Expert: Dan Griffin - 10/29/2008

Question
Hi Dan,

We have a 99 year old Craftsman in the SF Bay Area, and I'm fixing up the attic as a bedroom.  We're actually using it just now, as I slowly make the changes.  The floor joists are 2x4 (real 2x4, 100 years old, probably Doug Fir), 16" oc, with a span of 12'6" (to the load bearing wall under the middle of the floor).

I'm thinking I should probably beef that up, but I want to leave the floor joists at 4" tall.  When I come to do the floor (you'll laugh that I'm doing the floor last), my inclination is to either sister the 2x4s or add new joists to make them 8" oc, using either 2x4s or 4x4s.  

I've read through some of your other posts on the subject and looked at the span length charts I can find (like the American Wood Council's online one), but they're not very flexible regarding old wood (ie 2x4 not 1.5x3.5) or using non-standard on center dimensions.  I hope to do this with recycled (scrounged) wood from other old house remodel jobs.

After that preamble, do you have any suggestions for the best way to proceed?  I suspect I'm sol trying to find a span length chart that will work for me, partly because 100 year old 2x4s weren't graded the same way they are now (if at all).  My inclination is to simply sister the existing joists as best I can.

Thanks, Neil.

Answer
Neil,  I know that old undersized lumber has carried some huge loads for a long time.  Many old roofs were raftered with 2x4 spanning long distances.  Empirically I know that old lumber can carry more than the span tables that are used today.  All this being said, I am not aware of any span tables or calculations that make any allowance for old straight grained lumber.  You will need to satisfy an inspector on this issue, so my best suggestion would be to contact a structural engineer in your area that might work with you and either have alternative suggestions or know of some exception to current code requirements.  Even if you create a carrying beam at mid span, your 2x4s won't make the span.