Roof trusses are fabricated from individual framing members so they behave as a single object. They are typically engineered, but can also be home built. Home built versions can work great for shops and barns or anything else that must clear span (no posts or beams). Just know that if your project involves building inspectors, they will frown upon home made trusses.
There are two distinct methods for making your own roof trusses. On one hand you can prefabricate the trusses on the ground and then lift them into position, or you can build them right in place. It takes considerable climbing skills and temporary walls to build the trusses in position, so we will focus on prefabricating them.
It takes a good flat work surface to build an accurate truss without too much fuss. A concrete slab floor in a garage or basement will work perfect. Make sure you can get the trusses out of the door. So what we will do on our concrete work surface is,
- Establish the bottom chord and get a line or a series of marks on our work surface.
- Calculate the top chords and mark their positions.
- Situate the webbing and mark their positions as well.
Red chalk is permanent. If you chalk lines on your work surface, then use blue or they will he there forever.
Lets start with the bottom chord. It is under constant tension. Tension is only overcome by strong fasteners. Truss manufacturers use gang nails. We will use gussets made from 3/4″ plywood and shot with 2 3/8″ ring shank nails. The chord’s length must match the width of the roof. The bottom chord will have breaks in it. The ideal way to position the breaks is to match them up with the webbing.
The top chord or chords are the single most important elements of a roof truss. They will serve as rafters. On a typical truss they will be made from 2 x 4’s. For trusses with longer spans, or when they are spaced very far apart for purlins, use 2 x 6’s.
Click here for our rafter calculator. You will need to know the roof pitch and the width of the building. Once its open,
- Use the number boxes in the first section, “Roof Width”, to enter the width of the roof.
- Then select a roof pitch.
- Finally, select no ridge for the ridge.
The length of the rafter generated by the calculator is from the top to the HAP. This is very important. The HAP is where the rafter lines up with the end of the bottom chord. Let’s do an example,
- Open up the rafter calculator.
- Open up the number box below feet and enter 29.
- Open up the number box below inches and enter 5.
- Open up the drop down below fraction and select 1/2″
- Select 6/12 for the roof pitch.
- Select no ridge.
This sets the building’s width at 29′ 5 1/2″. The rafter length generated is 16′ 5 5/8″ but, it does not include an overhang.
Go ahead and make a rafter pattern. Use a framing square to add the tail. Be sure to mark a good plumb cut line at the HAP.
You can now use the rafters to establish the cuts at each end of the bottom chord. Tack them together at the top. Line up the HAP’s with the ends of the bottom chords and mark the back slopes.
The webbing in each truss serves as braces. They can be positioned in several different manners. I personally don’t like to span over 7′ with a 2 x 4 on truss, so I base everything on that. I other words, I could build my truss with 7′ 2 x 4’s if that’s all I had.
For our example, we will use our division calculator to divide 29′ 5 1/2″ by 4. The calculator generates 7′ 4 1/4″. This is a little further than I like to span. I checked it by dividing by 5, but that complicates things. My trusses will not be supporting any ceiling load, so I elected to stay with the first.
To make a long story short, I divide up the bottom chord into 4 equal segments. Then I triangulate down from the bottom of the top chords to the nearest division on the bottom chord. I go up now to a spot that is one-third the length of the top chord, which is 5′ 5 5/8″.
You now have patterns for the top chords and the bottom. You should have good clear marks or lines on your concrete work surface and every framing member should be labeled clearly. Now you can saw all of the framing members, then put then together with 3/4″ plywood gussets on each side. The gussets should be large enough to accommodate 5 nails in each framing member without crowding them. Consider using more nails on the bottom chord. This is where the most load is on any home-made roof truss.