how do I cut a birdsmouth in a rafter in the right place?

Question

i am building a summerhouse and at the stage of putting the roof on

Answer 1

Most importantly you have to know the pitch of the roof before hand, then with that info and the measurement from the peak to an overhang you can determine the proper angles, then make a sample cut on a piece of stock and test your measuring and mathmatical skills.

Rev. Steven

Answer 2

You need the pitch (Angle) of the roof, and the Going of the roof (Half the Span).
Alternatively the Rise of the roof and the Going.

Make a small scale drawing and project the lines as far as possible on the page. This gives you the Plumb and Seat cuts for the Birdsmouth. You can set your sliding bevel to these and transfer them to the rafter.

For the lengh, use Pythagoras Theorem (Not complicated)
Multiply the rise by itself and note the answer.
Multiply the Going by itself and add that to your first answer.
Enter this into your calculator (There is one on your computer) and press the square root function. This is the true length of the rafter.
Remember to subtract half the thickness of the ridgeplate for accuracy.
Make a pitch line on your rafter one third the rafters width(Not thickness) up from the bottom of the rafter. Make the Plumb cut on the rafter and measure down the length you calculated earlier.
The mark should be made on the pitch line ie one third up the rafters width.

This is the point of the Birdsmouth.
Use the Plumb and seat cutts on the sliding bevel to mark this out.

Hope this helps, but without a drawing it is difficult to describe.

Answer 3

offer the rafter up to the wall plate working down from the ridge and mark the vertical cut on the rafter at the bottom then mark it horizontally but do not cut too much out, work on this formulae. if the rafter is 100mm deep leave a minimum of 70 mm of rafter thickness after the birds mouth cut has been taken out, otherwise you will weaken the rafter too much. hope this helps

Answer 4

You need to know the angle of the rafter. Place the rafer on top of the wall plate and mark the inside edge on the rafter. Draw a line at the known angle to the depth of the wall plate and complete the triangle. Measure twice, cut once!

Answer 5

Since it don't cost but 30 bucks more and 2 x 4s are less than code minimum for that span, why not step up to a 2 x 6 that would have enough width to cut a birdsmouth?

Answer 6

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Answer 7

That's a hard one to explain without a picture. I suggest you go to home depot and get you a Swanson speed square or a framing square ,these will come with a book that has diagrams to show you how to cut a roof system.

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Answer 9

get your apex right first, then get your wall plate right, then offer up the wood and mark it by hand.

Not many are cut with mm tolerance, just get as much load bearing onto the wallplate as you can