Framing the Roof for a Shed Style Dog House with Air Conditioning
The slope of the roof is appropriate for the desert of Southern Nevada. If you live in an area that gets more rain and or snow, you will probably want a steeper roof. The rafters, like the wall studs, are spaced 16 inches apart.
All of the rafters were assembled on the ground into seven assemblies. Five of them were assembled into a roof frame module that was then lifted and set into place resting on the top plates of the side walls, and attached there with exterior screws. Then the two remaining rafter assemblies were attached with exterior screws to the top plates of the front and back walls. See photos below.
There is no ridge board. The 5/8 inch T 111 siding that will be installed on the outside of the rafters will provide plenty of rigidity to the finished roof structure. Gusset plates were cut from plywood to strengthen the joints connecting each pair of rafters.
It was decided to do most of the roof framing on the ground rather than constantly climbing up and down a ladder. Most of the roof was formed in a roof frame module that could be lifted into place upon completion.
While the roof frame module was being assembled, the two horizontal boards to which the rafters were attached were temporarily attached to another pair of boards cut to be the exact same length as the top plates of the front and back wall frames. When it was time to attach the roof frame module to the wall frame, the screws attaching these two boards were backed out enough for them to fall off, and then the module was placed into position and the four screws driven into the top wall plates. Then more screws were driven in between the other rafters to connect what was the upper top plate board to the lower top plate board.
The roof angle is 22.5 degrees, which is the same as a 5/12 pitch. The cosine of 22.5 degrees is 0.924, rounded to three digits. The back wall is 48 inches wide, at which point each rafter will cover 24 inches, or half the width of the wall. But the rafter will be angled at 22.5 degrees, and will form the hypotenuse of a right triangle. The length will be 24 inches divided by 0.924, or 25.974 inches long. Because my tape measure indicates fractional inches in sixteenths rather than in decimals, I cut each rafter for the back wall to be 25 15/16 inches long. Each cut was made using a miter saw set at a 22 1/2 degree angle.
Another cut was made on the outside end of each rafter, also at a 22 1/2 degree angle, to allow a 2 1/2 inch wide surface for the end of the rafter to set evenly on the top plate of the wall. The nominal width of the top plate is 3 inches, but the actual width is 2 1/2 inches. See the photos. You can also see the website referenced in the "Other informative websites below. It has a very good explanation and diagrams that I used in fabricating the rafters. The explanation provided there so excellent that I do not want to waste my time trying to improve upon it.
In theory, both the front and back walls should be 48" wide, but the front wall turned out to be about half an inch wider than the back wall. Because of this, I cut two pairs of rafters to be 25 15/16 inches long, two pairs to be 26 1/8 inch long, two pairs at 26 1/4 inches, and one pair at 26 3/8 inches.
Photos showing construction of rafter assembliesClick thumbnail to view full-size
Construction of the rafter assemblies
Please refer to the photos above and to the website referenced below in the "Other informative websites for shed roof construction" section. Each of the rafter assemblies was built by first connecting the two rafters at the joint that will form the roof peak, and then cutting out and attaching one or two gusset plates to reinforce that joint.
When completed, five of the rafter assemblies were attached to the upper top plates of the roof frame module. The other two were attached to the top plates of the front and back walls after the roof frame module was slid into position and screwed into place. I think I used nails to attach the rafters to the roof frame module, but used exterior screws to attach the front and back rafters.
Photos showing installation of roof frame moduleClick thumbnail to view full-size
Installation of roof frame module
Once the roof frame module was assembled, the boards the assembly was resting upon to keep the top plates properly spaced were removed and the assembly was slid over the top plate of the front wall frame. It was very carefully slid back until it slipped into place with each of the upper top plate boards resting between the top plates of the front and back walls and on on top of the lower boards of the top plates that were framed with the side walls. Screws were used to connect the assemblies as described above in the "Approach to construction" section.