Trussed Rafters or Roof Trusses are without doubt the most popular way to form roof scapes. Not only do they arrive on site as a fully engineered and certified product, they are considerably cheaper and much more versatile than a traditional “cut roof” and typically consume 40% less wood. Trusses supplied by Timber Frame in Spain are made in our factory from drawings prepared by our engineer specifiers. We take your house drawings and special truss design software is used to determine exactly what is required in each individual case. Once our design engineer is satisfied that the chosen calculations meet all the necessary regulations he will issue the approvals.
There are many different types of roof trusses known as Duopitch type or Monopitch type. This simply means Duopitch has two slopes and a central ridge, whereas Monopitch has only one slope and sometimes called “lean-to”.
The most common Duopitch are shown below with the average span, although greater spans can be achieved.
1. King Post Truss Spans up to 4.5m Uses: House & Garage Roofs
2. Queen Post Truss Spans up to 6.0m Uses: Usually Domestic
3. Fink Truss Spans up to 11m Uses: Most common and takes water tank.
4. Double W Spans up to 16m Uses: Commercial and Futuristic.
5. Howe Girder Spans up to 8.0m Uses: Supports other Trusses & loose infill
The most common Monopitch are shown below with the average span, although greater spans can be achieved.
- Mono 2/1 Spans up to 2.5m Uses: Hip Ends and Lean-to Roofs.
- Mono 2/2 Spans up to 4.5m Uses: Large Hips & Span on to Firewalls
- Mono 3/2 Spans up to 6.0m Uses: Back-to-Back Central Corridor
- Mono 3/3 Spans up to 8.0m Uses: On Monopitch Roofs.
The timbers permitted for use MUST be stressed graded usually TR26 and C16. Most of the timbers used in Truss manufacturing are from Europe, Canada and Britain. The common species are BS.5268: part 3
- Whitewood and Redwood – Europe
- Hem-Fir Canada & USA
- Douglas Fir-Larch Canada & USA
- Spruce-Pine-Fir Canada & USA
- Southern Pine USA
- Scots Pine UK
- Corsican Pine UK
Moisture: BS5268: Part 2 Stipulates that the moisture content of any timber must not exceed 18% although BSEN14250 does permit the moisture content to be 22% at manufacture provided will be dry to below the 18% value during construction.
Trussed Rafters are primarily made up of Tension and Compression members.
A tension force is a pulling or stretching force, like a tow rope and a compression force is being pushed or compressed, like a table leg of a support column.
So, if you look at a typical Duopitch Fink Truss configuration the Bottom Chord is a Tension Member and the two “Roof Slopes” are Compression Members. The two outer infill members are in Compression and the two inner members (stretching from apex to bottom chord) are in Tension. There are 7 pieces of wood joined together with a “pull and push” type relationship as 4 pieces are in Compression and the other 3 are in Tension.
Some roof designs require a mixture of different types of Trusses, which make them so popular, almost any shape and size of roof can be achieved using Trusses although in some cases, depending on the design of the building some alternative support such as Steel or Glulam Beams may be called for.
The use of RIR (Room in Roof) attic trusses are becoming very popular, as around 60% of the ground floor area can be utilised as living space at very little extra cost on new builds. In Spain a builder could legally add around 60m2 to a 100m build licence, again at little cost, and the sale value of the property increases quite considerably.