I'm converting the above garage attic into living space and am about to install insulation. The attic floor joists are 2"x 8"x 13 16" OC. To pack in more insulation I want to increase the floor depth by possibly attaching 2"x4"'s to the underside of the existing joists. My questions? Can this be done by toe nailing and glue and will the added weight of the 2"x4" 's plus 5/8 firecode drywall overstrain the existing joists?
Thanks.

This message has been edited. Last edited by: Paul SA,

The best way to add depth to the existing floor joists is to create a sandwich of 1/2" plywood on each side of the stacked 2" x 8" and 2" x 4". By adding the plywood, you create a flitch plate on either side of the stacked dimensional lumber, which not only keeps the 2" x 4"s from rolling off of the existing 2" x 8"s, but also considerably stiffens up the resulting 11" floor joists, making the stacked array behave as if it were a "short" 2" x 12".

Since the 2" x 8" is really 7 1/2", and the 2" x 4" is really 3 1/2", the stacked array will dress out to 11". Rip your plywood to 10 1/2", and use construction adhesive on the interfaces, plus plenty of ring-shank nails, being certain to catch both the 2" x 8" and the 2" x 4". Hold the plywood back from the top and bottom edges by 1/4" when you fasten it, to prevent interferance with either the drywall ceiling in the garage, or the sub-floor in the attic, and you will wind up with a very stiff floor, able to support any reasonable load you put upon it. Just make certain that the taller floor doesn't compress the ceiling above to the point where it no longer meets minimum code height.

If you are converting the space into living space you will need a minimum live load of 40 PSF. A two by eight at a 13 foot span is a little on the small size. You would need to identify your joist as doug. fir-larch with a structrual select(SS) or No. 1 grading. Other than those, not to code.
A more sound method would be to either add a beam mid span or sister two by tens next to the two by eights.

Thanks guys for your great responses.Two further questions.As the sub floor is already in place, I was wanting to attach the 2x4s to the underside of the 2x8s. Would the plywood sandwich still be as effective and if so should the plywood run the entire length of the 2x4/2x8 joint.
The existing 2x8 joists are douglas fir.
Thanks again.

Attach your 2x4 like you said then glue and nail a 1/2 piece of plywood the entire length of each joist.Don't worry so much about the insulation I would worry more about the load your putting on the joist.

Doing as W.J.Parker suggests will help the floor strength. Just wanted to chime in to clarify a few things for future reference. The plywood on each side will not be a "flitch plate". Flitch plates are different animals altogether. The plywood will act in shear to try to get the 2 x 8 & 2 x 4 to work together to make an 11" deep joist. The plywood will do little to add to the overall strength of the beam because it will not be continuous (only 8' long max.) and only the plies that run with the grain of the joists will try to resist bending - therefore the plywood will really act to hold the pieces together.

Since your span is only 13', an arrangement like this will probably work well. This is not a universal system that can be applied randomly. The 2 x 4's & 2 x 8's are different grades of Doug. Fir so each will handle load differently. Plywood typically comes in 8' lengths and with different # of plies.

It is a good suggestion for this case only. Be careful using this system elsewhere.

You only build it once, and you'lllive with it for a long time. Take up the subfloor and replace or sister the joists with 2x10;s and do it right. The labor saved will more than compensate for any additional cost, if any.Why invent the square wheel when round ones already exist?

Thanks again for the responses. Based on your comments and a quick look at lumber costs it will probably be less costly to install a new ceiling below the existing 2"x8" 16"oc one using 13'x2"x6"s spaced 24oc. I believe this will be strong enough to support the blown in insulation and 5/8 drywall.
If I'm off base here please let me know?
Thanks again.

quote:

Originally posted by Richard Hetzel:
You only build it once, and you'lllive with it for a long time. Take up the subfloor and replace or sister the joists with 2x10;s and do it right. The labor saved will more than compensate for any additional cost, if any.Why invent the square wheel when round ones already exist?

Thanks for your thoughts. Unfortunately the sub floor is well and truly screwed and glued in and the framing of the bedroom is completed so I really have no choice but to work from underneath the floor.

How many joists are involved in this project? You could skip the plywood cost and if space allows place a 2x10 in between each existing joist by notching the ends of the 2x10's to allow for the drop you need and place them on your garage wall top plate and attach what we call saddle buckets to the end of each 2x10, the buckets basically are joist hangers with strap like legs that wrap over and fasten to the plate. Just an idea!

Thanks PCBuilder.
There are 36 joists in all spaced 16" oc. As the current floor is not overly springy and the added depth is needed only to get in more insulation and support the drywall I was thinking of putting in a second layer of 2x6s spaced 24"oc directly below the existing floor joists. Spaced this way every 4th 2x6 would line up with the current joists providing a little extra support (can't hurt) and because the joists would be offset it should decrease the transfer of cold and sound from the unheated garage below. Now to find out if that spacing will be be ok to support the attachment of the fire rated gypsum ceiling.
I was thinking originally of attaching 2x4 to the current joists but I believe that would place more load on the joists and provide little if no support.
Thanks again.

The minimum requirements for 5/8 drywall on ceilings:
At 16 inchs o.c. the drywall can be hung either parallel or perpendicular to framing. At 24 inches o.c. it can only be hung perpendicular to framing.
Always double screw your ceilings 12 inches o.c. and gluing the drywall with adhesive is a good practice to get used to. Gluing is not recomended, but it is obvious to see the advantage in the added strength of gluing subfloors, so I HIGHLY recomened figuring in the extra cost of gluing the drywall to the framing.

I ment to say gluing is not required, but recomended.