How to Properly Cut Gambrel Roof Angles

When constructing a small building such as a shed or a barn, a gambrel roof design is very popular as it makes the most efficient use of the interior space of the building to maximize storage. Below you will be presented with a list of tools that you’ll need to properly build this type of roof followed by the steps required to make the required angled cuts. It’s important to note that the three angles involved if added together must equal 180 degrees.

Step 1 - Determine the Rafter Lengths Required

For this step, you will use the mathematical formula for solving lengths and angles in a right triangle. This formula is called the Pythagorean Theory and it states that a-squared plus b-squared equals c-squared, with a and b being the sides of the triangle and c is the hypotenuse or the longest side of the triangle, which for this project, will be the rafter joists. Use the formula to determine rafter lengths and then mark your rafter joists for cutting.

Step 2 - Make the Required Cuts with the Mitre Box: Wall Ends

The miter box will be your friend for this type of project as it will hold the wood for you and even specify where and how to make your cuts. Since the sum of the three angles in a gambrel roof is 180 degrees, the angles you will need are as follows: 45 degrees where the joists meet up with the top plate, or header and 90 where the joists meet in the middle at the ridge board. You will need to make 22.5-degree angle cuts on the rafter joist ends that mate up to the top plate or header. Remember to double the number of boards required for one side of the roof in order to have enough rafter joists to complete the project. When these rafter joists are nailed to the wall headers, they will form a 45-degree angle.

Step 3 - Make the Center Line Cuts

Once you have made the required cuts on the joists where they will mate up with the outer walls/top plate, you will need to make the cuts on the ends that will mate up to the centerboard or ridgeline. The total angle between the two rafter joists needs to be ninety degrees. Since you’re nailing two boards with angle cuts together, the angle cuts are additive. This means that you will need to cut the rafter joists at a forty-degree angle where they will be nailed up to the ridgeline. Thus, when they are assembled, the two forty-five degree angles will add up to ninety degrees.