How to calculate the rafters for the roof: determination of the length, cross -section and load on the rafters

Determining the rafters is an important step in roof construction. The roof structure is supported by rafter beams, which are angled beams that shift the weight to the walls. For the roof to be stable and long-lasting, the proper length, cross-section, and load on the rafters must be determined.

The pitch of the roof and the distance between the supporting walls are the two main variables that determine the length of the rafters. The angle of the roof slope, or pitch, determines how sharply the rafters will incline. Longer rafters are needed for a steeper pitch in order to reach the eaves, or the edges, from the ridge, or top of the roof. The rafters’ required horizontal distance is determined by the spacing between the walls. Building professionals can precisely cut rafters to the length required for a snug fit by precisely measuring these dimensions.

The term "cross-section" describes the actual dimensions and form of the rafter. The majority of rafters have a triangular cross-section, with the apex being the narrowest part and the blade, the thicker base. Environmental conditions, local building codes, and the weight of the roof covering (shingles or tiles) all affect the cross-section’sstrengthand size. Building materials such as steel, wood, or engineered lumber are used to select a cross-section that will not buckle or sag under anticipated loads.

The weight of the roof structure, including the covering, insulation, and any additional loads like snow or wind, must be taken into account when calculating the load on the rafters. This computation guarantees that over time, the rafters can securely support all of these weights. The total load that the rafters must bear depends on a number of factors, including location (different regions have different requirements for snow loads), roof shape, and roof orientation (which influences wind load).

Knowing how to calculate the rafters when building a roof is crucial to guaranteeing both structural integrity and safety. This entails figuring out the rafters’ proper length and cross-section as well as estimating the weight they will support. Gaining proficiency in these computations enables builders to effectively create roofs that satisfy the unique requirements of the structure, be it residential, commercial, or industrial, while also withstanding outside influences."

The specifics of calculating the rafter frame

The pitched roof, which serves several important purposes, is configured and has strength characteristics that are determined by the rafter system. This is an essential part of the architectural ensemble and a responsible enclosing design. As a result, the calculations and design of rafter legs should strive to eliminate errors and flaws.

Typically, design developments take into account multiple options and choose the best one. Selecting the best option does not require you to create a specific number of projects, carry out precise calculations for each, and then favor the one and only.

The meticulous selection of the design form and the size of the material for its construction is the very process of determining the length, mounting slope, and cross-section of the rafter.

For instance, the cross-sectional parameters of the material that is most appropriate given the material’s price are first introduced in the formula for determining the rafter leg’s bearing capacity. Additionally, the size of the lumber is adjusted until it reaches the highest level of conformity if the outcome does not meet technical requirements.

Tilt angle search method

There are technical and architectural factors to consider when determining the pitched structure’s angle of slope. Apart from a well-proportioned arrangement that aligns with the building’s style, an ideal solution should consider:

  • Snow load indicators. In areas with abundant precipitation, roofs are erected with a slope of 45º or more. Snow deposits are not delayed on the slopes of such steepness, due to which the total load on the roof, stopwood and the construction in general are significantly reduced.
  • The characteristics of the wind load. In areas with impulsive strong winds, coastal, steppe and mountainous regions, they build low-scatter structures of the streamlined shape. The steepness of slopes there usually does not exceed 30º. In addition, wind prevent the formation of snow deposits on the roofs.
  • Mass and type of roofing. The larger the weight and the elements of the roof, the cooler you need to build a rafter frame. So it is necessary to reduce the likelihood of leaks through the joints and reduce the specific gravity of the coating, which is per unit of the horizontal projection of the roof.

All of the aforementioned specifications must be considered in order to determine the ideal rafter inclination angle. The future roof’s steepness must adhere to both the roofing’s technical specifications and the climate of the roofing that was selected for the area’s construction.

It is true that property owners in serene northern regions should keep in mind that the amount of materials consumed rises as the rafter legs’ angle of inclination increases. The cost of building and arranging a roof steeper than 60 to 65 degrees will be about 1.5 times higher than that of building a structure at a 45 degree angle.

It’s not a good idea to drastically lower the slope in places where winds are strong and frequent in order to save money. Overly soft roofs are unappealing from an architectural standpoint and don’t always result in lower costs. In these situations, the need for insulation layers is usually necessary, which raises construction costs against the expectations of the economy.

The rafter’s slope can be represented in degrees, percentages, or dimensionless units that show the height of the ridge run installation divided by half a meter of flight. It is evident that the slope of the slope and the ceiling line form an angle that is defined in degrees. Because interest is so complex to perceive, it is rarely used.

Dimensionless units are the most widely used method by low-rise building designers and builders to indicate the angle of inclination of rafter legs. They express the relationship between the roof’s height and the overlapped span’s length. Locating the center of the future pediment wall at the facility and installing a vertical rail with a skate height marker is a simpler task than cutting the corners off of the ramp’s edge.

Calculation of the length of the rafter leg

After deciding on the system’s angle of inclination, the length of the rafters is calculated. It is not possible to link either of these values to the quantity of precise values, t.To. The steepness and length of the rafter leg may vary slightly during the load calculation process.

The primary factors influencing the rafter length computations are the kind of cornice roof overhang, whichindicates which

  1. The outer edge of the rafter legs is cut flush with the outer surface of the wall. The rafters in this situation do not form a cornice overhang that protects the structure from precipitation. To protect the walls, a drain is installed, mounted on the rabbits nailed to the end edge with a cornice board.
  2. Wrap -cutting with a wall of the rafters are built up with filly for the formation of a cornice overhang. Mares are attached to the rafters with nails after the construction of the rafter frame.
  3. The rafters initially cut through the length of the cornice overhang. In the lower segment of the rafter legs, cuts are chosen in the form of a corner. For the formation of the rollers, the rafters retreat from the lower edge to the width of the cornice. Cuts are needed to increase the support area of ​​the rafter legs and for the installation of support nodes.

When determining the length of the rafter legs, it’s important to consider the options for attaching the roof frame to the upper crown of the log house, the passage, or the Mauerlat. The calculation is done along the length of the upper rib of the rafters, taking into consideration the size of the tooth, if it is used to form the lower connecting unit, if the installation of the rafter is thought of by the flush with the outer contour of the house.

The length is determined on the upper rib of the rafters plus the overhang if the rafter legs are cut out with the removal of the cornice in mind. Keep in mind that while using triangle trash greatly quickens the rafter frame’s construction, it also degrades the system’s components. Consequently, the coefficient of 0.8 is applied when determining the bearing capacity of rafters with the selected angles.

The standard 55 cm was the average width of the cornice removal. Nonetheless, the dispersion may range from 10 to 70 or more. The projection of the cornice removal on the horizontal plane is used in the computations.

The manufacturer’s recommended maximum values are contingent upon the strength characteristics of the material. For instance, in order to prevent the snow mass that accumulates along the roof overhang from damaging the cornice’s edge, the manufacturers of shifters do not recommend placing the roof behind the walls of the walls at a distance greater than 10 cm.

Having large overhangs on steep roofs is not typical; no matter the material, the cornices do not make the roof 35 to 45 centimeters wider. However, buildings with a slope of up to thirty degrees can work well with a broad cornice, which will act as a sort of canopy in places with a lot of sunlight. If the roof design has seventy or more cornation spaces, those areas are reinforced with extra support racks.

How to calculate the bearing capacity

Rafter frames are constructed using coniferous wood-based pilomaterials. A prepared beam or board cannot be of a lower quality than the second.

Pitched roof rafter systems operate on the basis of compressed, curved, and compressed-bent elements. When it comes to resistance to bending and compression, second-rate wood performs admirably. The first variety is necessary only if the structure’s structure allows for stretching.

Raratile systems focus on the standard dimensions of the flow generated by the renewal of lumber and are constructed from boards or timber that is chosen with a margin of strength.

The two states that are used to calculate the bearing capacity of rafter legs are as follows:

  • Calculated. A condition in which, as a result of the applied load, the structure is destroyed. The calculations are carried out for the total load, which includes the weight of the roof pie, the wind load, taking into account the number of storeys, the mass of snow, taking into account the slope of the roof.
  • Normative. A condition in which the rafter system is bending, but the destruction of the system does not occur. It is usually impossible to operate the roof in this state, but after repairing it is quite suitable for further use.

In a streamlined computation, the second prerequisite equals 70% of the initial value. T.e. Computed values need to be trumpedly multiplied by a coefficient of 0.7 in order to obtain regulatory indicators.

The cards attached to SP 20.13330.2011 determine the loads based on the climate data of the construction region. Locating your city, the closest cottage village, or other settlement is the first step in finding the regulatory values on cards. From there, you can take readings of the calculated and normative values from the card.

The average data regarding wind load and snow should be modified in accordance with the house’s architectural features. For instance, the value extracted from the card needs to be split up into slopes that match the wind rise estimates for the region. The local weather service has printouts available for you to obtain.

Since there will be less snow on the windward side of the building, the calculated indicator is multiplied by 0.75. Snow deposits multiply here by 1.25 because they will accumulate on the leeward side. The leeward portion of the structure is typically constructed from paired boards, and the wrendered part is arranged with rafters of their single board, in an effort to unify the material used for the roof’s construction.

It is preferable to multiply by 1.25 if it is not clear which slope will be leeward and which, on the other hand. If the price of lumber does not rise significantly, there is no harm in taking a little extra precaution.

The card’s calculated weight of snow is still subject to change based on how steep the roof is. When snow is placed at a 60º angle, it slides off the slopes instantly and without any hesitation. The correction factor is not applied in the computations for such steep roofs. Since the snow can already linger with a lower bias, an additive in the form of a coefficient of 0.33 is used for slopes of 50º, and 0.66 is used for slopes of 40º.

The corresponding map determines the wind load in the same manner. Modify the value in accordance with the region’s unique climate and the house’s height.

Finding the maximum load on the primary components of the planned rafter system requires adding up temporary and constant values in order to determine the bearing capacity of those components. No one will fortify the roof prior to the snowfall, even though it would be preferable to install safety vertical spacers in the country house’s attic.

The weight of every component of the roof cake, including the insulation, the internal hem, the roof itself, and the crate placed atop the rafters, is necessary in addition to the mass of snow and the pressing force of winds in the computations. It is common practice to overlook the weight of steam and membrane-containing waterproofing films.

The manufacturer provides information about material weight in technical passports. When approaching, information on the bar’s and the boards’ mass is gathered. While the mass of the crate allowed by a meter of projection can be computed using the average weight of 500–550 kg/m 3 for a cubic meter of lumber and 600–650 kg 3 for a similar volume of OSP or plywood.

The load values expressed in kg/m 2 are those provided in SNiPs. Rafter, on the other hand, only senses and supports the load that applies direct pressure to this linear element. The sum of the natural tabular values of the loads and the mass of the roof pie is multiplied by the step of installing rafter legs in order to precisely calculate the load on the rafters.

The step, or the space between the rafters, can be changed to increase or decrease the load value provided to linear parameters. Achieve the ideal values for the load-collecting area in order to prolong the lifespan of the pitched roof’s frame.

Determination of the cross -section of rafters

The function of the raft legs on roofs with varying degrees of steepness is unclear. A compressive force is added to the analogues of steep systems, which acts primarily as a bending moment on the rafters of gentle structures. Consequently, the slope of the slopes must be considered when calculating the section of the rafters.

Calculations for structures with a slope up to 30º

Only the bending stress acts on the rafter legs of the roofs of the indicated steepness. They are computed with all possible load types applied at the maximum moment of bending. Furthermore, computations for maximum indicators make use of transient, or climatic, loads.

The center of the rafter leg is where the maximum bend occurs in rafters, which only have supports under both of their edges. The middle of both spans will receive the moments of maximum bend if the rafter is composed of two simple beams and is supported by three points.

The maximum bend in rafters with three supports will occur near the center support, but not exactly there. There is a support underneath the bending area; unlike in previous cases, it will point upward.

Two requirements must be met for the system’s rafter legs to function normally:

  • The internal voltage formed in rafters during the bend as a result of the load attached to it must be less than the calculated value of the resistance of the lumber to bend.
  • The deflection of the rafter leg should be less than the normalized deflection value, which is determined by the ratio of L/200, t.e. bending the element is allowed only for one two hundredth share of its real length.

These requirements will be met by additional calculations in the sequential selection of the rafter leg size. The section can be calculated using one of two formulas. One of them is used to calculate the board’s or timber’s height based on an arbitrary thickness specification. The thickness at any height can be determined using the second formula.

It suffices to use just one of the formulas in the computations; both are not required. Following computations, the first and second limiting states verify the outcome. An arbitrary indicator added to the formula can be decreased if the computed value shows an impressive margin of strength in order to avoid overpaying for the material.

An arbitrary value is increased if the calculated value of the bend’s bending is greater than L/200. The selection is made based on the common sizes of lumber that are offered for sale. Thus, choose the cross section until the optimal choice is found and the moment is calculated.

Let us examine a basic calculation example utilizing the formula b = 6wh². Assume that W is the ratio of m/rHut and that h = 15 cm. Using the G × L 2 /8 formula, we determine the value of M. Here, G represents the total load applied vertically to the rafter leg, and L is the span length, which is 4 m.

RHut According to technical standards, we take 130 kg/cm 2 of lumber from conifers. Let’s say we precalculated the total load and it came out to be 345 kg/m. Then:

M × 16m 2/8 = 690 kg/m= 345 kg/m

We divide the result by 100 to convert it to kg/cm, and the result is 0.690 kg/cm.

0.00531 cm is equal to W = 0.690 kg/cm / 130 kg/cm 2.

B = 7.16 cm= 6 × 0.00531 cm× 15 2 cm

We round the result as it should be upholstered and find that a 150 × 75 mm beam is required for the installation of rafters, considering the load in the load example.

We verify the outcome of both conditions and establish the suitability of the material with the currently calculated cross-section. F = 1.39; σ = 0.0036

For rafter systems with a slope of more than 30º

Roof rafters steeper than thirty degrees must withstand compression forces acting on them along their own axis in addition to the bend. In this instance, the rafters must be calculated based on the internal stress in addition to verifying the resistance mentioned above and the bend’s bending.

Similar steps are taken for T.E. actions, but a little bit more verification work needs to be done. Likewise, an arbitrary height or arbitrary thickness of lumber is established, and with its assistance, the second section parameter is computed. Next, the compliance of the aforementioned three technical conditions—including compression resistance—is verified.

If required, the arbitrary values added to the formulas increase in order to increase the rafter’s bearing capacity. Recalculating the height or material thickness is necessary if the normative deflection greatly surpasses the calculated value and the margin of strength is relatively large.

A table will assist in selecting the baseline information needed to produce computations that reduce the standard sizes of the lumber that we produce. Selecting the rafter legs’ length and cross-section for the preliminary computations will be helpful.

In order to calculate rafters for your roof, you must first determine their length, cross-section, and maximum load capacity. To start, calculate the length of rafters required by measuring the width of your roof from wall to wall. Because it determines the overall dimensions of your roof structure, this measurement is very important.

Next, think about the rafters’ cross-section, which is determined by things like the weight they have to support and the pitch of the roof. Wood, steel, and engineered trusses are common materials for rafters; each has a distinct load-bearing capacity and structural requirements. Your roof’s strength and longevity are directly impacted by the material and cross-section you choose.

It’s critical to comprehend the load on the rafters to guarantee the structural soundness of your roof. The total load that the rafters must bear depends on a number of factors, including wind resistance, snow load, and kind of roofing material. To guarantee compliance and safety, load-bearing capacity calculations should always take local building codes into account.

To sum up, rafter calculation entails exact length measurements, careful cross-section selection based on material and load requirements, and observance of safety regulations. You can design and build a roof that is not only practical but also resistant to changing weather conditions by taking these factors into consideration.

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Alexandra Fedorova

Journalist, author of articles on construction and repair. I will help you understand the complex issues related to the choice and installation of the roof.

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