A.Will Holladay responds: The problem probably isn’t one of calculating the length, but in the accuracy of your ridge height measurements. No matter how accurately you might measure the proper rise from the floor, a crown in the floor deck will boost the ridge height.

It’s not uncommon for the floor height to vary 1/2 inch or so in the middle of a big room. While small dimensional differences like this might not be noticeable at a lower pitch, a steep roof magnifies any discrepancies. To avoid vertical measurement errors on steep roofs (8/12 and up), I always measure the rise from a string stretched across...

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