Timber Roof Construction Details and Structure

A Guide to Timber Roofs

Timber has been the traditional choice of material for building roofs for many centuries. Today, it is still the top choice for roof material for contractors, housebuilders, and self-builders whether they are building in a traditional or contemporary style and irrespective of the roof style. Timber is used for both pitched and flat roofs and is a popular roofing material because it is widely available and is generally easy to work with. Timber is an economic choice of material and has an excellent strength-to-weight ratio. When sustainably sourced, it can be a very environmentally-friendly option and is also biodegradable.

Timber Roofs Throughout History:

In the past, timber was the most suitable and also the most available material from which to build the structure of the roof. When it came to determining the form of the roof, the roof covering was the dominant factor. Most of the time, roofs were pitched, with a slope from a central ridge. The degree of sloping or pitch was determined by the roof covering. The roof covering also dictated the size of the timbers; for example, stone slates that were often used for weather houses required extremely sturdy rafters and posts since they were much heavier compared to cheaper roof covering materials such as thatch or clay roof tiles.

Timber Roof Structure:

A timber roof has two basic components. These are the supporting structure and the covering. A timber frame for a flat roof is often the cheapest and the easiest to build when constructing a home. However, since it results in a stunted, blocky-looking appearance, this type of roof is not often chosen as it is less attractive compared to a sloped or pitched roof. In addition, the finishes available for flat roofs are limited and it does not provide a loft or attic space that can be used for storage or water tanks. Unlike pitched roofs, water is not ‘shed’ off the flat roof and it can also be harder to find and repair leaks.

Horizontal timber beams known as purlins support sloping secondary timber rafters in a simple pitched timber roof. The purlins run parallel to the edge of the roof and rafters are typically secured to fit over the purlins by notching with a ‘birdsmouth’ to prevent roofs from being blown off the top of the house in the wind. Masonry wall supports can be used to notch the rafters over a timber wall plate that is secured to the top of the blockwork or brickwork. Galvanised mild steel straps are typically used to tie them down. While the principles of a timber roof structure are very similar, there can be some considerable differences depending on the size, arrangements, and jointing methods used for any particular roof.

Timber Pitched Roof Construction:

There are two basic methods of construction for timber pitched roofs. These include:

• Cut roof: The traditional method of cutting timber onsite and building the roof up using rafters, joists, purlins, ridge boards, and other materials. The exact details will be determined by the size of the roof and timbers required.
• Truss roof: This is when the roof is made using factory-made trusses that are delivered to the building site complete before they are erected on the building.

Cut Timber Roof Details:

This type of roof consists of rafters and joists. The purpose of joists is to prevent the outward spread of the rafters while providing support for the ceiling below. The size of the rafter timbers used will depend on their length from the ridge to the wall plate, the type of roof covering used, and whether or not purlins are used. Keeping the cross-section of the rafters down is a more economical choice, however, larger rafters will be necessary if open roof space is required. Rafters are typically spaced at 400mm, but closer spacing may be necessary to allow small-section rafters and batten, which are fixed to rafters to fix tiles or slates, to be used. Rafter thickness will depend on the size of the gap between them.

• Rafters: These are typically nailed to wall plates located at the top of each supporting wall. They are normally around 100x75mm in size and are embedded on cement mortar at the top of the inner skin of the cavity wall or the inner part of a solid wall. Along the top of each wall, half-lap joints are used to join wall plate timbers where they meet. Each rafter has a small triangular section, called a birdsmouth joint, cut from them to enable neat fitting over the wall plate. Tops of rafters are cut at an angle and nailed to the ridge board, which is usually around 175x32mm in size and vertically mounted.
• Purlins: These are used to provide the rafters with additional support. They run at right angles to the rafters and have struts positioned under each fourth rafter that are attached to joists in the supporting walls beneath.
• Hangers: These are used in conjunction with longitudinal binders that run at right angles to the joist. Their purpose is to help hold the ceiling in place. Each hanger is nailed between every fourth rafter and the joist beneath. The need for hangers reduces with larger joist sizes.
• Collars: These are sometimes used on the ridges or further down the rafters to provide an additional binding between rafters on each side of the roof. They are typically fixed to occasional pairs of rafters along the roof when they are used.

Truss Timber Roof Detail:

Truss roofs are made up of several factory-made frames or trusses that combine joists, rafters, and struts. A modern truss roof will be designed and manufactured for a certain position in the roof structure. They are made up of timber lengths that are butt-joined and are typically nailed together with plate fastenings rather than a timber joint. Truss timbers can either be bolted together or pegged after being mortised and tenoned.

Benefits of Timber in Roof Construction Details:

Wooden cut or truss roofs are some of the features in both modern and traditional houses. Timber can be incorporated into any roofing products in both residential and commercial buildings, and builders have taken advantage of the many benefits of this material for centuries. Today, timber is still one of the most popular materials used for both cut and truss roofs and for very good reasons. Some of the main benefits of using timber in roof construction include:

Design Versatility:

The roof does not only fulfil functional purposes but also plays a large role in determining the buildings’ design and aesthetic appeal. Some commercial buildings may even incorporate their brand image into the design of the roof, and a well-designed roof can make all the difference to the curb appeal of a home. Timber is an easy material for builders to use to make unique designs since it is highly versatile. The material can be moulded into various different shapes making it an ideal choice for basic roof designs and intricately designed buildings.

Lightweight:

The weight of timber is one of the main factors making it a preferred material for cut and truss roofs. Wood weighs significantly less than steel; another material that is also often used for truss roofs. Although it is lightweight, timber is strong enough to accommodate the load and stress that it has to bear when used in a roof, but not so heavy that it puts pressure on the rest of the building.

Thermal Properties:

The thermal insulation abilities of wood are another benefit over other materials that has made it a popular choice of roofing material for centuries. Keeping a building with a timber roof warm is less complicated since the wood will not allow the heat to dissipate into the atmosphere. In addition, timber roofs can easily leave cavities that can be insulated in order to improve the warmth and the energy-efficiency of a building.

Cost-Effective:

For both construction companies and self-builders, the cost is always a dominant factor to consider when constructing any building. Opting for a timber roof is an ideal way to lower the cost of construction since it is widely available for a good price. It is much less expensive compared to steel since timber grows on trees rather than being engineered. Even when a roof is made with specially designed timber, it will usually cost less compared to putting up a steel roof. In addition, wood blends in easily with other common roofing materials, making it an ideal choice of material to offset some of the high costs involved in constructing a hybrid roof.

Issues to Be Aware Of:

While there are many benefits of a cut roof made from timber or timber trusses, it’s important to be aware of potential issues. Wooden trusses often do not last as long compared to other roofing materials since they are a natural material that cannot stand up to the elements as well. Warping and bowing are common problems with timber that can lead to structural damage to the rest of the building if not corrected quickly. Wood is also at higher risk of insect infestations and rot, so regular maintenance is key.

Timber has long been one of the most popular materials used in roofing for its cost-effectiveness, versatility, and durability.