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Upgrading attic floor joists to carry floor load


charlesmt's Avatar
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12-02-09, 04:29 AM   #1  
Upgrading attic floor joists to carry floor load

single story home with a 11 foot high hip roof with a flat top - it is basically a giant cave up in the attic.

would like to use this space, but the floor joists are only 2x4 16oc sistered joined in the middle over full wall length of support. its bouncy total length is 28 ft long, with support on ends and midway.

what is the best way to go about this without jacking up the roof to add 2x10s?

could i add sister 2x4s for the length of each joist, and then possibly add a laid flat 2x4 on top of it - it wouldn't lay over the end walls as the plate is there for the rafters. the deflection guides are not set up for this sort of calculation. maybe i could run another set of 2x4s joists between each joist ending up with 8OC spacing. obviously i will add horizontal bracing between joists.

thx for your time

 
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chandler's Avatar
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12-02-09, 11:54 AM   #2  
2x4's, even doubled barely will span 13', so your 14' span between supports will barely hold up sheetrock, much less a live load. It would be beneficial for you to call in an engineer to look at what you have and see what the feasibility of adding support for your room. Is your roof system trussed?

 
GBR in WA's Avatar
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12-02-09, 12:03 PM   #3  
You will need at least 2x10's and a permit for the usable space. You also need egress stairs to code, heat and ventilation, natural light may be required as per Official, and possibly other minimum safety things as per local code.

Be safe, Gary

 
charlesmt's Avatar
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12-03-09, 01:38 AM   #4  
i realize this seems like a bad idea

for one, there are no codes or anything here, no inspectors or permits, but i do plan to use common sense and copy code from other areas.

the roof is sorta odd in how its attached.
the joists below overhang the walls about one foot, and the plate for the rafters to lay on was nailed in on the outer edge of those rafters, so it is not over the wall plate. not sure if this construction technique has a name.

because of this, i was thinking about sistering something bigger to the current 2x4s up to the plate as they'll rest on the wall plate below.

i was just wanting to know if there was a way to build up the 2x4s or not or will i just have to put in new joists next to them to support the floor.

as far as truss support, there is none. i'll add some framing which will serve as walls. it does seem amazing this lonely 2x4s have supported this roof for nearly a century.

 
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12-03-09, 03:33 AM   #5  
Regardless if there are inspections or not, the job must be done right. Anything less is unacceptable.
With that said, the 2x4's were installed to hold up the ceiling, nothing else. Now you want to add a live load, and that will require substantial joist beefing (2x10's or LVL).
Get an engineer in to see if your plans will hold water or not. We can't see what you see.

 
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