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Gang-Cutting Rafters, continued

Birdsmouth. The birdsmouth is the last thing we cut before unracking the rafters. If the roof pitch is steeper than 6/12, I'll remove the triangular piece of waste material by making a pair of bevel cuts with worm-drive saws. I'll set up a standard saw for the heel cut and another one with an oversized blade and a swing cut table for the seat cut. The swing table is necessary because a conventional saw will not bevel more than 50 degrees, and seat cuts are often much steeper than that. For example, the seat cut for a 6-pitch roof is 63.5 degrees. The swing cut saw I use takes a 10 1/4-inch blade and will cut up to a 75-degree bevel.


A swing-table saw makes possible the steep angles required for birdsmouths.


The birdsmouths on 50 rafters are made with just two long cuts.

If the roof is 6/12 or less I use a dado saw to cut the birdsmouth in a single pass. I don't use it if the roof is steeper because the cut will be so deep that it would strain the saw to make it in a single pass.


Here, the author uses a custom-made dado saw to hog out birdsmouths in a single pass.

At one time you could buy a kit that allowed you to put dado blades on a worm-drive saw. No one has sold anything like this for years. I got a local machine shop to take a standard worm-drive and turn it into a dado saw. It's not worth doing unless you make your living cutting roofs. It's cheaper, safer, and only a little slower to make all the seat cuts using a swing table.

Rafter Cutting Tools

A standard circular saw will do for some ganged cuts, but you'll need specialized saws to make the others. The head and tail cuts are too deep to make with a circular blade. The only way to make them is with some kind of chainsaw. If you're going to production-cut the birdsmouths, you'll need a circular saw that bevels more than normal and has an oversized blade.

Cutting with a chain. At one time I used a Prazi Beam Cutter (Prazi USA; 800/262-0211;, a $140 chainsaw attachment that bolts on to a circular saw (see photo). I haven't used it, but a similar product is available from Muskegon Power Tool (Linear Link VCS-12; 800/635-5465; I rarely use a Beam Cutter anymore because the chain comes up through the work and splinters the layout line. These days I use a gas-powered chainsaw with a specialized shoe and beveling attachment. I cut with the top of the bar; that way, the line remains intact and the chips and dust go down instead of up in my face.


A Prazi Beam Cutter can be attached to a standard worm-drive saw. This one is being used to make tail cuts. The saw pulls the blade up through the work, which is why there are so many chips piled on top.

A friend of mine uses a $265 off-the-shelf attachment called the Headcutter (Big Foot Tools; 702/565-9954; I had a local welder custom-fabricate a similar device out of aluminum for me. The Headcutter bolts onto the bar of the chainsaw. The attachment I use is more stable because it bolts right through the bar of the saw.

Specialized circular saw. Another tool that's necessary for gang-cutting rafters is a swing-table saw. It consists of a standard worm-drive body equipped with an oversized blade and specialized base. The blade increases the depth of cut, and the base tilts well beyond the usual 45 or 50 degrees so you can make the steep bevel cuts required for birdsmouths.

Big Foot makes swing tables and swing-table saws. Their 75-degree 10 1/4-inch swing-table kit costs about $300 and fits Skil and Bosch bodies. You can buy a complete saw with swing table and guard for between $425 and $525, depending on which body you choose. Big Foot also makes a 14-inch model called the Big Boy, which comes with a 72-degree swing table and costs around $800. It's available with a Bosch body.

Pairis Products (760/868-0973; sells a swing table that can be added to an existing saw.

Some of my saws have been rewired to run on 220 volts. It gives us a little more cutting power because there's less voltage drop at the end of a long extension cord. — J.H.

Hip and 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. Most of the houses I frame have both hips and valleys. I gang-cut the fill as common rafters and then cut a hip jack and valley jack out of each one. The bevel cuts are made one at a time but taking the pieces out of commons allows me to gang-cut the heads and tails.

Hip jacks first. There are usually more hips than valleys, so it's necessary to cut more hip jacks than valley jacks. When I need extra hip jacks, I'll make tail cuts on both ends of some boards and cut two hip jacks out of each one. The birdsmouths are cut from above, so they have to face up on both ends of the boards. The bevel cuts are made one at a time with a worm-drive saw. It takes two cuts and there will be a small piece of waste from the middle of the board.


Hip and valley jacks are usually cut from commons. In this case, the author only needs hip jacks, so he has put tails on both ends and will get two hip jacks out of each board.

Valley jacks only. It's rare around here, but sometimes a roof will have commons and valleys but no hips. In that case, I will gang-cut heads on both ends of some boards and get 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. The other way to do it is to use two bevel cuts to get the valley jacks and make the head cuts closer to the ends.

Layout tips. It's easy to lose track of which hip and valley jacks have already been cut. To avoid this problem, I lay out all the bevel cuts while the boards are still in the rack. That way I can look at the pile and see if all the 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 to indicate which way the bevel runs. The jacks are laid out in pairs, so 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 only a small amount of waste in the middle.


Hip jacks are laid out in pairs, lefts and rights.


The three shortest pairs for two hips have been marked. The offcuts will be long enough to produce the next three pairs.

Cutting the upper end of hip jacks is simply a matter of rolling the boards down one at a time, marking the roof pitch on the side, and making the cut with a saw that has been set to the proper bevel. I never have to guess which way to make the bevel cut, because it's already marked on the bottom edge of the board. It's a very fast and efficient way to cut hip jacks.


The bevel is ready to be cut on this pair of hip jacks. The birdsmouths have already been cut on the other end.

The tools and techniques are important, but not as important as having 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 soon becomes second nature.

John Harman is a roof cutter and framing contractor in Northumberland, Penn.