trim 2x8 rafter sisters to 2x4?

16 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
16 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
16 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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Exposed rafter tails is a complicated system, especially with a truss layout. Even if you add the tails to the trusses, you are going to be subject to the layout of the truss in whaich you sister onto. Were doing a project, with 3 inch cedar tails (24 inch overhang), and they are thru bolted eight times in 6 feet of length onto the truss. Truss layout had to be manipulated for the final exterior appearance to be balanced and spaced equally. Even if your trusses are layed out with equal spacing, it will be skewed when you factor in the thickness of the tail, depending on which side you sister to. Along with the added expense of the tails, you will be looking at additional thickness of the roof sheathing in the overhang area, and possibly even a decorative pattern in the bottom side. The additional thickness is so that the roofing nails wont protrude thru the sheathing, into your decorative overhang. I think you are only getting started with the expense of a exposed rafter tail with decorative sheathing. The overall layout and final appearance shouldve been thought about months ago, not after the trusses were set.
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here's a suggestion, but remember, it is never very wise to design by e-mail or on the telephone - I can't really see what's there, and you may misconstrue my words. If the bathroom is quite small, you might be able to double the floor joists above so they are 48"oc instead of 16"oc. This is like framing a stair opening. If you currently have 2x8 floor joists this would add 7" of ceiling height in the middle. Then you would have to make sure your floor above was strong enough to span the 48", which might mean new 2" decking - so there I've lost you another inch... You also might be able to take out the floor above and frame it again with 2x6's 12"oc, or 2x4's 6"oc if the span is short enough. Your town might ask for an engineer's stamp for that. If you have walls on the floor above the bathroom, you might be able to hang the first floor from the walls over the bathroom area, instead of bearing on the floor joists. Then you would have no joists and more height. If you have 2x10 floor joists, 16"oc, consider 2x8"s 12"oc.
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how many nails are in the face nailed connection of the bottom chord and the rafters? Are there any splits or are the nails pulled? The chords definitely have some functionality keeping the bottom of the rafters from spreading but I can't see the location being so critical that lifting those tie members 6 in. to win clearance will have any significant effect. Also, are you saying the chord members are 15ft 2x4's? Replacing them with a 2x6, 2x8, or 2x10 at 6 in. higher and adding more nails to the connection would be an improvement over the 2x4 connection. You might also want to put some blocking on the ends between the new members and the top of the wall so the ends "bear" on the wall and also provide a little shear support at the roof edges next to the rafters. Have fun with your new opener.
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User
16 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
16 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
16 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
16 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!

trim 2x8 rafter sisters to 2x4?

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trim 2x8 rafter sisters to 2x4?

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worthy

jegr

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ron6519

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lwylie
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