Our 1938 cape cod has an attic that was converted to master suite (who knows how long ago), but we've been living there for over a year. We have since decided to gut the entire upstairs and put in a bathroom. In doing this we realized they built the "attic" on 2 x 6's. They are 16" OC apart. They go across the room probably 6 feet and then are screwed into another 2x6 to continue the rest of the width of the room. We don't want to say goodbye to our whole project. Would you recommend sistering the joists in the bathroom with 2 x 8s and leaving the rest with 2 x 6s? Or should we just had more 2x6 joists throughout the entire upstairs? Do both? We weren't planning on putting in new flooring, but if we have to we will. We won't have a tub in this bathroom. It will be a small, tiled shower, vanity, and toilet. Would love to hear any thoughts!
I suspect where they are screwed together, they overlap and there's a load bearing wall beneath. Is that correct?
You say the span of the joists is "probably 6 feet" but that the critical measurement. If they're really only 6' long, 2x6's at 16" on center are fine, in fact they'll go a bit further under normal conditions.
If they are older dimensional lumber, back when a 2x6 was actually 2" by 6", they'll go even further.
Have you noticed any deflection (bounciness) or sag?
The Code (ICC Table R301.5) an attic floor or a sleeping room floor shall support a minimum of 30 lbs. per square foot plus all dead loads (carpet, floor sheathing, joists, ceiling finish below, etc. )
I’d estimate your total load at about 12 lbs. per square foot dead load plus 30 lbs. per square foot live load for a total load of 42 lbs. per square foot.
2x6’s at 16” o.c. that support 42 psf can span about 11’ - 12’ depending on species and grade of wood.
2x8’s sisters to 2x6’s at 16” o.c. Can support 42 psf can span about 16’ - 18’ depending on species and grade.
If you decide to sister joists, I’d sister all of them and fasten them together with 10d at 16” - 18” o.c. In the middle third of the span and 24” o.c. Staggered on the remaining one-third ends.
The reason you need to sister all joists is because the subfloor is designed for 16” spans...not 32”.