Build a Simple Wood Truss
Trusses are often used to build a structurally sound roof system in a variety of buildings. Often, in simple projects like small sheds, storage buildings, or other structures not governed by specific engineering requirements or building codes, it's possible to build your own trusses from ordinary lumber.
Steps
- Determine if homemade trusses are suitable for the project you're building. Since your truss will support the roof of your building, it should not violate local building codes and must be designed to meet wind load, snow load, and other structural requirements.
- Design your truss. You will need to know at least the basic following design elements for your project.
- Select the appropriate lumber for the truss you'll construct. High density softwoods like southern yellow pine or fir are preferred over lumbers like spruce, lodge pole pine, and white pine. Other things to look at when selecting individual boards include the following:
- Size of the lumber. 2X4 inch (5cm x 10cm) nominal boards are suitable for lightweight trusses if the bracing and member connections are well designed and fabricated.
- Quality of the lumber. Straight grained, dense boards with few knots are preferred for this project. Try to select lumber with no obvious splitting or barked edges.
- Straightness of lumber. Twisted, warped, or crooked boards do not lend themselves well for building trusses. Lumber with a slight curve, or crown may be used, as long as the crown is facing up when it is installed.
- Lumber should be kiln dried or seasoned, as green lumber may shrink and/or warp after the truss is assembled and installed.
- Set up sawhorses or another flat, stable work place to build your trusses. If possible, draw out a full size truss on a floor to help you establish the lengths of each member and the angles of the various cuts required. If you cannot do this, use sawhorses and set the bottom chord board on them to begin the process of marking pattern pieces.
- Place the bottom chord board with the crown up, then place the boards the top chords will be cut from over it, so each member laps where they will intersect when the truss is built. Scribe lines for the cuts at these overlapping places, cutting the bottom chord ends first if required.
- Mark the top cuts on the top chords where they will meet at the roof peak. The angle of this cut can be determined with a speed square if you know the roof pitch. Mark a common rafter pitch cut on each top chord, or use a string to make a mark from the center point of the bottom chord perpendicular to that member across the two top chords where they overlap each other.
- Determine whether the top chord or bottom chord will overhang the eave of your building, and then choose the appropriate bottom cut of the top chord. For a plumb eave line, you should be able to use the same angle you established to the top cut of the top chord. If you want the fascia board (if you anticipate installing one) to slope inward, use a sharper angle.
- Locate the position of the intermediate diagonal bracing members you'll need to complete the truss. There are specific engineering guidelines and even online resources you can use to determine load bearing and span applications for your project, but this article only applies to simple trusses where the general rule of thumb is one brace from the one third points of the bottom chords to the center of the top chords.
- Divide the length of the bottom chord by three, then measure the resulting distance across the chord, marking the two locations where these braces will intersect the chord.
- Mark the center point of each top chord, then place a board long enough to overlap each chord so the angles of the intersection can be scribed on each end.
- Use one of each unique member you have marked and cut as a pattern to mark subsequent members for each truss you plan to build. Be careful to make sure top and bottom chords are placed so their crowns are up when the trusses are built, and take care to be as precise as possible when marking and cutting each piece, as fitting them in the truss will be difficult if the lengths or angles are not correct.
- Determine if toe-nailing the members will create a strong enough truss for you project. Likely, you will want to use reinforcing plates, gang-nailers, or gussets for all the connecting points of the frame members. The illustrations included here show 16 gauge galvanized steel plates, cut from salvages scrap metal being used, but if weathering isn't a consideration, 1/2 inch (12.7mm) plywood plates can be used.
- Nail each member of the first truss together, supporting it at intersections with additional boards or posts to keep the assembly flat while it is laying on it's side. Use clamps to hold the fitted pieces tightly together while fastening them, and to work out any twists that are present in the lumber you're using.
- For metal or plywood reinforcing plates, use plenty of nails of a length that will not penetrate through the member you're fastening, or you will have to bend the protruding ends of these nails over before putting the plates on the opposite side of the truss.
- Use the first truss you assembled as a template to assemble the remaining trusses. Make sure all the joints are supported by temporary legs if you're working on a sawhorse or a table that is not large enough for the complete truss to set upon.
- Stack the completed trusses on dunnage so they lay as flat as possible, and in the best alignment you can achieve to prevent them from warping prior to installation. Placing the trusses stacked in this manner will also allow you to observe how well the trusses match each other. Keep in mind that unless you have perfectly sized and straight material, you can expect minor variations from one truss to the next.
- Install your trusses on the project you're building. Attach them to the top plate securely, and in accordance with applicable building codes.
Tips
- Take care to saw cuts accurately, and to square cuts so that the joints will fit tightly.
- Choose the best available lumber for your truss building project. Kiln-dried number 2 grade lumber of sufficient strength will help insure your project is successful.
Warnings
- Follow all local building code requirements, which should include factors such as wind loads, snow loads, or other environmental conditions.
- Safety glasses are suggested when using power saws and hammers.
- Wearing work gloves will protect your hands while handling the lumber.
Things You'll Need
- Lumber suitable for building a truss that meets your needs
- Nails or other fasteners of your choice
- Plywood or sheet metal for reinforcing plates
- Saw, hammer, square, and measuring tape
- Table, sawhorses, or other work surface
Related Articles
- Build a Shed
- Build a Saw Horse
- Repair a Pitched Roof
- Build a Brick Wall
- Repair a Tear in the Roof of a Metal Storage Shed
Sources and Citations
- Videos provided by Re/Max Associate Realtors: Pratt Kay
