A gable roof (or simply a pitched roof) is one of the most popular and time-tested roofing options. It is known for its reliability, relatively simple construction, and low cost of erection. However, the key to successfully arranging a gable roof is the correctly selected and installed rafters. Below you will learn what a rafter is, how to properly calculate the rafter size for the roof, what the mauerlat should be like, and you will also get answers to common questions about the rafter system.
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What is a rafter and what role does it play in the roof structure
A rafter (or rafter leg) is the main load-bearing element of the roof structure, which perceives and distributes the load from the roofing onto the load-bearing walls of the house. Rafters are usually made of coniferous wood (pine, spruce, larch), and less often oak or other hardwoods are used.
The main functions of a rafter:
- Load-bearing: accepts the weight of roofing materials, snow, and wind.
- Structural: redistributes the load onto the mauerlat, which in turn redirects these forces to the walls.
- Forming: defines the shape and slope of the roof, affecting the appearance of the house.
Mauerlat: what is it and why is it needed
Mauerlat is a wooden beam laid around the perimeter of the external walls of the house. It serves as an intermediate support between the rafters and the wall itself. The mauerlat is securely fixed to the walls with anchor bolts or special metal fasteners.
Typical sizes of mauerlat
- The most common cross-section of mauerlat: 100×150 mm or 150×150 mm.
- The length depends on the perimeter of the house: usually, these are solid beams along each wall or spliced with overlap.
In wooden houses, the role of the mauerlat can be performed by the upper crown of the log or beam. In brick or block houses, the mauerlat is fixed to the reinforced belt or directly to the wall (depending on design solutions).
Rafter sizes: how to choose the cross-section correctly
The strength and durability of the entire roof depend on what the rafter size for the roof should be. The size of the rafter (its cross-section) is chosen based on:
- Span (the distance between support points – mauerlat and ridge beam or other supports);
- Rafter spacing (the distance between adjacent rafters);
- Weight and type of roofing (tiles, metal tiles, ondulin, etc.);
- Snow and wind load of the region;
- Additional structural solutions (the presence of braces, girders, purlins).
Examples of typical rafter cross-sections (for reference)
- For a span of up to 4 m: 50×150 mm or 50×200 mm.
- For a span of 4–6 m: 50×200 mm or 60×200 mm.
- For a span of 6–8 m: 50×250 mm or 60×250 mm.
If the roof slope is steeper, the load on the rafters is distributed somewhat differently, but usually, it is recommended to increase the cross-section when the length of the rafters (span) significantly increases. The rafter size can also be adjusted depending on the planned insulation (for example, for a thick layer of mineral wool, higher rafters are needed).
How to determine the length of the rafter
To understand how to determine the length of the rafter, you need to know:
- The width of the house (the span over which the roof is mounted).
- The height of the roof rise to the ridge (slope angle).
The simplest way is to use a triangle. If half the width of the house is one leg, and the height difference (from the mauerlat to the ridge) is the other leg, then the length of the rafter is the hypotenuse. The formula for a right triangle:
where:
- A – horizontal projection (half the width of the house),
- B – height from the mauerlat to the ridge (according to the project).
For accuracy, the overhang of the rafter beyond the wall (eaves overhang), which usually amounts to 40–60 cm, is taken into account. The corresponding value is added to the obtained result, and a small margin (5–10 cm) is made.
Main types of roofs and rafter systems
1. Gable roof
The most common option. It consists of two planes that converge at the ridge. Rafters rest on the mauerlat and can be supported by additional elements (girders, braces).
2. Shed roof
Has one sloping plane. Mostly used for utility buildings, garages, and extensions.
3. Hip roof
Consists of two trapezoidal and two triangular slopes. More complex to install, but better withstands wind loads.
4. Broken and mansard roofs
Involve a more complex configuration of the rafter system to increase the usable area of the attic. Used for arranging living spaces under the roof.
Types of rafters and their features
-
Simple rafters
- Rectangular cross-section (most often 50×150, 50×200, etc.).
- Rest on the mauerlat without additional supports.
- The most common type for small houses with relatively short spans.
-
Shortened rafters
- Shorter in length, usually used to form gables of the roof or junctions.
- Often found in mansard constructions.
-
High rafters
- Extended elements that form a steep slope.
- Used when it is necessary to increase the space under the roof or have a larger slope angle for regions with heavy snowfall.
-
Cantilever rafters
- Do not rest directly on the mauerlat.
- Attached to the walls using special brackets.
- Used to create large spans or complex architectural solutions.
Stages of rafter installation
1. Preparing the mauerlat
- Installing the mauerlat – is fixed around the perimeter of the walls. It is usually leveled horizontally and secured with anchor bolts or metal pins.
- The mauerlat is treated with antiseptics to protect against rot.
2. Making and installing rafters
- First, a rafter template is made (one rafter, from which all others are cut).
- It is important to provide notches for the mauerlat (the so-called “lock” or “recess”) for reliable fitting.
- Attaching the rafter to the wall and mauerlat is done with metal brackets, bolts, or nails.
- Check the verticality and parallelism of the rafters, adjust the spacing between the rafters.
3. Installing girders, braces, ties (if necessary)
- Girder – a horizontal beam that connects two opposite rafters (reduces thrust on the walls).
- Brace – a sloping board propped under the rafter to reduce deflection.
- Tie – a beam or board that can replace the ceiling slab (in a mansard roof, it is installed higher to increase space).
4. Battens (lathing)
- After installing the rafters and additional elements, battens (a frame of boards or battens) are installed, which serve as a base for roofing materials.
- The spacing and thickness of the battens depend on the type of roofing (metal tiles, ondulin, ceramic tiles, etc.).
5. Laying the roofing
- Depending on the chosen material, hydro-, vapor-, and thermal insulation may be installed.
- The roofing material (sheet or tile) is installed, after which the ridge, ventilation elements, and drainage system are arranged.
Tips and nuances
- Be sure to use treatments (antiseptics, fire retardants) for wood. This will extend its service life and protect against fungi and insects.
- Check the geometry: rafters should be of equal length, with the same cuts, installed at the same spacing.
- Do not skimp on fasteners: it is better to use quality bolts, nails, brackets, and clips – this is key to the reliability of the entire structure.
- Consider snow and wind loads for your region. If you are unsure about the calculations, consult specialists.
FAQ (Frequently Asked Questions)
1. What is a rafter?
A rafter is a wooden beam (rafter leg) that is the main load-bearing element of the roof. It transmits the load from the roof to the walls of the house through the mauerlat or other supports.
2. What should be the size of the rafter for the roof?
The optimal size of the rafter depends on the span, spacing between rafters, type of roofing, and snow-wind loads. Common sizes: 50×150 mm, 50×200 mm, 60×200 mm, 50×250 mm, etc. The exact value is determined by the project or calculations.
3. How to determine the length of the rafter?
The length of the rafter is equal to the hypotenuse of the triangle, where the legs are half the width of the house and the height of the roof rise to the ridge. Additionally, the overhang (eaves overhang) is taken into account, and a small margin is made.
4. What is a mauerlat and what is it needed for?
A mauerlat is a wooden beam that is laid around the perimeter of the walls. It serves as a base for attaching rafters and evenly distributes the load from the roof onto the load-bearing walls.
5. What types of roofs are the most common?
The most popular are shed, gable, hip, and broken (mansard) roofs. Each has its own features in the construction of the rafter system.
6. How to attach rafters to the wall?
Usually, rafters are attached to the mauerlat, and the mauerlat is secured to the walls with metal bolts or pins. For greater reliability, metal angles, plates, and other fastening elements are used.
7. How often should rafters be installed (what spacing)?
Typically, the spacing of rafters is 60–120 cm. The specific value is determined by calculations: taking into account loads, span length, and type of roofing.
8. Is a girder or brace needed?
A girder and brace are needed when the length of the rafters is large or there are excessive loads on the roof. They help reduce deflection and thrust of the rafters.
9. Can you do without a mauerlat?
In wooden houses, the upper crown of the log or beam can serve as a mauerlat. In brick and block houses, a separate beam (mauerlat) or reinforced belt is necessarily installed to reliably distribute the load and ensure a strong attachment.
10. What types of wood are best suited for rafters?
Rafters are usually made from coniferous species (pine, spruce, larch). This is an affordable and sufficiently strong wood that, with proper treatment, can withstand high loads and last for many years.
Properly designed and installed rafters are the key to a reliable roof for decades. Consider the snow and wind characteristics of your area, choose quality wood, and do not forget about antiseptic treatment. If you are unsure about the calculations, it is better to consult specialists – this will help avoid mistakes and guarantee a safe and durable structure.