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One of my builds this year will be a simple shelf that I'm gonna put on the roof of my front porch for pukkins. My question is....how do I find the right angle to cut my boards (legs) at for the slope of the angle of the roof? I'm sorta a rookie woodworker but quick learner so if anyone can put it in durt-da-durr terms thatd be great! Like always Happy Halloween!!
 

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Take a contractor’s level at least 24 inches long and a tape measure. Place one end of the level against the edge of the roof and lift the other end until it becomes level. At that point measure down from the raised end back to the roof surface. Grab yourself a trusty math calculator and then use the tan-1 on the calculator of the measured distance divided by the length of the level. That will give you the angle for your cut.
 

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Most roofs are in the range of 4/12 pitch to 8/12 pitch. If you know your roof pitch, you can use a carpenter's square to mark the angle on the wood. That being said, I assume you don't know how to do that so here's a super simple method.
Up on the roof, lay a short length of 2x4 on the slope with the 3-1/2" side vertical. Take a second short length of 2x4 and stand it straight up with one edge touching the roof and the 3-1/2" side against the 2x4 that is on the slope. Put a level on the upright 2x4 to plumb it and mark the angle where it's up against the 2x4 on the slope.
 

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Here's a quick vid showing how to use a speed square to mark roof pitch. Of course you need to know the roof pitch to do this.
 

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Here is a way to do it with a just a level. Lay the 2×4 on the roof so its 3.5 in high (if your roof is very steep you may need a larger board). Take the leg ( it must be extra long). Stand the leg up next to the 2×4 and level the leg. Then trace a line from the top of the 2×4 to the leg. This give you the roofs angle (pitch) on you legs. Cut on the line. Take the shelf place it on the roof where you want it, level it place the new leg next to the shelf. Trace the line on the legs where you want the legs to be. There you go. Remember that the legs need to be exrta long you are going to cut off at least 3.5 inches. So make the sure the legs extra long.
 

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What cmerli said will will work but you dont have to do the trigonometry. Just do what hudtechllc described and hold a level up beside a 2x4 so it is plumb to the roof and take another 2x4 and lay it on the roof. Then draw a line on the plumb 2x4 using the edge of the one laying on the roof as a straight edge. You could also cut an approximate angle on your supports and then put one screw in a 2x4, or whatever you want to use, as a foot and sit it on the roof the way you want it then drill another hole for a second screw to hold it at exactly the right angle. That way you don't have to worry about minor inconsistencies.
 
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