Thread: How to cut beams for roof pitch?

Exactly like it reads: what's the best way to cut the top of a wood beam to taper for a roof pitch? As opposed to birdsmouthing?

Are you talking about rafters or beams that support them?
We'll go with rafters. The top cut (at the peak) is related to the birdsmouth, that is the birdsmouth is laid out at 90 degrees to it. Roof framing is tough for me because I suck at math. But you can frame complex roofs using a good framing square (16" x 24").

You have to find the pitch, that is how many feet it rises in the run, a 4/12 pitch rises 1' every 4', 8/12 rises 8' every 12', and so on. So, you measure to the peak from the same level where the rafter bears on the wall, then you measure from the far wall where the rafters will sit (birdsmouth end).

Example, wall to the center of the peak is 20', top of the peak is 8', then you have a 4/12 pitch. To make the top cut hold the fat blade of the square on the rafter near the end. Then raise it so the small blade is at 4", this is your cut.

Hpoe this helps, good to have abook about it, but that the general idea.

I guess I should clarify that I want to taper the top side of the beams, so I don't have to birdsmouth the rafters to sit on them.

So how the heck do I cut a 10 degree angle in the top of the beam?

What is the size of the beams?

On my rafters I'm going to use a birdsmouth at the ridge beam and just using lags into the top plate at the other end. 6x10s are being used for the rafters (20' lengths).

If you cut birdsmouths on the rafter to fit onto the beam, the force is transferred straight down. If you cut the beam then the force is transfered sideways, unless you have some good way of attaching them.

I guess if you are cutting an angle on the beam you would snap a chalkline and cut to it. A whole lot easier said than done if the beams are round, you may have to make up a jig or something. How big are these beams, how long, are they round or square? What is the diameter?

The plumb cut at the top of the rafter where it meets the ridge board is the same angle as the plumb cut of the birdsmouth at the bottom where it meets the wall plate. Its plumb.

Originally Posted by gink595

What is the size of the beams?

8"x10"

Edit, good point about the load transfer of the birdsmouth.

I was hoping for something a bit more interesting, like tapering the beams for the 2x6" rafters. 9x27' firewood shed with a single plane roof.

I know the log home builders would just snap a chalk line and cut it with their saws. I may do this, but I'm not as well practiced as most of them. I might have to horse around with a jig, I'm just wondering if anybody's done anything like this.

8x10. Easy, snap a chalkline, set the angle on your skilsaw and go. If the cut isn't deep enough finish it off with a sharp handsaw. As long as you have a flat surface for the saws bed, it is pretty simple to do. Take your time, check and check measurements.

That'd be pretty easy. I may rip the 8x10's in half to 4x10's then laminate (bolt) them together to get more strength over the posts. Since pretty much everything I milled was only 10' long and I need to span ~29' in total with posts every 9', ripping them down would let me make a few passes with the circular saw to cut the tops off the beams.

8x10 is plenty strong, you can cut enough for the rafters to have good bearing easily. No need to cut them up.

Do you have a mini-mill? Just cut some wedges the angle you want on your miter saw, place them under the guide board and screw in down. When you run the mill down the board it will cut the angle that you set.

I guess this could work on an Alaskan style setup too if the angle is low enough.

Cutting the birds-mouth would probably be your best bet. As stated earlier, it transfers the load much better plus it will help hold your wall strait without as much frogging around. I like to figure the first rafter, and then use it as a template for making the rest. I think it would make a better structure IMO to use the birds-mouth.

Originally Posted by jimdad07

Cutting the birds-mouth would probably be your best bet. As stated earlier, it transfers the load much better plus it will help hold your wall strait without as much frogging around. I like to figure the first rafter, and then use it as a template for making the rest. I think it would make a better structure IMO to use the birds-mouth.

Perhaps you're right. What's the best way to mass produce birdsmouths then? measure, cut repeat? I found a neat little site for you to figure out your cut angles and depths, it's just that I'm not looking forward to making ~20 identical cuts.

Thanks for the mini-mill idea kicker, I'll save that one for next time. Should have just done that first.

Originally Posted by MR4WD

Perhaps you're right. What's the best way to mass produce birdsmouths then? measure, cut repeat? I found a neat little site for you to figure out your cut angles and depths, it's just that I'm not looking forward to making ~20 identical cuts.

Thanks for the mini-mill idea kicker, I'll save that one for next time. Should have just done that first.

Measure one carefully, lay it out with a framing square. Make sure it fits and then use it for a template. Make the two cuts with your skilsaw but finish them with a handsaw so you don't weaken the rafter. Good enough, but if you are doing it right you are supposed to do them one at a time, like stair stringers.