How to build a roof

Roof carpentry: Lean-to's, Gables, Hips, Rafters, Barges, Fascias and Soffits

All the pages in this section take you through how to build a roof from start to finish. Most of the information on the web assumes the reader already knows a bit about roof carpentry.

Instead, I'm going to start with the very basics of roofing a house, shed or other structure before moving onto the much more complicated stuff. I'll use lots of detailed illustrations too so you should be able to understand it all even if you've no roofing experience whatsoever. If you already know a bit about roofing you can skip down through the easy stuff and find the roof carpentry info you need. There are links in the right column to help you find some of the areas covered as well.

Roofs are triangular in shape. Forming a triangle during roof construction is important because it gives the roof it's strength. Roofs also add strength to the walls of a building.

The diagram above shows the components that go into making up a small gable roof.

You may remember using trigonometry in school. I didn't enjoy it then and I still don't now! It's complicated and easy to make a mistake. Whilst the maths used to work out the lengths of roof members is trigonometry, you won't need to learn or do any trig work - I'll show you why later.

However, when building a roof it is important to understand that to work out rafter lengths you need a 90° right angle triangle. A lean to roof already forms a right angle triangle, like in the picture below;

Lean-to roof triangle

By knowing the span of the roof, and the pitch (the pitch is the angle at which the rafters rise) we can work out the length of the rafters.

Gable roof

If the roof is not a lean to but a normal gable roof we have to divide the roof/span by two in order to form two right angle triangles. A gable roof is basically two right angle triangles (or lean to roofs) up against each other back to back. Importantly, we halve the full span to arrive at the distance each rafter will span on either side of the roof.

The wall plates that the rafters sit on and are fixed to need to be perfectly square and level. To be square they must be parallel - the same distant apart at each end of the roof, and everywhere in between.

To check for square measure from corner to corner (they should be identical) or you can use the 3 4 5 method to make sure your wall plates have perfect 90° corners.

To level the wall plates I find the straightest joist, lay it from one wall plate to the other and put an 1800mm spirit level on it.

Adjustments may need to be made before you start the roof by measuring your span. The span is the measurement from the outside of one wall plate to the outside of the opposite wall plate.

Once your wall plates are square and level you're ready to calculate the common rafter lengths. Common rafters are the main timbers used to build a gable roof and are in almost every hipped roof

How to build a roof related pages