Dropping Shed Roof Rafters on a Beam

In the preceding blog we talked about figuring the length of a porch rafter using a formula you can program into your calculator.  In the next illustration we show what to do if the porch rafters are sitting on a beam which has been dropped. To do this the amount of drop divided by the rise of the high pitch roof = run in feet to subtract from the original run. Then just multiply the decimal portion by 12 to convert to inches. For example....

drop = 3.5

high pitch = 7

3.5/7 = .5

.5 x 12 = 6"

original run - 6" = new run

then enter the new run into your formula from this page to figure your new length of rafter (lor)

Remember that since we're dealing with straight lines and rafters have depth then the depth of the rafter at the plate ( I call it height above plate or HAP) must be kept the same or accounted for when laying out, figuring or cutting your rafter.

Click Image to enlarge. At left you can see how the theoretical  distance "a" changes when the beam is dropped essentially making the distance for the rafter shorter. If you find this information useful leave a comment.

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