Looks to me like the light blue triangle is not big enough. When you sew it on it should hang off the top by 1/4 inch. Then when the 2nd triangle is attached, it will sew over the roof and the 1/4 of the blue to make the top corner. That will then leave a 1/4 of the blue for top seam.
I'd make the blue at least 1/2 inch bigger and sew from bottom of roof to top. You can always cut off the extra at the top.
Just MHO though!