trim 2x8 rafter sisters to 2x4?

18 years ago

Hello GardenWeb-sters,

Seems like there's great collective wisdom here, so thought perhaps someone would have answers to a structural question I'm facing.

My house was built in 1920, is 1 1/2 stories, and half the upstairs hasn't been finished. It's a pretty big space, 12' x 23' , which I plan to make it into a small bathroom and bedroom. The issue at hand is with the rafters.

The original rafters are 2x4s, 30" OC, and were sagging. Under the influence of a former SO, I had them sistered with 2x8s for strength, and to accommodate installation of R20 insulation batts. Now I regret doing this, as I've lost 4" of headroom that I could really use for the proposed bathroom (which will be tiny as it is), as well as the rest of the space. The bathroom headroom will just barely meet the minimium required by local code, and the kneewalls will now be about 4'8" high, which is short of the 5'0" code requirement. I asked the building dept. how strict they are about the 5'0" kneewall height, and they said they're fairly understanding about older houses, and that I could apply for a waiver. But I'd hate to start down that path and then run into problems.

So, my questions are:

1) Can I trim the face of the 2x8 sisters to make them nominal 2x4s (or maybe 2x6), and gain back some headspace (realizing the insulation would need to be redone to R13)?

2) If so, would I need to also add collar ties?

3)Should I have even used 2x8's in the first place, or are they too heavy (will sag over time)? BTW, the ridge beam isn't structural, looks to be a 1x4.

I have contacted two engineers about this, but they're both booked up for the next two months. In the meantime, perhaps someone here on GardenWeb has already "been there, done that". Thanks in advance for any advice.

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Comments (6)

worthy
18 years ago
I can't find a span chart for sistered 2x4 pieces. But I would never use 2x4s for rafters anyway.

Here is a link that might be useful: Span Calculator
jegr
18 years ago
you need to wait for the engineers or find another. Ask your building department for a list of local good engineers and architects who know old buildings.
2x4's are not strong enough for a roof even if they were 12''OC, unless the span was very short, the roof very steep, and the roofing material light weight.
I think you should not depend on us on-line who cannot see your house in person for more than some direction.
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User
18 years ago
I'd absolutely suggest posting on the forums at oldhouseweb.com. There are loads of other old house owners who have faced situations like yours...and some have engineering backgrounds. Head on over there!
ron6519
18 years ago
No, you can't do that. Using 2x8's for a 12' unsupported span is marginal with out the bathroom. If you want the headroom, put in a dormer and do it correctly. Structure shouldn't be compromised because it's inconvienient.
Ron
worthy
18 years ago
I thought the OP was talking about rafters, because that's what he calls them. But as ron6519 implies, I think he's talking about 2x4 floor joists. In which case, absolutely ridiculous!

Once, on site, a neighbour asked me about how to build outside third floor stairs, then did it his way--all with 2x4s. The tenants just can't step "too hard," he said.
lwylie
Original Author
18 years ago
Thanks for the advice. I will continue to wait for the engineers to be available, and in the meantime check out the oldhouseweb.

Yes, they really are 2x4 rafters, not joists (the joists are 2x6). In temperate parts of the country like the Pacific NW where I live, 2x4 rafters were the norm back in the 1920's. The wood was old growth fir, and I've been told it's better quality and stronger than what's available nowadays. But after 87 years the rafters are sagging, so definitely need add'l support of some kind.

I looked into a dormer, but it's not economically feasible for me. The structure/layout of the house is such that even a small dormer would require major reinforcement (sistering floor joists, adding a structural ridge beam, and a support post in the middle of the room that would extend down two floors to the foundation). At an estimated cost of $50K (from two contractors), that comes out to about $900 per sq ft unifinished, and I can't justify that ROI (and don't have the $$). Thus my carving out a bathroom from the existing space, which will work, albeit with tight headroom. But it would be bigger than some bathrooms I've experienced in Europe during my college days. Europeans are able to fit bathrooms in the most amazingly small places, out of necessity.

It will be interesting to see what the engineers say. Wish I'd consulted them originally instead of listening to the former BF. Oh well, live and learn!