There are two basic types of roof construction (structurally speaking), rafters and trusses. Rafters are the “old style” of construction, while trusses are the more modern method. The main reasons why trusses have replaced rafters are:
- Trusses are stronger
- Trusses can be custom ordered to the building’s specifications
- Because they are pre-fabricated, trusses require less time to install
- Trusses are typically cheaper, because smaller dimensional material can be used
Trusses are an “engineered wood product,” but not in the same sense that plywood and OSB are. It isn’t the material itself that is engineered, but the configuration of that material, which makes it into a structurally strong assembly.
Before You Begin:
What makes trusses strong is the fact that they are made of triangles. Structurally speaking, triangles are about as strong as you can get. By comparison, a square is extremely weak, and won’t stand up under its own weight. That’s why corner bracing is necessary for the framing of a house.
You can see this natural strength in the triangle by taking some 2x4s and making two shapes, a square and a triangle, both nailed together with 16 penny nails (we’re just talking 2 dimensional shapes here, but if you do it in 3 dimensions, the difference is even more pronounced). If you push against the triangle, along the plane of the shape, it won’t budge; but if you try the same thing with the square, it will collapse.
While most builders order their trusses pre-fabricated from a truss manufacturer, through their building center, you can make your own. The architect who did the drawings for your home probably included a truss plan as part of the roofing plan, showing how many of what truss design are required. This won’t usually provide any details, other than the overall dimensions and layout of the truss. This is enough information for you to build your truss.
When a truss manufacturer builds trusses, they put the truss together with metal plates. These connector plates have built in “nails” that are pressed into the wood with a huge hydraulic press, firmly connecting the pieces of wood together. For your homemade trusses, you can accomplish the same thing with 1/2” thick CDX plywood and 6d nails.
How to Build it:
One requirement for trusses to be effective is for them to be consistent. So, the first thing you want to do is to make a pattern for your trusses. Find a large enough area to work, such as the garage floor or the driveway. You can either mark your pattern directly on the cement or place pieces of OSB, butted together on the floor, to draw your pattern on. This OSB can later be sued as your roof sheathing.
The “top chord” and “bottom chord” sizes are important, as they affect the overall strength of the truss. You might end up using either 2 x 4s or 2 x 6s for these parts of your trusses, depending upon the span, roof pitch and span that you need. Your architect should be able to provide this information.
The exact locations of each stringer aren’t as important as having the overall pattern correct. A few inches, or even a couple of feet difference, won’t significantly affect the strength of your truss. However, the roof pitch is extremely important, and has probably been decided on by the architect for the weather where you live (areas with a lot of snow need steeply pitched roofs). Another thing that the roof pitch affects is the ability to use your attic space for storage.
You’re going to have to be pretty good at cutting exact angles and dimensions to be able to make your trusses. With your pattern laid out, you can measure the exact length you need each piece to be, and the angle of the cut at both ends. You want your truss stringers to butt up against each other well, to insure that the weight is properly transferred from one piece to another. Poor cutting and inaccurate angles will weaken your trusses.
If you have a radial arm saw or a large enough power miter saw, that’s the best way to cut your truss stringers, because you can be assured of consistent angles. Once you’ve made one truss, to figure out your angles and make sure you’ve got everything working right, you might want to cut all the material for all your trusses, marking each piece, and then put them all together. This will save you time on having to constantly readjust your saw’s angle.
The pieces of plywood that you are going to use for your connectors don’t have to be cut accurately. You’ll want at least 6” of plywood overlapping each piece of wood that you are connecting together, so cut a bunch of pieces that will be at least that big. As much as possible, make your pieces universal, so that you can use them in a number of locations.
To assemble your truss, lay out all the pieces, and make sure they are butted together solidly. Lay the plywood joiners over the joints and nail them to the stringers, with a minimum of 6 – 6d nails in each of the pieces of dimensional lumber that is being connected together. You want to be sure that your plywood connector pieces don’t overlap to the outside of the truss (okay on the inside). If they do, cut off the excess with a saw.
Once one side of the truss is totally connected together, it can be flipped over, and the plywood connectors attached on the other side. Be sure to not miss any of the joints, and nail with the same minimum number of nails that you used on the first side.
What You Need to Know About the Building Code:
The building code is very specific about the dimensions of lumber required for roof construction, depending upon the snow load in your geographic area. As roof pitch and spacing also affects the strength of your rafters, we are unable to provide actual tables or dimensions on this site.
Tricks & Tips:
If you’re going to have any cathedral or inset ceilings on the upper floor of your home, be sure to frame them in as part of your truss design. This may require that you have several individual truss designs, but it will save you lots of headaches later on. In this case, to avoid mixing up the trusses, number each one, starting from one end of the house.
You’ll probably want a buddy to help you build your trusses. If nothing else, having someone to help you flip them over and move them is a great help.
To prevent the chords and stringers of your truss from moving while you’re working on it, attach some scrap blocks of dimensional lumber to your pattern, on the outer side of the truss, to hold those pieces in place. They can later be removed to use the pattern material as roof sheathing. You can do the same thing to scab pieces of the pattern together and keep them from separating.
If possible, cut your plywood joiners from scrap material, left over from subflooring and sheathing. This is a great way of using up your scrap, while saving money on not having to buy as much material. Don’t use OSB for these joiners, as it won’t be as strong.