I may need to sister some of the rafters in my house. They’re douglas fir 2×6 from the 50s. Actual size, 5-1/4 to 5-3/8. I would try for full length sisters with ends resting on the top plat of the wall and the ridge beam. I picked up some spruce 2x6x16. That’s only a foot or two over the full length of a rafter. These things flex up and down noticeably when I hold them upright (narrow side down). Are these good enough to reinforce the old rafters if they’re not even strong enough to fully support their own weight? Do I need to find some southern pine or LVL?
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Spruce is springy - that's why they used it for airplanes: Springy and strong. I'd immediately consider more Doug Fir, but I'm out west.
I'm in the northeast. I've not had good luck finding douglas fir around here. I was looking for 2x8s last year.. Have to order a whole palette or get flooring quality boards. Southern pine is not totally out of the question and LVL is around.
why do you think you need to sister the rafters? another way to strengthen it would be to add a 2x4 to the bottom edge to create an I-beam - might be easier than sistering
Adding a 2x4 along the bottom edge of a cracked rafter does nothing. The rafter is still cracked.
Can always install purlins as well. Installing purlins is usually less time consuming and requires less material, but this requires load bearing points below.
You've had some good suggestions with the purlins and the 2x4s. What's the span you're working with?
One tip if you do end up sistering is to put the upper edge of the sisters in first. Then when you're beating them into place, the only friction points are at the ends. Don't ask how I know this.
Andy, How do you know that?
I think I need to sister the rafters because they are cracked to varying degrees. I don't think the roof is going to fall down and it's not sagging noticeably but I want to install insulation and that will add weight by itself and through snow load. I'm thinking sistering because that seems the closest to just replacing the broken components without major structural changes. It's a finished attic. Everything is torn out right now which is how I know the rafters are cracked so nothing is completely off the table. I hate to miss an opportunity to fix things right while everything is opened up even if it takes more time, effort, and expense. I've made that mistake before and I'm regretting it.
2x4s along the bottoms are doable but I wanted to do something to interrupt thermal bridging. I can't do purlins with supports because it's a finished attic. There are already knee walls holding things up some. Would solid 1.5x2.0 (true dimension) purlins every 16 in with no supports improve strength or just add weight?
Andy_Engel: Thanks for the tip. Do you have to clip the sharp end at the top of the upper edge so the sister can rotate? And it's 12 foot horizontal span, 9/12 pitch so ~15 foot rafters ridge to wall.
Two of your comments "cracking to various degrees" and "not sagging noticeably" leads me to suggest you hire a Carpenter experiences in structural repair to take a look at the cracks and check for sagging. One possible solution would be 3/4 CDX plywood ripped to full depth of existing rafters and glued and bolted both sides with the joints in the plywood sheets offset.
Plywood does nothing to reinforce a cracked rafter.
Sorry for the delay in answering. No, although slightly beveling the top of the whole rafter helps.
thinking you could glue & screw (long structural screws /a pan head) the cracked rafters to put them back together, then use a piece of rigid foam between the 2x4 on the edge to provide a thermal break & support- the 2x on the bottom will also hold up either fiberglass or rockwool insulation - could also hold the netting if you want to use blown in dense pack celluose
guessing you're doing all new wiring while it's open as well
good luck
I'm trying my hand at adding a full length sister to a rafter and I didn't expect it to be easy but it is a colossal pain! I have to cut all the protruding nails in the rafter bay flush so I don't bump them while inserting the sister and end up with a nail head sticking up under the shingles. I can't seem to measure it right and have to keep swinging a 14' 2x6 around in an attic to get it back to the miter saw to trim it up. I thought I could turn it upright while it's oriented diagonally in the bay but that hasn't worked out so far. Pounding it upright is going to be pretty intense.
just some thoughts- bend the nails over rather than cutting them and maybe wear a hard hat (hot but keeps the blood were it belongs...), use a piece of 1x furring strip to make a template/ pattern for the 2x stock, use a short piece of 2x stock to make a template/pattern for the ends, keep a piece of 2x stock to use as a "pounding block" w/a 2 lb hand sledge (rather than a claw/framing hammer)-distributes the impact load rather than denting the 2x, get some short pipe clamps to squeeze the sister into place, construction adhesive and structural screws to fasten the sisters into place
the suggestion of 3/4 plywood on each side of the rafters sounds real good, use a gusset/plate where they join, add a 2x ripped to match the "finished" thickness on the bottom and you have a makeshift I beam
sorry about being so long
good luck
Would it be a code failure if I ended up cutting the rafter ~1/4 in too short and shimmed between it and the ridge board? I've got composite wedge shims or plywood. I'd toenail through to keep everything in place. Haven't had a chance to get back to this. I take a week off work to deal with this and now I'm dealing with random meetings I can't miss and unrelated emergency errands 100 miles away.
Thank you to everyone that replied. I appreciate all your advice.
If you cut it a little short, why not just drop the rafter down onto the plate? It doesn't matter if it's contacting the sheathing.
If many of your rafters are cracked (for whatever reason), check whether there's been any damage to the birdsmouth cuts where the rafters rest on the exterior walls (a natural weak point and simultaneously a natural stress point). You may be able to sister damage further up the rafter, but it will be tricky to address damage to that junction without de-skinning the roof.
could you post some pictures of what you're dealing with? My understanding of the problem is cracks in the roof rafters w/sistering to "re-inforce" them, so I'm not sure if "the code" covers it
Having said that, I'm thinking more and more that sandwiching the rafters w/3/4 plywood and adding a 2x across the bottom edge give you the best easiest solution
adding a "short" sister and shimming the bottom would probably work as well but adding plywood sort of eliminates the potential for cracking as the plywood is a laminate product and has no lines to crack along
just some late night thoughts before walking the dog
good luck
good luck
It may be a little unsightly, but a lot of wood checks (cracks) from drying/shrinking. The structural integrity is hardly effected by checking usually. A big knot in the middle of a board will often make a board weaker than a checked board. If the cracks are perpendicular to the grain it is a bigger issue and structural integrity is comprised. Hard to say w/out seeing it.
Ok, some pictures. These are the worst, I think. There is more checking that doesn't reach the edge of the board. In no particular order and referring to the numbers in the upper left corners of the photos:
1. Cracks are hard to see in the photos but they follow the red lines. That outlined piece of wood is split off starting at the big knot. There's a wavy crack that reaches the top above that too. Not sure this one is that bad or that I was going to sister it.
2. Big knot eats up most of the cross-section of the rafter. Right in the middle too. This one is getting sistered.
3. A crack starting at the top corner of the birdmouth cut. Looks ok otherwise, wasn't sure I was going to do this one. That 2x on the right is floating on strapping, not attached to the rafter.
4. Big crack, rafter splitting. This is 1-2 feet from the top. Intend to sister.
5. This is the one I'm working on now. Difficult to see in photo but at the spot where the arrow is pointing, there is a crack across the rafter and the edges are twisted some. Looks like a ribbon torn part way. The length-wise crack runs in some form or other all the way to the top, lots of checking, and the top is split where it was end-nailed through the ridge beam.
I managed to get the sister upright next to #5 by beveling an upper edge and putting that in first (thanks for the tips, Andy) but it also ended up being 1/4 in too short if I press up on the upper end to straighten it out against the underside of the roof. It does rest on both ends if I pull it in a bit. Are the shims a bad idea? I thought keeping it as high as possible would help because a lot of the damage on the rafter is toward the top.
I can't criticize adding plywood on the sides and a 2x on the bottom but it sounds complicated with lots of "gotcha's" and something I'm not qualified to put together right. A full length sister is a pain but straight forward.
It's probably not critical that the new rafter contact the plate. It's better, but if you fasten it well enough to the existing rafter, it should be fine.
Plywood does nothing to reinforce a cracked rafter.
I really would not worry about sistering the entire rafter.
cut the sister to extend as far as you can past the nasty parts, and glue and screw or nail together. use a couple C clamps till they bond if you like. use some good poly construction adhesive.
That would not meet code where I live and work.
There's actually a sound reason based in engineering on why you want to sister a cracked member for its entire length, particularly a rafter. I can't tell you the number of times a year I get called in to assess a prior repair, and tell the owner to re-do it properly.
One down. Ended up 1/4 in short by accident and shimmed it with composite shims at the ridge. Toenailed to beam through the shims with 2 16d nails. Loctite PL premium and 2 Strong-Drive SDWS structural screws every 1 foot. Looks pretty solid and is just about flush with the old rafter so I may not have to plane anything down.
Now I'm fighting with #4 above. This rafter doesn't have much sag but it's bowed along the roof sheathing. It's curving left and right, not up and down. Measuring along its face gives me a longer sister than would fit straight into the bay. Should I set the sister straight and shim between it and the curved rafter or try to bend it to match the rafter?
don't measure the rafter length-measure the peak to plate length - sister/connect where you can and fill the open spaces where it curves to have a "straight" rafter in place - everything you're doing should be strengthening the roof support system so I don't think you need to try to "match" the existing - to some extant you're "re-framing" the rook
good luck
Thank you. How can I fill the open spaces?
glue & screw the sister to the flat surfaces of the rafter and use or cedar shingles or 1/4 inch plywood to fill/build up the void spaces - glue them as you put them in and then use some structural screws to "hold/laminate" the filler strips in the void spaces - if you use shingles you can slide them into place to "fill in" the void spaces
good luck
How straight does an SPF 2x6 need to be for sistering? I've been letting the 2x6x16s sit in the attic for a few days and exchanging the ones that twist. I'm assuming it's fine if it has some bend across the wide face, a 2x6 bends pretty easy that way. Bend across the narrow face hasn't been too bad and I put them in crown up. Is some twist acceptable?