Hi I am more of a woodworker and finish carpenter than framer and have a question about fixing my home. I have a 1910 craftsman with a huge attic that is framed with full dimensional 2x4s 32 o.c. for roof rafters and 2x4s 16 o.c. spanning 14′ (to load bearing wall at approximately half the span) as floor joists for the attic or ceiling joists for the home. I would like to make the attic more usable and thus need to strengthen the joists (and sooner or later fix the roof). I would like to lose as little headroom as possible and am thinking that there is probably a way to use steel framing or steel c-channel to sister onto the 2×4’s. Ideally, I’d like 6″ tall members if that’s doable. I was thinking they could rest on the top plates or even on a small furring block above to stay clear of the plaster keys and either run the full width of the house (27′) or overlap at the midwall. Again, they’d be sistered on to the 2×4’s. I’ve never seen this type of sistering and asked an architect friend who said she hadn’t either but that it made sense. Is this done and if so where can I find info and span tables for different gauges of steel joists, tracks, or c-channel?
Thanks
Rob
Replies
the words you're looking for is "Flitch Plates". There are design guides and ratings, so you can get a given load and deflection out of it.
you're going to screw a piece of metal to the wood member and make a sandwich. really good for replacing a swiss cheesed old beam/joist. but at 2x4, if your ceiling really is that (are you sure it's not 2x6?), you're very marginal for anything. 14ft, you're looking at 2x10 in many applications if you're supporting floor loads. and with floor loads you now need to make sure the walls underneath are good shear strength wise.
BUT....with a lot of them, I think you'd be better just reframing everything, joists and rafters. We had a 2x4 rafter attic, spanning about 16', overloaded with extra tiles, and supported at the outer 3rd. only skip sheathed. it was bouncy like a trampoline. up and down about 4" when a heavy guy walked on it.
reframed with i-joists, structural ridge supported by posts running to basement, and bonus ability to put in a bunch of insulation and condition attic. it wasn't as radical as you think, maybe less than a week with the roof off and 2 guys.
Thanks Allan. Yes I am sure they are full dimensional 2x4s for both rafters and joists. I know about fitch plates and have one in teh home from a 1950's addition. I was taught that fitch beams were popular before they realized or were able to mass produce I beams or c-channel. Again, I was taught they did not offer the same stiffness as these other steel members. Also, I'd like to bring the joists up to 6" height so as to get more insulation as well as a stronger floor. The idea is more storage and a chair in a front dormer to read in nice sunlight but not real living space. In a few years when things have settled down some, I may try to rip off the roof and reframe it with proper i-joists and plywood sheathing (it now has 1x4 skip sheathing. Fortunately, I'm in California in an area with no snow (but earthquakes).
Thanks again
Rob
yup, california cheapo tract home here too. $100k house on $3m of dirt.
our attic floor was 2x6, but hadn't seen skip sheathing in other areas of the country. even on old houses that they had sheathed before plywood, they didn't leave gaps
i would just wait until you could do it properly. attics always get used for more than bargained, and even at 2x6, you're really marginal for a floor. someone always decides they're going to store book collection, or scrap copper and steel (my grandfather), or something up in the attic if the space is halfway usable.
it goes something like
10psf attic (what you have)
20psf attic storage (if you can put a box up there)
30psf habitable attic
40psf normal floor
btw, design ROOF loads at tahoe where I am partly, are 200psf! we had 17ft fall over xmas this year.
Hi again and thanks again. My real question is can I, using 6" tall steel c-channel, floor joists, or heavy gauge tracks (from framing), get the floor to that 30PSF? The span would be a bit under 14 feet.
Thanks again
Rob
You are in the right range, even if you are not connected to the existing 2x4 joists. Sistering the members should help with deflection, but I would not count on load-sharing between the steel and wood for strength unless you have someone think through the details.
You may not need it though. Here are span tables for light-ga joists. (https://www.clarkdietrich.com/sites/default/files/imce/pdf/SupportTools/Catalogs/Structural/CD_Floor_Joist_Span_Tables_08-13-21.pdf)
It looks to me like you are really close to working with 6"x16 ga. metal studs @ 16" o.c. by themselves. Connections to bearing walls and bridging need to be through through, but you are in the right ballpark..