Cutting Rafters


Old 11-03-03, 11:19 AM
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Cutting Rafters

Can you tell me how to cut the angle of a rafter using a carpenters square and also the "birds beak". I am trying to cut a 4X12 pitch. Thank you in advance for your help
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Old 11-03-03, 03:47 PM
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These should help, especially the first one. Hard to explain this even though I have used it many many times but a good visual is always the best.

Hope this helps!
Old 11-03-03, 06:34 PM
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Don't remember -- I haven't been able to build my own joist and rafter systems for 30 years!! Gotta buy trusses. (One of the best things that ever happened around here -- they are so much cheaper, faster and stronger than the old joists and rafters!!!)
Old 11-05-03, 07:11 PM
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Use either the inside or outside scale....don't use one of inside and one of outside....

For you 4/12 pitch.......look at the 4 on the inside of the 16" leg of the square.........then look on the inside of the 24" leg ( the long leg) at the number 12".........One the SAME edge of the 2x6 or whatever you're making the rafter of, let the 4 and the 12 meet the edge....the SAME edge........the long leg makes about a 30 degree angle with the wood.....the short leg makes about a 60 degree angle. Strike a pencil mark on the short leg.....either inside or out, doesn't matter, parallel lines.

This mark is your "plumb" cut....the cut that will meet the ridge board OR have the facia ( gutter ) board nailed to it on the tail of the rafter. The plumb cuts are parallel cuts on each end of the rafter that are true verticle lines when the rafter is positioned.

To cut the birds mouth, you have to know HOW to calculate the rafter length.....IF you don't understand that, post back and I'll give ya a lesson in rafter math.

But assuming you KNOW where to position it, again, I mark a plumb cut line that would be the outside of the building wall ( with sheathing ) you have 3 plumb cut lines on your board......the ridge cut, the outside wall and the tail of your overhang ( assuming you have overhang.....otherwise the wall is the end of your rafter).

Then I flip the square around holding the outside corner and one leg against the plumb cut line on the wall outside. Move the square up this line until the perpendicular measurement is what your outside wall thickness 3 1/2 + sheathing......say 4" with 1/2" sheathing......Mark from the plumb cut line perpendicular 4" to the edge of the rafter. That little triangle is your "birds mouth" cut.......You will be dropping the rafter that 2 or so inches along the plumb cut when you take this out.

I make two rafters, check them with a pc of ridge board thickness material first.....assuming all is well, I then use one for a "pattern" and mark all subsequent rafters from it.

Remember to "crown" you lumber......look along the edge and turn any bowed lumber with the bow UP as you mark them so you don't end up with dips in your roof.
Old 11-05-03, 08:21 PM
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Or just order the trusses and put them on the top plates of the walls when they get there!! ;-)

Andy, what's you doing here?? (It's OK -- the more the merrier!! Just figured that between the doors and the windows, ... guess I figured wrong!)
Old 11-06-03, 04:52 AM
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What am I doing here ? NOW that's a real why do they have an "interstate" highway in Hawii ? ahahahahaha

Nah, trusses sure have their place.....easy, convenient, give you clear span inside the bearing walls.......lots of advantages.....but rafters have their place also. If you just want to add a simple gable porch roof for instance, cheaper and quicker to just cut out some rafters and be done.......or a cape cod style story and half house with living space upstairs ( my personal favorite house style.....lot of living space for the money and look good )......

Both have their place.......and cutting rafters is becoming a lost art.....just like a good set of stairs.

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