Timber trusses are hewn out of cut logs and utilized for making a truss for a roof structure. Most of the time the timber trusses have aesthetic finishes besides the required strength. Heavier timber trusses can used as single beams. Occasionally, the thicker beams are made by glulam or laminating the thinner pieces of lumber together with the glue.
Types of Wood
Seasoned fir, red cedar and yellow pine are the most common types of wood used for producing the timber trusses. There are other different types of wood used all over the world in different regions.
Construction of Timber Trusses
Generally, there are two types of constructions of timber trusses. Timber trusses perform the function of transferring the load from the roof through the walls to the foundation of a building. Closed trusses constitute a tie beam forming a triangle with the rafter. The two rafters are the primary elements of a timber truss. The rafters are attached with a tie beam to make a triangle. There are other vertical and inclined members fixed in those triangles, which provide the strength and rigidness to the complete structure. Open beams generally have no tie beams. You can find the timber trusses in many barns, covered bridges, businesses and homes.
There are several methods employed for joining the elements of timber trusses. Many timber trusses are joined with mortise and tennon joints in joinery mills, while others make use of hardwood pegs fixed into tight holes. The trusses are also created using the gussets or steel plates and bolts to join the timber elements to form the structure. Some people use webbling or steel ties for even distribution of load on trusses.
Types of Timber Trusses
Most common designs of different timber trusses comprise king post, queen post, parallel chord/bridge and scissor. Every type of truss provides different load-bearing benefits and designs in addition to a rustic look through its distinct type of finish. The different finishing alternatives are rough sawn, smooth, hand-hewn or scalloped, wire brushed for an antique look.
1. King Post Truss
It has a vertical king post joining the middle of its tie beam on to the ridge of the rafters. These trusses normally use angle struts as extra members for joining the king post with the rafters. It is a closed type truss.
2. Queen Post Truss
The queen post truss mainly comprises of two vertical queen posts which are connected at equal distances from the middle of its tie beam to its rafters. A horizontal member further connects them at the heads. The queen type truss is more suitable for greater span widths as compared to the king post truss.
3. Pratt Truss
There are many vertical posts between its tie beams and rafters. The angle strut connects them in between in the direction of the rafters. One of the examples of a Pratt type truss is a mono pitch variation.
4. Howe Truss
Howe trusses are almost similar in construction to the Pratt trusses. They have beams fixed with the angles in just the opposite direction to their rafters. The most common use of Howe trusses was in bridge building where the metal as well as parallel type variants were widely used.
5. Scissor Truss
A scissor truss is almost similar in construction to a king post truss. The only difference is that its tie beam is substituted with two angled members, which join with the rafters. This provides an extra height in the middle of the room and you can make a pitched ceiling. These types of trusses require more support at the walls as they spread out in the absence of a tie beam. This beam has a natural tendency to spread out without a tie beam. A scissor beam is an open type of beam.
6. Hammer-Beam Truss
The hammer-beam truss has a stepped profile in the center by the horizontal members. The central tie beams are removed. The use of these beams provides a huge volume in the room. Because of the large space created, it adds to the size of the area. This type of beams is used in big buildings like churches and huge halls. These trusses are additionally supported by their external structures, generally called buttresses, which offset the lateral forces acting on the truss.