How to Build Wooden Roof Trusses
You may refer to a truss as the rafter, but it's basically the skeleton of the roof, carrying the weight of the frame and supporting the walls of the building. Trusses are very important to preventing the walls from bending or flexing, more so important for buildings and homes with more than two stories.
The building trusses are usually connected together with galvanized metal plates and connected together in a triangle shape. Most of the time, for smaller buildings and houses, trusses are made of timber because it's reliable and easy to work with, but it's not uncommon for trusses to be made of aluminum, steel, or another material. Most of the time what material the trusses will be made of will depend on the budget of the project.
If you're making your own truss, you need to make sure that you lay out everything before you get started. You want to make sure that you have all the right tools, materials, and equipment, and you need to make sure that you know what you're doing.
If you have any doubt, it may be safer for your to higher a contractor to make and set up the trusses.
There are a few different methods of building trusses, below you'll find basic instructions to build a hip roof truss, which is going to be your average building truss. In order to get started, make sure that you have the following things:
- 2x6 or 2x8 timber
- 2x4 timber
- Galvanized steel connector plates (gusset plates)
- Screws (at least 1-1/2 inch)
- 3 inch deck screws
Choosing the Timber for the Trusses
- The thickness of the timber that you want to use will depend on how much weight the truss will support. Generally, 2x6 or 2x8 size timber is used, but for smaller home additions, sheds, and buildings, 2x4s may be sufficient.
- Purchase the highest quality timber that you can for the top chords (two pieces of timber that determine the roof's pitch) and the outer segments of the bottom cords (bottom pieces of the truss that form the bottom of the triangle) of the trusses. These will support the most weight, causing the most stress on the wood.
- Check to make sure that the lumber is straight when you look from one end to the other. If the lumber is curved, you want to make sure to put the curve facing downward, so that when the weight is put on the truss, the curve is flattened out. You don't want to have the bottom of the curve on the top, as it will just bend further with the weight.
Measuring, Cutting, and Building
- Measure and cut the timber that you'll use for the triangle portion of the truss to the appropriate size of the roof that meets the proper building codes for one truss.
- Once all of the timber has been cut, fit the pieced together to make sure that the dimensions butt tight together. If the pieces fit properly, use this truss as a pattern for the rest of the trusses required for the building.
- Use galvanized steel connector plates (gusset plate) to secure the timber in a triangular shape with screws that are at least 1-1/2 inch long.
- Add a 2x4 brace to dissect the center of the triangle to add more support at the 90 degree angle from the bottom of the center of the angle to the top of the truss. Depending on the design of the truss, the length and angles of the braces will vary.
- Secure the truss to the frame and leave an appropriate overhang. The overhang will depend on the size of the building or house and the pitch of the roof.
- Attach the truss to the frame every 2 feet, securing them with 3 inch deck screws. Brace the trusses in place and check for plump prior to securing the frame.
- Sheathing added on top of the trusses will provide final support to the roof trusses.
Building Trusses to Code
When building trusses, you need to make sure that you comply with any architectural specifications. Make sure that your lumber is according to code. For the most part, you want to stick with timber that contains between 7% to 19% moisture and is fire retardant.
You want to check building codes to make sure that the:
- The lengths and measurements for the truss are accurate
- The pitch and angles are appropriate
- The interior braces are screwed into the correct parts of the triangle and at accurate angles
- The size timber you're using is recommended for the size building or addition
- The truss design is adequate for the roof design in accordance to the National Design Specification for Wood Construction, the National Design Standard for Metal Plate Connected Wood Truss Construction, and the Code of Jurisdiction.
Installing a Truss
You have to install a truss manually, either by a forklift or crane depending on the size of the truss and height of the building. The truss should be carried and moved vertically in order to reduce stress on the joints.
Trusses that are installed manually should be slid into place over the sidewall and rotated into place using poles. The longer the span, the more people that you'll need in order to reduce strain on the truss. Make sure that the trusses are supported at the joints and the peak while being raised and moved.
As mentioned above, you want to use one truss for every two feet, unless otherwise specified.