All right, it’s time to face the most involved part of the project. No longer straight cuts, we are dealing with angles now!
Tools needed: framing square, framing nailer, measuring tape, miter saw, table saw, circular saw (optional)
Materials needed: OSB (oriented strand board), lumber, collated nails, Simpson hurricane ties.
My shed is 12×16, so the rafters span more than 10 feet. This means I have to build rafters with the bottom chord to straddle the walls. Now, we have a large triangle, and we need to figure out the angles, mark them on the pieces of lumber, cut precisely, so the pieces align with each other, and then nail together using gussets made out of OSB.
The plans already included everything I needed to cut accurately, but it’s a good idea to understand how rafter square works to mark proper angles and pitches. Typically you choose a pitch for the roof, and calculate everything off that using basic geometry (allowing for material thickness). Plans specified 10/12 roof pitch which means that for every foot (12 inches) of run (horizontal), rafter gains 10 inches in height. This translates to 40 degree angle. Again, no need to calculate, all marks are already set on the rafter square. People have been building roofs for ages now, and everything around you in construction is standardized. You need 10/12 pitch? It’s marked on the square, and 40 degree mark aligns with it.
10/12 pitch for common rafters equals 40 degree angle.
In general, construction is like coding: you use patterns and functions. Whenever you build a particular block, you use patterns previously shown to work well (framing pattern, roof pitches, sequence of steps) and functions to minimize the error: instead of measuring the same stud length every time with a possibility of being off on each one, you measure one stud carefully, cut it, and then mark everything off of that. This guarantees the same lengths, and even if you made a mistake on the first one, at least it’s uniform across the board.
Here’s another pattern: cutting gussets. You may be tempted to use miter saw to cut the angle, but then you need to draw the lines on each piece of OSB you want to cut, rotate the blade back and forth (or mark the template on both sides). Instead, a table saw is an excellent tool for this. Set the fence to the required distance, rotate the guide to the appropriate angle (remember, you rotate the piece, not the saw, so subtract your angle from 90 degrees and set the guide to the remainder) and go wild. Don’t have to think: full automation with your hands loading the pieces in and guiding them to the blade.
No pencil marks needed, just let ‘er rip.
When all the pieces are cut, assemble the triangle and test fit it on the floor to make sure both sides overhang as you expected (we want the roof to extend past the walls to drop off rain water). Here is where I ran into an issue: with a 10/12 pitch the rafters are enormously tall, over 5 feet in height. This will be awesome for when I’m building my royal Victorian palace, but right now the shed is going to stick out like a huge fuck you to all the neighbors. Let’s not aggravate the city inspectors and moderate the roof a bit.
Giant rafters propping up a giant tree
So now I am going to calculate the roof pitch in reverse. Municipal code states that shed structures should not exceed 12 feet at the apex, so my roof heigh should not exceed 3 feet or so. I could calculate all that and ruin a few pieces of lumber while test-fitting, but, of course, there’s StackOverflow-driven construction as well. This site is called blocklayer.com, and you simply plug in any known figure for the roof, and it calculates the rest. In this case I know the height (3 feet) and the run (6 feet) which comes out to the pitch of about 4/12. I adjusted this to an even 20 degree angle with 1 foot overhang, and got all my measurements ready for me:
Ready to build!
Now we have to disassemble constructed rafters. If you ever tried to pry out galvanized nails driven in by a nail gun, you know how much fun that is. But eventually we are back to square one: re-cut the pieces to the new angle and assemble them together. Test-fit and begin setting them up. Set up end rafters first, and then hang a tight piece of string between them to line up the rest of the rafters.
End rafters set up.
I toe-nailed the end rafters to the top plates and used Simpson Hurricane Ties to hold the rest of the rafters in place.
Rafters set up
The last piece is to install sub-fascia on both sides, but first we need to do sheathing, or sub-fascia will get in the way. So, with roof framing complete, we can move onto the next step: putting up wall sheathing.
