Paying someone to build a shed roof can be expensive, especially since it is a project you can DIY. Roof framing may look complicated but is based on fairly simple concepts. There are many uses for a garden shed, which include; storage for tools and garden implements, a workshop, a home office, a family room, and many more.
Step 1 - Determine the Dimensions
Before starting construction of the roof frame, some measurements need to be taken. Measure the span—the distance from the outside of one wall to the outside of the other across the shed. Then measure the rise, which is the distance from the center of the span to the top of the roof.
The line length is measured from the center of the roof to the outside of the supporting wall. The rafters are constructed looking like 2 triangles back to back with the rise and half the span making up the right angle of the triangle and the line length making up the hypotenuse with the other triangle a mirror image of the first.
To determine the pitch, which is the angle of slope of the roof, there are 2 common factors used: 1/4 or 1/3, this relates to a 4-inch rise over 12 inches or a 3-inch rise over 12 inches. Any pitch may be used over 12 inches but determining factors are snow bearing or other weather factors or local authority regulations.
Step 2 - Cut the Timber
Making the rafters requires 3 basic cuts, the top cut where the rafter butts against the ridge plate; the tail cut which is the outside edge of the overhang for the eaves; and the bird’s mouth which is the notch that enables the rafter to stand on top of the supporting walls. If the pitch has been determined to be 1/4, then using the square, the correct angles may be drawn on the timber by aligning the 4-inch mark on the short outside of the square with the edge of the timber and aligning the 12-inch mark on the long outside of the square on the same edge of the timber, then marking the cut with the pencil.
Check your measurements at every stage. Use the old carpenters’ maxim of “measure twice, cut once.” Cut the rafter pieces and hoist into place to check the fit. If the rafter is correct, use the first one as a template to cut the rest.
Step 3 - Erect the Frame
The simplest way to erect the rafters is to make 2 braces out of scrap timber. These braces should be the same height as the walls plus the rise. They should be constructed from 2 pieces of timber kept apart by a spacer forming a "U" or saddle at the top into which the ridge board is placed. This holds the ridge board while the sloping side of the rafter is being nailed into position.
After all of the rafters have been nailed to the ridge board, the braces may be removed and the rafters will support the ridge board. Add any collar ties if necessary to prevent the rafters from spreading when the roofing is applied. Once the rafters and ridge board have been assembled, the roof covering may be fitted after which the flashings can be nailed on to the finished roof.