I like using a Construction Master because it allows me to calculate rafter cuts without resorting to trigonometry, referring to a rafter table book, or converting fractions to decimals. But one thing it doesn’t do is work in 3-D. Unless you take into account the actual width of your rafter stock, your calculations will be off, either by a few degrees or a few inches.

This becomes a problem when trying to frame to a specific height. On a shed-roof addition, for example, the rafters are hung from a ledger that often must be placed so that there is adequate clearance between the new roof and existing windows above the addition (see illustration). Or if the site has a building height restriction, you need to...

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