air or leaks. Modern antiseptic impregnations can protect the material for up to 50 years, but only if applied correctly.
Processing is best done by immersion or heavy application of the brush in two layers with intermediate drying. Particular attention is paid to the ends of the boards, through which moisture is absorbed most intensively. If the roof is already installed, you can use corrugated compositions that allow you to visually control the quality of processing.
Regular inspection of the attic space after each winter season will reveal the first signs of leaks or mold. Timely elimination of small defects of waterproofing will save load-bearing structures from rotting and loss of strength. Remember that replacing a rotten truss leg is a complex and expensive process that requires partial disassembly of the roof.
Folk methods vs Chemistry
Although there are folk methods of protection (oil working, saline solutions), they are either fire-hazardous or ineffective against fungus. Professional flame-protective compounds form a protective film on the surface of wood and penetrate into deep layers, crystallizing there and preventing the development of microorganisms and fire.
Can a smaller section be used for rafters if you reduce the pitch?
Yes, reducing the pitch of the rafters allows you to reduce the load on each individual leg, which theoretically allows you to use a material of smaller cross-section. However, this increases the overall weight of the structure and the consumption of wood. The economic feasibility of such a solution requires accurate calculation, since the gain in the price of the material can be lost due to an increase in labor costs and the amount of fasteners.
What is the minimum angle of roof inclination allowed for the flooring?
The minimum angle of inclination for roofing from the flooring is usually 12 degrees. At smaller angles, the risk of leaks at the joints of the sheets increases sharply, so the use of continuous waterproofing and sealing of locks is required. The optimal angle is from 15 to 30 degrees, which ensures a good descent of snow and water.
Do I need to make a ventilation gap between the insulation and the rafters?
Yes, the presence of a ventilation gap (ventilator) is critical. It provides air circulation under the roofing, removing moisture that condenses on the inside of the proflist. The lack of ventilation will lead to wet insulation, rotting of the rafters and shortening the life of the entire roof.