Cutting a rafter sister

[JHZR2]

I’m going to try to put this in words...

I have a garage, and I’m just about done rebuilding it... one last rafter had a bit of old termite damage, and I want to sister it. I don’t want a birds mouth, and I don’t want a tail. I just want full bearing of the rafter on the upper plate. I might do the next neighbor too, so the potential for four total measurements and cuts.

I have a mitre protractor, but it’s not the right size for doing this accurately. too big at the end. I have a speed square, but it doesn’t fit to measure either.

What’s the best way besides trial and error to get the right length so the sister goes down along side the existing rafter, and fully engages with the upper plate?

I think this is simple, but I was cutting a template today, and just couldn’t get it exactly right and tight. Nothing is square of course...

 

[PimTac]

This is assuming you have enough wood on the damaged rafter to sister it to.

Find the angle for your cut on the upper plate. If you have a iPhone you have a angle finder on it. It’s in the compass app. Other smartphones may have it too. Other than that determine your roof pitch by measuring out one foot and then down. Typical pitch is 4 in 12 or 5 in 12. You can figure the angle from that.

Once the angle is cut on your new rafter measure the old rafter from the heel of the cut to the ridge. That should get you length.

Is this a truss? I’m thinking you might have to sister some support to the ridge of the new rafter piece.

 

[JHZR2]

Rafter is solid, just has signs of a little termite damage at the very end. The damage was in a beam (that serves as the upper plate in walled sections, but also provides full support across the sliding door openings) right below, and since it was rough cut lumber before, and the rest of the 120’ span still is perfect, I made a flitch beam to match the dimensions. It is not a truss. It’s a gable roof building approximately 120 feet long, with sliding garage doors on one side.

Since the end that sat on the old rough cut beam is where there’s a bit of damage signs, I want to sister the whole thing (both sides), and get the maximum interface at that point to the beam.

 

[PimTac]

Okay I think I can picture it now. It doesn’t sound too bad actually. A simple fix might be to sandwich that part of the rafter with scrap plywood and use metal ties to attach to the plate.

 

[JHZR2]

Okay I think I can picture it now. It doesn’t sound too bad actually. A simple fix might be to sandwich that part of the rafter with scrap plywood and use metal ties to attach to the plate.

That would likely be fine. My issue is still getting that measurement with “sufficient” accuracy, that isn’t trial and error. I don’t have the right protractor, and I think I need a bevel square(?).

 

[PimTac]

A bevel square would be handy.

 

[JHZR2]

A bevel square would be handy.

Ok.

Will get one. I have a big starett plastic protractor, but it’s really for measuring outer corners and doesn’t get me consistent reading on this inconsistent setup

 

[dlundblad]

Can you post a photo of what needs done?

 

[JHZR2]

Next time I’m at the building.

Should just be accurately measuring an angle that lets the rafter end rest in the upper plate/beam. Don’t need a birds mouth, because I don’t need the tail, it’s already there.

 

[JHZR2]

Can you post a photo of what needs done?

Not much to it. Just want to cut it to end on the beam.

 

[dlundblad]

Measure a good beam from a non rotted area for the correct dimensions. Make 2 pieces then sandwich them together with the old in between them. You might have to make a smaller piece just to tap into place to support the roof if there has been any sag. While it’s supported, install the opposite side and screw in place. Remove the small piece and install the second one.

Make them as long as you can going towards the peak and use some 3.5” screws to fasten them together. Bolts every 2 feet or so may not be a bad idea either.

 

[JHZR2]

The beam I got a pic of is not rotted/badly termite damaged. There are signs of an old trail (everything has been heavily treated and we didnt see any sign of live termites in any of the wood we pulled down). My issue has been getting the right angle/measurement for the cut with the rafter in place. seems so easy, but for some reason, each time I take a measurement and cut a template, it’s just not right. I haven’t had a lot of time to spend on it, I don’t work on this every day, but all the more reason why I’m looking for suggestions on how to better get the measurement or template it so I can make it right.

Im going to make the sisters out of 12’ long stock and sister along the entire length of the rafters, both sides, because the material cost with me doing the labor makes it an inconsequential thing.