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How do I figure out what angle to cut for a sloping board?

534 Views 5 Replies 6 Participants Last post by  miamicuse
I want to build a detached back patio cover over my patio and I’m trying to figure out what degree angle I need to cut the side boards at.

The rear of the patio cover will stand at 8 feet high and the front of it will stand at 7 feet high. The beam which will connect the rear tofront is 8 feet, minus the little bit that will be need to shaved off to make the angled cuts.

I drew out two pictures (I’m no artist lol) to try to show what I’m talking about. I circled the angles I’m trying to figure out. I tried to figure it out and I think it’s like 83°, but I want to know if there is an easier way to figure it out or a specific equation that you guys use?


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We change that to the rise in 12 inches so if you divide the rise by the feet we get 1 1/2 per 12 inches and would write that as 1 1/2 /12
We mark that with a square


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Not sure what board your are cutting but what is going on top?

My concern would be stability, 8' high needs significant bracing or anchor into the ground. Splaying the legs out implies you are wanting to stabilize the assembly, but that is not enough.

Tell us more.

As for the angle, I would lay some test pieces on the ground and mark then and use them as a pattern for the actual frame.

Not sure what board your are cutting
OP would need to cut the side boards, because the front is shorter than the back.

I just CADded up an 8' run and 1' rise and the angle is 7.125º. However, in practice, that will be less than 8' run because you'll be cutting off a bit off each end.
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With the front and rear headers attached to the posts, lay a beam across on top of those. Draw a plumb line on each end of the beam using the inside of each header as a guide. This will give you the angle and length to cut. Do each beam because if i know anything about lumber and you want a snug fit the distance between the front and back headers will vary a smidgen with each.
You can calculate the angle but knowing the angle does not necessarily mean it's easy to transfer that in the field.

But to calculate it, from your sketch, you are looking to drop 1 foot over an 8 foot span. Rise over Adjacent is the tangent of the angle.

So the actual trigonometry is:

Tan(A) = 1/8 where A is the angle in radians.

A = Inverse Tangent (0.125) and converted to degrees it is 7.1250163489.
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