Beneath every roof is a structural system to support its weight, plus the force of all the snow, wind, and people that might stand atop it. Here are four support possibilities, along with lots of reasons to use them (or not).

1. Simple rafters. Dimensional lumber is easy to buy, cut, and erect. It might not be the cheapest way to span a given distance, and it’s not even an option for long spans: 2×12 is the maximum depth you can get, and anything longer than 16 feet requires a special order. But the convenience makes simple rafters a great choice when you need to build it NOW.

Liam’s house.

2. Built-up rafters. This category includes more sophisticated types of lumber, like glulam and LVLs. Another example is an open-web beam, which looks like a cross between an I-beam and a truss. This stuff is more expensive than dimensional lumber, and might require a longer lead time. But you can get a longer span with built-up rafters, as well as greater structural depth for placing insulation if that’s where you choose to insulate.

Triforce built-up rafters. (from https://www2.buildinggreen.com)

3. Trusses. Prefabricated and delivered in one piece, trusses are usually the most economical way to span a distance. They save you the trouble of cutting a bird’s mouth and a precise peak angle, assuming you order them correctly and the factory gets the order right. And they’re easy to work with, although hoisting them into place might require some mechanical help. The main disadvantage of trusses is that they chew into your headroom, making the attic strictly a storage space.

Trusses make the attic uninhabitable… no harm done if you planned to insulate at floor level.

4. Sister beams. If you want to retrofit a roof without demolishing the existing structure, you can make the existing structure stronger instead. Cut dimensional lumber as you would for simple rafters and nail them to one or both sides of the rafters you already have. The advantage here is you get to keep your old rafters, and the sister beams can be shallow (say, 2×8 rather than 2×12) since they don’t do all the work themselves.

Bob’s roof: old timbers, new sisters.