# Carpenter/joiner help on how to find a angle

Discussion in 'Trade Talk' started by sammpennington, 17 Oct 2019.

1. ### sammpennington

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I am wanting to build something very similar to this out of 18mm ply I am wanting to know how to find the angles to cut the ply for the roof top pitch and also where the roof will meet the side walls the back will be a piece I put in after and just pencil round once the roof is on. Help would be appreciated thank you.

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• Thanks x 1
4. ### chris1982

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That is easy. The sum over the three angles of a triangle is 180 degrees. As your triangle is symmetric, the two bottom angles are of same degree, say a, and let's call the ridge angle b.
So 2*a+b=180, thus a=90-b/2.
If you do the ridge as a right angle, b=90, and the side angles are a=45.

5. ### John D v2.0

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45 degree mitre cuts at the ridge and 22.5 degree at the eaves will give you a 45 degree roof pitch.
Make sure the length of each roof plane is 0.707 times the length of the base- but be consistent and measure either the internal or external sizes everywhere to avoid errors from the thickness

6. ### JohnD

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Or, in the picture, 90 at the top and 135 at the bottom, depending where you measure from.

8. ### John D v2.0

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Thanks - for clarity, these are the actual angles of the joint, i gave the difference from a zero degree perpendicular cut, which would be useful if you're using a circular saw or table saw

9. ### JohnD

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If you try to build the sides and roof with no back, it will wobble terribly, and rack or break any joints. If you build the sides and back first, with a brace across the front, it will hold its shape enough to add the roof.

The one in the pic might be bracketed at the back to the house wall to add rigidity, or maybe the "terrace" of "houses" braces each other.

10. ### Notch7

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A difficult joint to do

Tricky to cut accurately enough
Tricky to add any mechanical fastenings

11. ### foxhole

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I would just cut square and use piano hinges to join all three edges, the back keeps the structure rigid.

12. ### EddieM

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Ones in the picture have glued batten inside if you look closely.

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5. ### And this bloke is a joiner?

HERTS P&D, in forum: Trade Talk
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