Why is my frame not square?

[leostar20]

Hi there! 

 I’m trying to build a wooden frame for a vent on my outdoor pergola. My plan is to then put polycarbonate roofing on this vent to finish off my pergola. The roof is at a 22° angle so I have cut each piece of wood at a 45° angle and at a 22° bevel, however as you can see in the picture below no matter when I bring them together It’s it’s not coming out square! What am I doing wrong? 
 The top as in the bottom picture is what I am trying to achieve, 

what I’m trying to achieve :

 

 

[JacobZ]

Hi @leostar20,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

I could only take a guess at what is causing the issue, but two things come to mind.

The first is that your cut is very slightly out. If you are using a mitre saw, it could be ever so slightly out of square, or you could be experiencing some blade deflection. Try cutting the timber at 44° or 46° and seeing how they sit.

The other thing that could be causing the problem is the screws. You can see that your screws are located on the inner side of the join. When they are screwed in, they will pull the inner part of the joint together. As it is pine, which is a softwood, it could be causing the wood fibres on the inside of the join to compress, which would make the outside open up.

Try clamping the timber to a square, then predrill some holes for the screws. When you drive in the screws, don't overtighten them. If they just sit tight without pulling too hard, they shouldn't open up the joint like they currently are.

Allow me to tag @Noyade, @AlanM52 and @Nailbag to see what thoughts they have..

Let me know what you think.

Jacob 

[Nailbag]

Hi @leostar20 

First thing is to check it its your fastening method or the mitre cuts. Assuming your using a mitre saw what’s the brand and model and can then advise on how to check and adjust to square both mitre and bevel. 

Nailbag