Learning how to build a shed roof often slows people down when it comes to shed building. It seems that the walls are easy because they are square and stand straight up but when it comes to sloping roof angles and rafters that hang over the edges of the walls or cutting the birds’ mouth on a rafter that many people get stumped. This article outlines the basic steps to building a shed roof with a 4/12 pitch and a peak in the middle to hopefully take some of the mystery out of shed roof building.
Plumb The Walls
The first step is to make sure that all the walls are plumb and square which means that they are standing straight up and down and that the corners are square. These two items are important because once the roof trusses or rafters are installed the walls will be permanently stuck in whatever position they are in.
- Put a level on the side of the shed and moving the walls until they are straight and then bracing them in place with pieces of scrap wood. Having crooked walls is more of a problem on larger sheds because there is more distance for things to get out of square.
- Check the shed walls for square by measuring diagonally from corner to corner. Push the corners in or out until the two dimensions are the same.
Design The Shed Roof Rafters
After squaring up the shed walls the next step in learning how to build a shed roof is to design the roof rafters. This is simpler than it sounds. If you do not have a drawing of the roof trusses in your shed plans then the easiest way to design a roof rafter is to draw it out full size on the floor and then cut out the rafter and fit it to the full size drawing on the floor. The most common roof slope for sheds is 4/12. What this means is that for every 4 inches that the roof goes vertically up into the air it goes 12 inches horizontally across the wall. A 4/12 slope is the minimum slope that is allowed by shingle manufactures so that they can guarantee their product. These are the steps to draw a 4/12 pitch roof rafter on the ground.
Draw The Shed Roof Rafters On The Ground
- Make a straight line on the ground as long as the width of the shed. We will call this This is the Width Of Shed Line.
- Measure the exact width of the shed on the top plates of the walls and put two marks on the Width Of Shed Line showing this distance.
- Measure in 4 inches from the outer marks on each end and put a second set of marks. We will call these the inner and outer marks. You will draw from the two inner marks at the 4/12 angle toward the center to designate the bottom of the shed roof rafters.
- Find the center point on the Width Of Shed Line between the two outer marks and mark it on the line.
- Draw a line from the center mark perpendicular to the Width Of Shed Line. Draw this perpendicular line a foot or two long. It may need to be extended later if you have a shed that is fairly wide.
Draw The 4/12 Pitch Line
The 4/12 pitch line will be the line and angle that the roof rafters will sit on. You will be drawing a triangle on the Width Of Shed Line.
- The simplest way to do this is to locate the two inner marks on the Width Of Shed Line and measure 1 foot towards the center.
- At the new 1 foot mark you will use the square to make a perpendicular line straight up 4 inches and put a mark at the 4 inch spot.
- Use a straight 2×4 to draw a line from the mark that is 4 inches in through the point that is 4 inches up and to the perpendicular Width Of Shed Line. Then repeat this process on the other end of the Width Of Shed Line. When the two angled lines meet they should intersect on the center line that you drew earlier. If all three lines do not intersect then you need to double check the measurements and correct your lines.
The two angled lines represent the bottom edge of the roof rafters and the great thing is that all the angles used to cut out your roof rafters are now drawn out on the ground. You simply need to transfer them to the first rafter and you are on your way to framing the shed roof.
Transfer The Rafter Lines From The Ground To A Rafter
Transfer the angles and lines onto a 2×4 piece of wood by putting a 2×4 that is long enough to extend past the marks on the Width Of Shed Line and the marks where the three lines intersect on the perpendicular line.
- Place the board on the upper side of the line, the side outside the triangle.
- Use your square to transfer the marks on the floor onto the board.
Cut Rafter Birds Mouth And Peak
The two ends of the board are cut differently.
- The top or peak end will be cut off so that the two rafters can butt up to each other where they meet in the middle. Use a circular saw to cut the board at the peak.
- Put the board back on the lines to make sure the peak lines up well with the vertical line and the bottom of the rafter line.
- The lower end will get a special cut called a birds mouth cut that will allow the bottom end of the rafter to sit on the shed wall and at the same time extend past the shed wall to form a overhang.
- Draw the birds mouth shape on the lower end of the rafter by tracing 4″ horizontally along the Width Of Shed Line toward the outer mark and then down and off the board.
- This shape will create a “seat” in the rafter that will sit on the shed wall. After you draw the shape on the lower end of the rafter you will take the circular saw and cut the shape out. You will need to use a hammer and chisel to finish off the cut.
- Place the rafter on the drawing on the floor and check the cuts against the lines. When you are happy with the shape place the newly cut out rafter on a second piece of 2×4 wood and trace the cut lines so you can make a second rafter. Cut the second rafter out and then place both rafters on the lines of the drawing and check for a good fit against each other and the lines on the ground.
Attach A Gusset
A gusset is a piece of wood that attaches the two rafters together. Use a Piece of Plywood or O.S. B. to make a gusset and attach the two rafters to each other at the peak.
- Make sure that both of the rafters are exactly on the lines on the ground.
- Cut out a gusset from a piece of O.S.B. The gusset should extend down the rafters from the peak about twice the width of the 2×4 rafter or about 7 inches.
- Attach the gusset to both of the rafters. Put three nails through the O.S.B. into each of the rafters.
Test the shed roof rafters by putting them up on the shed walls and verifying the birds mouth cuts.
If you are happy with the rafters then you are ready to use them as a template to build the rest of the shed roof rafters. Build enough shed roof rafters to have one set at each end and then every 24″ along the shed walls. When you have all the rafters built you are ready to start framing the shed roof.