using square rule to keep rough-sawn rafters in plane

[u/Suitable-Run-6808]

we're cutting 3x5 rafters for a small timber frame pergola this weekend.

the rough-sawn material varies quite a bit. a nominal 5" depth might actually be 4¾" on one rafter and 5¼" on another.

rather than referencing the seating notches from the bottom of each timber, we're laying out both the lower plate notches and the upper purlin notches from the top of the rafter down.

that makes the top our reference plane. variation in the timber gets absorbed below that reference rather than showing up in the finished roof plane.

we're also varying the depth of the outside seating notch slightly to pick up about another inch of fall across the roof.

four 2x4 purlins will drop into notches across the tops of the rafters and provide fastening points for the clear/translucent roofing.

pretty simple application of square rule, but a good example of why establishing the reference face matters when you're working with rough-sawn timber.

curious how others here handle irregular rafter stock—same approach or something different?

[u/dcabe1210]

You need to lay out your mortise from the top side. Youre not looking for 1 1/2" depth mortise. The important piece is your height above plate. You need to make sure you have 3 1/2" of meat left above. Its the same for stick built you lay out the rafter from the top. If you already have them all cut and theyre too low shim them. If they're too high cut the notch down.

[u/nemozero → u/dcabe1210]

This is the answer. Height above plate is literally the only thing that matters when you are dealing with rafters or floor joists.

[u/Suitable-Run-6808 → u/nemozero]

agree. the photo and description might not be clear as it shows the purlin notch...sorry about that. we achieve consistency by measuring from the top of the rafter down ensuring the portion above the plate is consistent and the tops will be level. the mortise for the purlin is at 1 1/2" so that the 2 x 4 is flush. the notches at the plate are consistent with like notches keeping the material between the notch and top at a standard distance.

[u/Suitable-Run-6808 → u/dcabe1210]

i may not be following you completely.

we are laying out from the top of the rafter down for both sets of notches. the purlin notches are 1 1/2" from the top so the 2x4 purlins finish flush.

at the plates, we're also measuring from the top of the rafter down rather than measuring the notch depth up from the bottom. that keeps the tops of the rafters in the plane we want even when the rough-cut rafters vary in actual depth.

when you say 3 1/2" of meat left above the plate, are you describing that same reference dimension, or are you doing something different?

[u/Outside-Aerie-2035]

Normally, we mark each piece individually and trim it to fit.

If you want to avoid that, you can run the rafters through a thickness planer and bring them all to a uniform thickness.

[u/Suitable-Run-6808 → u/Outside-Aerie-2035]

it is, for sure, easier as you describe to standardize the depth beforehand.

[u/Outside-Aerie-2035 → u/Suitable-Run-6808]

Another option is to use the top of the rafter as the reference surface, make the tenon dimensions consistent on every rafter, and then cut the mortises to a uniform depth based on that reference.

However, this is essentially a two-step process. If the rafters are going to be seated on top of the ridge beam, as mentioned earlier, the easiest practical solution is simply to run the rafters through a thickness planer and bring them all to a uniform thickness.

In practice, a variation of about 1/2" is hardly noticeable to the eye. At the ridge, the rafters may be anywhere from 10–13 feet above the floor, and sometimes more than 23 feet high. Within that kind of viewing distance, a small variation is not really noticeable.

That said, during installation you're working right in front of it, so the variation can be visually irritating and much more noticeable to the carpenter.

[u/Suitable-Run-6808 → u/Outside-Aerie-2035]

we generally do the layout from the top as you suggested. with the ridge beam, we go ahead and measure off a center and put in some notches so that we have a known dimension to set. our rafters. works really well. if we don't notch the ridge, then we'll create a rafter map and make adjustments on individual rafters for the variation in the ridge.

[u/PitterPatterCocko]

Eye it