Rafter system of the broken roof: the nuances of calculations and design + examples of construction

Welcome to "All about the Roof," our in-depth discussion of roofing system nuances. Today, we look at the broken roof’s rafter system—a vital component of roof design that blends practicality and visual appeal.

In order to maintain stability and distribute the weight of the roof structure evenly across the walls, the rafter system is essential. The design of a broken roof is more complicated since it has angles and intersections that call for exact math and careful planning.

Understanding a number of variables, including the roof’s span, the load it must support—including snow and wind loads—and the kind of roofing material to be used are necessary for calculating the rafter system. These estimates guarantee that the roof will continue to be structurally sound for many years to come in addition to having a pleasing appearance.

When designing a broken roof rafter system, angles and joints must be carefully taken into account. Each rafter needs to be cut exactly right and placed so that it fits together with the other rafters and the overall geometry of the roof. Achieving the intended architectural style and maintaining structural integrity both depend on this accuracy.

Let’s look at some real-world instances of malfunctioning roof rafter systems in the building industry. Each example demonstrates how various configurations can improve a roof’s functionality and aesthetic appeal, from simple hip and valley designs to more intricate traditional gable roofs.

Calculation of a browned rafter system

A pentagonal structure with a distinct variation in the slopes’ corners is the most vivid and expressive example of the class of broken roofs. You can tell that it is set up in two tiers that are supported by one another without even getting into the technicalities of its construction. The attic that gave rise to the second name, broken roofs, is located in the lower solid tier. The shape of the structure in the skate area is determined by a less voluminous tier-top that crowns the lower part.

Briefly about the specifics of the rafter design

The rafter frame for both parts of the attic roof is built according to the rules dictated by the usual technology for the construction of the pitched roofs. The lower part of the frame of the broken roof is built by installing layered rafter legs. In the upper device, both layered and hanging rafters can be used. The bottom of the layered rafter has the right to rely on the Mauerlat or on the beams of the ceiling. The support for the top is most often a wooden frame, at the same time playing the role of the frame of one of the walls of the attic. The upper tier device is focused mainly on the convenience of work for the Contractor.

According to tradition, the angle of slope of the slopes of the bottom of the broken roof is much cooler than the upper. They create a break-a visual indicator of the use of broken technology in the construction of the roof. However, the steepness of the upper and lower parts of the slopes can be equal, which is why the broken structure will look like an ordinary gable. But they are standard for broken roofs by the method, because the frame of the lower tier is obliged to ensure the possibility of organizing the operated space. T.e. The rafter system should have the necessary elements with a given bearing ability to arrange walls and ceiling insulated or cold attic.

Types of loads and their totality

We won’t bring him for the construction companies’ designers’ computation of the rafter system of the damaged roof. Without us, builders know them. These kinds of basic details are completely unnecessary for those who have chosen to add one or two roofs to a suburban area. Numerous programs are available that carry out intricate mathematical calculations to determine the section of beams, supports, and rafters in a matter of seconds. We more thoroughly examine the data that will need to be entered into the program, the kinds of loads that will be present on the overlap, and the rafters of the upper and lower tiers that need to be considered.

Download the program itself by clicking this link (work requires Excel only). After that, we’ll provide samples of our work done especially for her.

Why are the limits needed

Different kinds of loads will act on every component of the roof’s broken rafter system. The weight of the loads shouldn’t cause distortions and damage that need to be fixed. Two limit values are considered in the calculation of carrying constructions in accordance with the rules. These are:

  • The maximum strength is a state, the excess of which leads to the destruction of the construction structure, to the loss of endurance or stability.
  • The maximum deformation is a state, the excess of which leads to unacceptable deflections, as a result of which the geometry of the structure not only changes, but nodal joints are violated.

Designers are computed for both kinds of specified marginal conditions. A self-employed roofer is not very smart. They already have the formulas built into them taken into consideration on the Internet. They are incorporated into the computed algorithm as signal values of the following kind:

  • Ntr. strength – the size of the element of the rafter system, the reduction of which will lead to the state of loss of strength.
  • N tr. deflection – the size of the element, the decrease of which will entail threatening deformation.

When using automated computing support, it’s important to keep an eye on these numbers. Since this is the limiting minimum, real design values ought to be higher.

The weight of winter precipitation, wind power, one’s own mass, the weight of furniture, and people using the attic are among the loads pressing on the roof. Loads can operate concurrently, alternately, or in any combination, such as snow + person + furniture or snow + wind + t.D. In an attempt to provide the likelihood of the maximum load, the calculations are performed as thoroughly as possible.

How to determine the weight of the snow cover

The weight of the snow cover can be calculated without the need for specialized knowledge. It is accomplished by making it clear that the construction site is a part of a particular "snowy region." Looked at the sign to see how much snow would be pressed on the horizontal surface after finding the area on the map with its assigned number.

For rafters of the upper and lower tiers of the broken roof, the snow weight indicators will be different. The slopes of the broken roof in most cases are not unequal in the angle of inclination. Solid precipitation has more opportunity to linger and lie down on a laid top close to the steep slopes of the lower part. It should be taken into account that on slopes by steepness up to 30º Snow weight is taken equal to the unity of the average value adopted in the region on the basis of long -term observations of the weather services. It is believed that on slopes with a steepness of 60º or more snow is not delayed at all, t.e. equal to zero. The weight of the weight of the snow in the interval between the indicated inclinations is found by interpolation. For example, if the angle of inclination is 45º, then the tabular indicator should be multiplied by a coefficient of 0.5, for 50º by 0.33 and t.D.

How to find a wind load

The wind load is required to determine the rafter system’s stability. Once more, we use the regionalization map, which has already been created based on the wind pressure values, to ascertain it. Due to the potential for tearing off and carrying off by gusty winds and steep banal overturn, this indicator is required for the rafter legs of both roof tiers. The coefficient created for various terrain types is multiplied by the wind power information to correct it.

In areas where there is a lot of wind, the rafter legs are fastened to the walls more frequently by twisting wire than by passing through it. The quantity of wind ties—struts, supports, boards, or rails fastened to three or more rafters—increases for sustainability. When determining the overall weight of the roof structure, their weight needs to be considered.


Roof weight is a prefabricated attribute with customizable settings. Actually, underneath the coating of a solid or rarefied crate, this is a mass of roofing cake specifically designed for a specific insulated or cold structure with a particular type of coating. It is computed using a roofing area of one meter.

It is possible to determine the average coating weight values based on the plate. It must be remembered that the weight of the snow cover needs to be increased by 10% when using embossed roofing materials. It is important to keep in mind that snow cover in the crevices of slate or corrugated board, for instance, can accumulate and lay down for an extended period of time.

The kind of coating affects the crate’s weight. The soft roof device needs sheets of moisture-resistant plywood, OSP plates, or continuous flooring made of board. Slate, clay, and profiled tin tiles are set atop bars that are put in place using a specific procedure. When diagonal wind ties are installed in areas with high wind loads, the weight of the crate will increase. Separate calculations are made for the mass of insulation and the rafter system, which includes struts, supports, runs, and other components.

There are indicative averaged indicators available for quick calculations:

  • weight of a wooden crate from 10 to 12 kg/m²;
  • the weight of layered rafter legs with a run of 5 to 10 kg/m²;
  • The weight of the hanging legs of the rafter farm from 10 to 15 kg/m².

Program-calculated readings shouldn’t deviate significantly from the provided values. When loading an insulated attic, the weight of the casing needs to be added back to the load list. It is acceptable to disregard the insulation’s mass when using it because its heat conductivity coefficient—which is not much different from 0, 04 W/m × ° C—is used.

We demonstrated where to look and how to obtain values to enter into computational systems. According to the design data, all additional information is introduced for the mathematical calculation of the cross-section of the rafter legs, beams, and supports. The sizes of the elements should be increased if the calculated system alerts the user with a warning that "the condition is not fulfilled" or that the bearing capacity is not guaranteed.

Gaining an understanding of the rafter system of a damaged roof requires delving into the complexities of calculations and design in addition to real-world construction examples. This article provides insights into critical computations for figuring out load-bearing capacities and required reinforcements, as well as how to evaluate the structural integrity of a roof’s rafter system following damage. We demonstrate practical design techniques and material selections that guarantee both durability and safety by looking at actual construction situations. Knowing these basics will enable you to make wise decisions and produce enduring results, whether you’re building a new building or fixing a damaged roof.

Construction of a broken rafter system

Prior to moving forward with the rafter system device for the eventual broken roof, you must create a project and perform structural element calculations. Assumed to be past the design phase.

Examine a common illustration of an attic layout featuring two levels of stacked rafters, elevated above a brick enclosure. The rafter system will be fastened to Mauerlat using a wooden frame made from a 150 x 200 mm beam that is cut flush with the inner wall perimeter. Along the box’s exterior is a row of bricks that conceal Mauerlat and relieve some of the spacer load. The upper plane of the Mauerlat should be two to three centimeters higher than the brick strapping.

Installation of ceilings beams

The installation of extreme beams establishes the ceiling’s structure; their removal determines the width of the cornice overhang. Next, we install intermediate elements with a step equal to the distance between the rafters along the extreme beams that stretch between the extreme beams. It is advised that steps for insulated roofs be made to match the width of the thermal insulation slab in order to ensure that the dense insulation is positioned in the intersoperian space that is meant for it. In the case of unstable structures, the step is computed to ensure that every rafter is positioned with equal spacing between them.

The floor construction form size is 100 x 200 mm. If strictly aligning the horizon with the beams does not work, we align their upper plane. The chip’s lining or Mauerlat tint under the beam serves as the alignment tool. In order to create a plane for end cornice overhangs, we fasten a short beam to each beam’s end after attaching the beams to the Mauerlat. It doesn’t matter how far apart the short beams are; it can be as little as one meter.

The construction of the walls of the attic

We place markers on the line where several supports for the lower tier’s rafters are arranged to overlap. In addition to their supporting role, they also serve as the attic’s wall framework.

We behave as follows:

  • Install the corner supports, for the manufacture of which we use a beam of 100 × 150mm a length of 10 cm more than the final height of the attic ceiling. We verify the verticality of the supports with a plumb line, we will fix it only after we make sure of the impeccability of the installation. For stability, we fix their position with temporary slants. By analogy, we mount the supports in the middle of the front -line walls.
  • We connect the corner supports with the lace to indicate the installation site of intermediate racks. For the manufacture of intermediate supports, a 50 × 150mm material with an equal angular support height is suitable.
  • On top of two rows of supports we put runs made of boards 50 × 150mm. Temporary spacers will no longer be needed, the built walls of the future attic are stable and without them.
  • We install the board on the runs by the edge, it will form a ceiling overlap of the attic.
  • On top of the ceiling of the attic under construction, lay a 25 × 150mm board. It does not need to be installed along the axis of the structure. It is better to lay in parallel, retreating from the axis 20-30cm.

The attic’s completed frame and the supports needed to install the upper tier of the rafter legs are the end products of the labor.

Installation of the rafter of the lower tier

The conventional dashing method is used to construct and install the rafters on the lower tier of the damaged roof:

  • The board is 25 × 150mm of the required length we apply to the end of the construction of the structure and with a pencil in fact, mark the lines of the upper and lower backbone. This is a template that can be used for the manufacture of all rafters of the lower tier if there is no deviations in the geometry of the rafter system.
  • If there are doubts about the impeccability of previous work, set only the extreme legs and pull the lace between them. According to the template on the rest of the rafters, we make only the upper backbone. We will celebrate the lower line in fact, combining the upper plane of the workpiece with the cord landmark.
  • Install the rafter legs. We fasten them to the beams of the overlapping with brackets or metal corners, and at the top to the runs of two or three nails.

One board is often insufficient to completely overlap the lower slope along its length. In these cases, rafters are mounted from two short boards that are stitched together with a trim of material having a comparable cross-section and a minimum length of one meter. It is true that in order to avoid loosening the structure with stitched areas, it is preferable to order lumber of the necessary length.

Installation of the rafter legs of the upper slopes

The central axis must be identified before the upper rafterins are manufactured and installed. This can be accomplished by strictly vertically beating the inch trim to the attic’s highest ceiling board. When one of the pruning’s ribs precisely lines up with the damaged roof marked on the plan of the rafter system’s central axis, as follows:

  • We try on an inch to the end of the template to the end and mark on it the lines of the backbone, the upper of which is carried out directly along the designated cutting of the axis.
  • We make a pair of rafter legs of the upper slopes according to the template. If we have no doubt in the geometry of the constructed frame, we make several blanks at once. Otherwise, we do the same as with the lower brothers.
  • Install the first pair of rafters, calling for the assistance two pairs of workers. Alone not to cope with the installation, because they do not have the upper support. So that the just hoisted rafter farm does not fall, we support it with a brace.
  • We mount the rest of the rafters of the upper tier. Sign them with struts after 3-4 pieces. The angle of inclination of the struts should be more than 45º. The direction of their inclination should be alternated.

Note: Every upper rafter farm needs to have a board suspension system installed, measuring roughly 25 x 150 mm, to stop the attic ceiling boards from bending and deflecting.

The photo selection will become familiar with the details of the nodal joints of the rafter system of a broken type:

Carriage of pediments and the construction of the crate

Regardless of the intended type and installation step, the crate is made continuous along the skate and slope fracture lines. Two boards are nailed across the direction of the rafter legs, with a gap of two to three millimeters between them. If there are any yendov, they are designed with a similar continuous flooring around the attic windows and chimney pipe passageways. When using softer types of roofing coatings, the crate is placed continuously across the slope’s surface.

Installing the remote bar allows the capture to be placed ahead of the crate in cases where the insulation thickness is greater than or equal to the width of the rafters. A space must be created between the roofing material and the waterproofing. Connect the system’s external remote bar to the rafters’ ribs. There is no need if the thickness of the thermal insulation plates permits you to leave a ventilation gap without using distance tricks. Additionally, it is not required for building an unscarried roof.

Cornices and pediment walls—known as tong-built wooden houses—are sheathed once the rafter system installation is finished. The time for roofing is then when the short overhangs next to the pedimental walls are equipped.

Anyone working on a building or remodeling project needs to understand the rafter system of a damaged roof. It serves as the roof’s structural support system, bearing the weight of the roof covering in its entirety and ensuring its stability over time. Builders can design and construct roofs that are not only long-lasting and aesthetically beautiful, but also functional by understanding the subtleties of rafter calculations and design.

There is more to calculating the rafter system than just simple math. Raiser dimensions and spacing are determined in large part by local building codes, span, snow loads, and roof pitch, among other factors. These factors must be carefully taken into account when designing a sturdy rafter system to guarantee that the roof can withstand weather and safety regulations.

Rafters, for instance, might need to be positioned closer together in areas with significant snowfall in order to support the weight of the accumulated snow. On the other hand, the design may concentrate on forming a streamlined profile to reduce wind resistance in regions that are prone to strong winds. These instances show how flexible rafter design can be when it comes to meeting particular environmental demands and structural specifications.

Examples of constructions further highlight how versatile rafter systems are. A variety of materials and techniques are available to builders, ranging from conventional timber framing methods to contemporary engineered wood products. Every method provides distinct advantages concerning affordability, eco-friendliness, and simplicity of implementation, permitting creative freedom in architecture without compromising structural soundness.

Video on the topic

The attic h.1 – calculated scheme, load, RSU

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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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