2x6 Floor Joists

[brisk]

My house was built in 1901 and remodeled in early 60's when the attic was made into a bedroom and storage. It has been used as such ever since. I would like to add some wall partitions and a small bathroom (just shower no tub).
I have just realized that the floor joists are only 2x6 16"O.C. spanning up to 16', boards on top.

What is the best and most economical thing to do? Someone suggested to just sister the joists with 2x6's. Would that help? If I sister 2x10's can they be flush on the bottom?

Any other suggestions?

Thanks.

 

[SteveBausch]

2x6 in what wood? Turn-of-the-century oak is stiffer than today's "big box" lumber. I assume 'true' 2x6, not nominal.

I suspect there is a (load-bearing) wall or two below the attic, if so that's good, if not, that is a mark against your proposal.

What do you plan for walls? Most folks consider dimensional lumber suitable for flexure and compression, tension is only considered when prefab trusses are involved.

However, your plans are immediately below the rafters, and the potential to strengthen the floor by stressing the rafters is a possibility.

Last time (years ago) I looked at a code book, seems that 2 x 6 were considered for short spans.

Sistering the boards with identical lumber will double it's strength. Halving the span will quadruple it's strength. Boxing in the bottom of the joists with plywood would also help, but I doubt you care to strip off the existing ceiling to achieve that.


Currently, how shaky does that floor feel? You might be surpised what level of deflection is tolerated by code.

 

[RASmithPE]

2x10's sistered to each attic joist is the usual method of conversion. Use plywood for flooring(APA)span rated for the joist spacing. The bottoms of the new and old joists should be flush so that the new will reach the exterior wall plates.

Kneewalls can help support undersized rafters common in 1910 gable roof framing. Interior wall headers may require reinforcing. (can be cut in flush with the joists

The beam in the basement is now carring extra load and may need reinforcing.

Try to locate any new stair as to cut as few joists as possible since the joists resist the outward force from the rafters at the eaves. Also a new stair located along the gable end wall will leave this wall unbraced.

Before you start, you may want to consider a new dormer. This, however, will require a knowledgable builder, engineer or architect since it may need a new ridge beam.