As someone with more than 30 years of experience in building timber houses, I'm here to give you some useful tips about the use of timber trusses in roof construction.
In Australia and United States timber trusses are extremely popular in single family homes and multifamily dwellings. What type of truss we need to choose depends on the purpose of the building, its size and span, as well as climatic conditions such as wind strength, rainfall and snow accumulation time.
Trusses are generally made up of logs, beams, purlins, battens and boards which can be either twisted or nailed directly onto the structure. Pine wood is usually used for this kind of construction, while spruce can serve as a good alternative. Smaller elements like dowels and inserts, however, should be made out of hard species such as oak, acacia or similar materials. It's important that all of these materials are dried properly (with no more than 20% moisture content) and protected against biological and fire risks. Solid wood pieces can have lengths up to 7 meters long, whereas longer ones require laminated wood. Steel reinforcements may also be added when larger spans are needed or if wooden cross-sections aren't effective due to their size.
Timber Trusses Strength
When it comes to timber roof trusses, a common mistake is assuming that lighter roofing materials such as metal, felt or shingles require a more delicate or lightweight structure. In fact, the roofing material only has a minimal impact on the required truss structure. The selection of a roof truss really depends on the combined load of the roof itself, insulation, finishing and external weather conditions.
I need to take into account many factors when designing a roof truss that can withstand strong gusts of wind and hold up the weight of snow, which can be up to 100-150 kg per square meter in some areas and even exceed 200 kg in mountainous regions. My process includes static and strength calculations based on various parameters like roof pitch, distance between walls, type of roof covering, support structure capabilities, and wind zones relevant to the building's location.
For example, let's say I'm designing a timber truss for a home located near mount Hood in Oregon, USA. This area is known for heavy snowfall, so my truss needs to be able to handle over 200 kg of snow per square meter. Additionally, due to the high winds in this region, I'll need to use extra thick timbers and an increased amount of fasteners to ensure the truss stays secure during these gusts. With careful planning and design, I can create a durable and reliable timber truss that meets all the requirements of this particular project.
Rafter Truss
The simplest and most common is the rafter truss, which works well when the span of the external walls of the building doesn't exceed 7 m and the pitch of the roof is between 40-60°. This type consists of two rafters connected at the ridge, spaced approximately 70-100 cm apart. At their lower ends, they can be attached to a horizontal tie beam that also serves as a load-bearing structure for the ceiling or to purlins placed on top of the ceiling or even knee walls. It's important to make sure these connections are strong and secure, since the stability of the roof depends on it! Each single rafter can have a maximum length of 6m.
The advantages of this solution include ease and speed of execution, low cost, and wide application. However, this type of roof truss may not be ideal in every situation. Surface curvature is likely due to long stretches of unsupported rafters, as well as forces pushing on the walls. With timber trusses, it's especially important to consider all applicable factors before installation.
Rafter Beams
As a homeowner, I understand the importance of selecting the right roof truss for my building. When it comes to timber trusses, a rafter-beam type is often used when the span of the external walls does not exceed 6-7 meters. This type of truss consists of two rafters connected at the ridge and supported by a horizontal beam made of wooden flooring. The beams can be directly supported on the wall or on the wall plate, with an interval spacing of 80-120 cm between each pair of rafters. To ensure structural integrity, these rafters should not exceed 7 meters in length.
Purlin Trusses
Purlin trusses are easy to construct, cost-effective, and have fewer limitations than rafter trusses. For these reasons, they are very commonly used in house construction. They are built for wall spans ranging from 5 to 12 meters and roof pitch angles of 25-67 degrees (a minimum of 35 degrees is recommended).
A purlin truss is a variation of a rafter truss to which a horizontal element, called a purlin, has been added. It is usually placed halfway up the length of the rafter (or slightly higher) to connect and brace them. It is generally accepted that the lower part of the rafter (from the wall plate to the purlin) should not be longer than 4.5 meters, and the upper part (between the purlin and the ridge) should not be longer than 2.7 meters.
Different types of purlin trusses are used depending on the roof span. A classic purlin truss is typically designed when the purlin length is no more than 3.5 meters. If it is being constructed in a house with a habitable attic, the bottom edge of the purlin must be at a level of 2-2.4 meters above the floor. In the case of roofs with a low pitch angle, it is necessary to build knee walls reinforced with reinforced concrete structures (a ring beam and columns) to provide sufficient headroom.
Horizontal purlins are often used as elements of the attic ceiling structure. The recommended spacing between the rafters in this solution is 70-100 cm. For roofs with a span of 9-12 meters, a purlin-rafter truss is also used, consisting of pairs of rafters supported by purlins, which are then supported by one or two additional gable walls. These walls, necessary due to the long purlins, provide stiffness to the entire roof structure and are set on a foundation and connected by a intermediate purlin at the top.
When the roof span is 7.5-10 meters, one purlin is used, and when it is 9-12 meters, two are used. The angled elements, called swords, serve to brace the gable wall structure and provide additional support for the purlins.
Purlin Braces
When the wall span is over 11 meters, I use a purlin-brace truss. This type of roof structure evenly distributes the pressure on the whole building and provides an even roof surface due to its rigid supports. The downside is that there's some limitation to free space in the attic. Still, this solution is versatile enough for both flat and steep roofs (with angles ranging from 6-70 degrees) and spans up to 16 meters.
Plus, I get superior strength and stability with timber trusses. They are also lightweight and easy to install — perfect for any project where design freedom and cost savings are important!