Cutting rafters one at a time works, but it's slow. With the right tools and layout tricks, I can gang-cut entire roof systems accurately and quickly.
Racks. To get the rafters' top edges in plane, I build low, stable racks (usually 14 feet long) on a fairly flat surface from 2x10 and brace the ends. Next, I rack the entire roof package, including commons, hip jacks, and valley jacks. Put one rack a foot in from the head cut; put the other under the seat cut. Use a third rack (if necessary) for short pieces.
Rack rafters tightly and crown down. Use a hammer to beat them together; protect exposed rafters with scrap. Nail blocks to both rack ends.
Laying Out Cuts
I figure my cuts at home the night before to prevent distractions or wasted time on site, which may cause an expensive mistake. On site, I lay out and mark head cuts on the two outside rafters then snap lines between them. Next, I determine rafter length, seat, and tail cuts from there.
Different Lengths. Use 12-inch spacer blocks to separate different rafter groups. This speeds the process because we can fill racks and cut more in each setup. This also allows us to make the same cuts on one end but different cuts on the others.
I make head cuts first, tail cuts second, and bird's mouths third. For all cuts, I use a 2-by fence to guide the saw. For heads and tails I use an adjustable table attachment on a gas-powered chainsaw. I cut with the top of the bar so the teeth plunge into the work, keeping the cut line intact and waste exiting down. My custom saw table is welded from aluminum and bolts through the saw bar (not onto it). Big Foot Tools makes a similar unit.
We make head cuts first so we can double check the angle and re-cut if necessary. We cut tails next, then bird's mouths. Note: Cutting the bird's mouths second removes stock where you'd otherwise nail the fence for tail cuts.
Bird's mouth. For pitches steeper than 6-12, I remove the bird's mouth with wormdrives. A standard 7-1/4-inch saw works for heel cuts. A Big Foot Tools swing table attachment with a 10-1/4-inch blade and 75-degree bevel (the seat cut for a 6-pitch roof is 63.5 degrees) makes the second cut.
Hip & Valley Jacks
There are different ways to gang-cut hip and valley jacks. How I do it depends on the combination of parts I need. I gang-cut hips and jacks as commons first, then cut a hip jack and valley jack out of that. While I make bevel cuts one at a time, taking the pieces out of commons allows me to gang-cut heads and tails.
Hip Jacks First. There are usually more hips than valleys. When I need extra hip jacks, I'll make tail cuts on both ends of rafter stock and cut two hip jacks out of each one. I make the bevel cuts one at a time with a wormdrive.
Valley Jacks Only. Sometimes a roof will have commons and valleys but no hips. In that case, I gang-cut heads on both rafter ends, getting two valley jacks out of each one. If you figure the overall length correctly and make the head cuts parallel to each other, you can get two valley jacks with only a single bevel cut and no waste in between. Another method is to use two bevel cuts to get the valley jacks and make the head cuts closer to the ends.
Keeping Track. It's easy to lose track of which hip and valley jacks have already been cut. To avoid this, I lay out all the bevel cuts with the boards still racked. Then I can look at the pile and see what pieces are there. For example, if there are two hips, there will be four of the shortest hip jacks, two lefts and two rights. I'll take these pieces out of the first four boards in the rack. Measuring up from the heel cut, I draw a line across the bottom edge of the board to indicate where the rafter will end and mark it with a slash indicating bevel direction. Laying jacks out in pairs means there will always be the correct number of lefts and rights. The next shortest set of jacks comes from the next four boards. I continue to lay out the jacks this way until the marks approach the midpoint of the boards. At that point, I reverse direction and start measuring off the other end of the group. This allows me to get two jacks out of each board with very little waste.
Cutting the upper end of hip jacks is simply a matter of rolling the boards down one at a time, marking the pitch on the side, and making the bevel cut. And, I never have to guess which way to make the bevel cut; it's already marked on the bottom edge of the board.
Expertise. The tools and techniques for gang-cutting a roof are important, but not as important as a good understanding of roof framing and how parts go together. Gang-cutting may seem complicated at first, but if you start with simple roofs and work your way up, it becomes second nature.
–John Harman is a roof cutter and framing contractor in Northumberland, Pa. This article was reprinted with permission from The Journal of Light Construction, a sister publication of Tools of the Trade. For subscription information, call 800-375-5981 or visit www.jlconline.com.