Quick question about birds mouth for rafters

[Brian W Evans]

I am building a very small lean-to style shed and was cutting rafters today. I cut all the birds mouths just fine through trial and error but I have a question about one dimension of the joint: I know how to determine the angle of the short side (roof pitch), and the length of the joint (sill width), and how to use a speed square to lay it out, but I don't know how to determine the correct length for the short side (see below).

birdsmouth.jpg

Is there a correct measurement here? Or is it more important that they're all the same. I know it shouldn't be more than 1/3 the width of the rafter material.

Thanks.

[Ken Combs]

I am building a very small lean-to style shed and was cutting rafters today. I cut all the birds mouths just fine through trial and error but I have a question about one dimension of the joint: I know how to determine the angle of the short side (roof pitch), and the length of the joint (sill width), and how to use a speed square to lay it out, but I don't know how to determine the correct length for the short side (see below).

birdsmouth.jpg

Is there a correct measurement here? Or is it more important that they're all the same. I know it shouldn't be more than 1/3 the width of the rafter material.

Thanks.

Unless I missed something in your description/question, the angle and seat width will determine the depth. if you change one of the first two, then and only then will the depth change.

[Jack Dickey]

the depth should be one third the width of the rafter , most cut the depth to 1.5" or two inches on a two by four , the length from the top of the cut to the edge of the rafter is the width of the rafter or 3.5" ..

[Bill Dufour]

the depth should be one third the width of the rafter , most cut the depth to 1.5" or two inches on a two by four , the length from the top of the cut to the edge of the rafter is the width of the rafter or 3.5" ..

Not for a 2x6 wall. It also depends on the width of the rafter and the desired width of the tail. I have seen many 2x6 rafters cut so the part outside looks like a 2x4. I have also seen them notched so their is still a 2x6 outside.

[Brian W Evans]

Unless I missed something in your description/question, the angle and seat width will determine the depth. if you change one of the first two, then and only then will the depth change.

I don't think this is true. If I draw the short side - let's say it's 1" long - I can then draw a 3.5" line from any point on the short side to the edge of the rafter. This does change the angle, but I have not been able to find an explanation of how long to make that short side to make the angle correct.

[Larry Edgerton]

The plumb cut is always square to the seat cut. Your layout looks good for the pitch you are using. Cut away.

[Zachary Hoyt]

I have never measured the angle of the birds mouth, I just figure out the first one manually using a framing square and then cut the other rafters to match the first one, once I've made sure it fits.
Zach

[Robert Engel]

What you've got looks fine. You're just trying to get a flat seat to match the top plate.

I go 3/4 -1" deep on the short side for a 4 or 5/12 pitch roof.

You have to consider the roof pitch too. For example on an 8/12 pitch vs. 5/12, you're cutting much deeper into the rafter. So you need to go with a 2x6 on a steeper pitch.

[Tim Bueler]

Unless I missed something in your description/question, the angle and seat width will determine the depth. if you change one of the first two, then and only then will the depth change.

Yes, this is correct. Both cuts of the birdsmouth are always perpendicular to each other. This is easy and evident if you use a framing square instead of a speed square. Say for example your roof pitch is 6/12. Put your framing square on the work piece, with you standing "inside" the L. Put one leg on 6" and the other leg on 12". The 6" leg will be the correct angle for the plumb cut, the 12" leg will be the correct angle for the level cut.

Level cut should never exceed the width of the wall plate. Plumb cut should never exceed 1/3 the rafter width. Doesn't matter if you're using 2x4 or 2x12. Rafter tails can be configured in any number of ways (ask 5 carpenters, you'll get 10 different answers) and can depend on what look you're going for and whether or not the soffits are enclosed.

[Bill Dufour]

It is just simple trig depending on the roof slope. It is a right triangle so the angle is the slope and the depth is the opposite length. The adjacent side is the width of the sill.
So tangent of the slope is opposite/adjacent. You know tangent and adjacent. So tangent x adjacent = opposite=depth of cut
You could aslo calculate the hypotenuses and use that length to layout the cut once you have done the trig

[Bill Dufour]

And many folks cut the rafter tails vertical on the end which meas they are taller then most fascia trim which is often 4" material. Others cut them at 90 so the trim board just fits, but no overhang. This also reduces the gutter capacity since it will overflow from the front edge sooner. But it does make it easier to use the metal drip edge which is bent at 90 degrees. You do not have to hand bend it to beyond 90 for each section.

[Mark Wooden]

+1 with Larry and Tim
And while speed squares are handy, best tool to layout roofs is a good "framing" square with cut tables tables on it, although I don't know that they're even available anymore.

[Tom M King]

Framing squares are still available. It's all I've ever used for the many roofs I've built. I don't even own a speed square. The same squares are used for laying out stair stringers, as well as rafters. edited to add: Even a Really good one https://chappellsquare.com/?gclid=Cj...4aAvHpEALw_wcB

For the drip edge issue: I always let the roof sheathing overhang the fascia a little, not only to solve the drip edge square shape question, but I allow a little extra so water actually does drip off it, rather than run down the fascia. I don't have any rules to follow. I always figure it out after cutting the rafter tails.

If you need drip edges to a specific angle, and even color, places that make metal roofing panels fold it to your specifications, with many colors to choose from. It costs some more, of course, than standard drip edge, but makes a much nicer looking job. I'm fortunate to have a roofing metal fabricator right down the street from the other building suppliers, so the cost difference is negligible.

[Brian W Evans]

The plumb cut is always square to the seat cut. Your layout looks good for the pitch you are using. Cut away.

Thanks. I suspected this but couldn't find it anywhere.

I can see, as others have said, that a framing square would be much better than a speed square for this. Next time, I'll do that.

Thanks again.

[Mark Wooden]

Thanks for the link Tom; I'm glad to see that someone is still making the second most valuable tool a carpenter can have. I don't own a speed square either, but have two stainless steel Stanley squares with full tables and scales I bought over thirty years ago. I've worn out probably half a dozen aluminum squares as I rarely take the ss tools to the job.
One thing I think I like about the Chappel square is they have updated the tables for more modern methods. I'd like to see one in person sometime. Most likely won't get one but it would be nice to see