Attic conversion joist span suggestions

Old 12-06-05, 04:20 AM
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Question Attic conversion joist span suggestions

Sorry for the Looong post. "Even the impossible is possible with a little thought!"

I am remodeling a 1939 bungalow with an unfinished attic. My next phase of construction will include adding a bedroom / bath upstairs. I would like some suggestions as how to proceed prior to submitting for my new permit.
Here are the constraints:

Currently the celing joists are 2x4 24o/c with 1x6 center-match on the ceiling below. 2x4 are 2-5/8 x 1-5/8. (this is suprising stout even for my 250lb frame)

Span is 14' 9" wall to wall

I know that this will not support the requirements for a bedroom above but here are the additional constraints I have been trying to work with:

The ceiling height in the attic is just sufficient to get the 50% headroom for an attic space although the local inspector is easy to work with he has stated on times past that existing structures are grandfathered as far at the headroom clearance in other submissions I have made.

I would like to maintain as much of the headroom as possible beings it is quite limited. The roof pitch is 7/12 and the ridge for the gable roof runs parallel with this paticular room. Therefore the room would be centered over the span of the lower rooms. Attic space is 14 x 30 and both rooms below are ~14x15.

I really do not want to put beams accross the room below due to the work involved in tearing up a perfectly good room and changing the period details of the rooms below.

Easiest thing here I know would be to sister the existing joists with 2x8s and put other 2x8's between to have 2x8's 12"oc but that would eat up 4 inches of precious headroom.

Here are some of my thoughts:

First I would triple up the existing 2x4's by sistering them together:

Second I would build a triangle panel wall (2x6 with glued and nailed 3/4 plywood on each side) at the midpoint above the span below to divide the room into a bath and 1/2 of the bedroom. This would "box beam" would go from roof rafters to floor joists. The bedroom would span the same way on the other side of the house with the bed acutally resting on the center load bearing wall. Likewise on the other side I would build a simular panel wall with a closet on that end.

The delima here is that I need a doorway in the center of each wall to enter the bath / closet which compomises the box beam built from above. I have sufficient room above the door to make another header again as a box beam.

My thoughts are to use 4x6 or 4x8 as the base of the box beams and frame the doors with 4x6s then lag bolt this to each other and to each of the triplet of sistered joists. I would also extend these out over the other rooms to take advantage of the the cantelivered effect of these 4xs.

Lastly I would glue and screw the 3/4 plywood to the floor on the joists.

As I do not have any experience with steel I would welcome suggestions that may include using angle iron or steel box beams that could keep within the 3-1/2 limit of the existing joists.

I am looking for ideas here. I will most likely have to go to a structural engineer and get this designed but need options others may have tried.
Old 12-06-05, 05:15 PM
Join Date: Dec 1999
Location: South Dakota
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Contact a professional engineer. Good Luck

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