I need to repair some rotten 5"x3.5" (actual size) rafters (the one in the pic is the worst). I'm thinking about using a pair of unequal length 2x6s to create a sort of half-lap joint with the cut-back timber, and adding a trim board. Is there a better way? How do I make clean, square cuts to the rafter?
I think your staggered-joint idea is a good one. From the look of it you'll have a solid enough base to get another decade or two from that tail.
I'd cut back just enough to get solid wood at the center, then notch the sides back to wherever you can comfortably fit the saw. Something like so:
| | |__ __| | | | | | | | | | | | | | | | | | |--| | | | | | |__|__|__|
Fasten the cedar or pressure-treated repair pieces with good exterior-grade adhesive and trim screws. (You can even fasten the most conspicuous side from the inside to keep the surface smooth.) Caulk against the wall, and stain or paint to seal the new wood.
What should I use to fill gaps when mating the new wood to the old? Some of the cuts are bound to end up less than perfectly flat. May 5, 2016 at 23:08
Since it'll be painted, use any exterior product you like. Something that hardens and is sandable would be better than latex caulk, for example. May 6, 2016 at 2:04
How do I make clean, square cuts to the rafter (tail)?
You may want to try to gently insert shims to add a little extra space between the rafter and the 1x4 roof boards. I would use a circular saw to start; you will not be able to cut all the way through (5") with a common circular saw, but it will give you a clean cut through a few inches. You'll need to use a reciprocating saw (and care) to finish the cut. You might be able to wrap the tip of the saw blade with tape to help prevent damaging the roof boards. Alternatively, an oscillating (bare) tool might be helpful.
Is there a better way?
It would be ideal to remove and maybe replace the 1x4 roof (above the rafter), but I suppose that this house doen't need a new roof any time soon? The most expensive but obvious solution would be to replace the rafter(s).
I would use a combination of approaches for this. To cut away the rotten wood I would use a small door jamb saw. It's handle is offset - it will make working in that space easier.
You probably won't be left with much good wood. To shore up the rotten wood, I would inject epoxy. The West system is ideal for this. It's not cheap, but it's cheaper than taking the roof off.
The West system is designed for wooden boat repairs - that should make it a good fit for this application.
1West System is a fantastic all-around system that will have 1000 uses in a workshop, and is well worth the investment on that basis alone. Git-rot is another system specifically designed to reconstitute rotted wood. The penetration of Git-Rot is much better than West because it is much more watery. Of course that also means it'll leak away if you don't contain it. There are many competitors to Git-Rot. May 6, 2016 at 0:50