Rafter and Joist Woes

16 years ago

Hello.

I am remodeling a bedroom above a tuck-under, 16x20 garage. I have it pretty well gutted at this point. I have a few concerns and problems, however:

1. Floor joists are 16' span, 16" OC, 2x6's. Floor is quite bouncy. (I posted about this a half a year ago, but haven't solved my problem(s) yet) The floor joists run transverse to the garage below, which probably rules out an intermediate beam, unless I used some special joist hanger to tie it into the garage door header (which may not support the extra load).

I should sister with 2x10s to get L/360 deflection, but I don't have this kind of space to work with - only about 2-2.5 inches to the garage door rails. I have thought about an 1/8" steel strap on the bottom of the joists (as recommended by brickeyee), or sistering with a 2x8, or maybe 2-2x4 sisters (on the sides of the lower portion of the 2x6's), or even a flitch beam (labor intensive).

The joists have drywall on the underside, and I don't know if I want to take on this reinforcing project at this time, with winter fast approaching. I have thought I would fix this problem "later", but if someone came up with a miracle solution, I am open to it...

2. Rafters are 2x6's, and I could use about R-49 insulation here in SD. I have flirted with furring out with 2x4s, adding layers of r-15 batts, plus a ventilation chute made of 3/4 foam insulation, for a grand total of about R-35. We would also use 6 mil. poly vapor barrier. I have also thought about multiple layers of pink extruded polystyrene, to the tune of about R-40 (a bit expensive, though), and also spray in isocene foam (I would assume most expensive of all) Thoughts?

3. We have thought of using grooved engineered siding (4x8 sheets) for the interior wall finish. This has its drawbacks, cost, labor, etc. We are now thinking of using 1/2" drywall, taped and textured. -- My only concern about the drywall is the potential for cracks to develop?? due to the problem with the undersized floor joists. Is this a valid concern??? other thoughts?

4. One final one - The window on the south wall has no header - only a 2x4 on weak axis where a double 2x6 or 8 should be. It is about a 4.5' wide opening. It is a bearing wall, the rafters tie in just about 6 inches above the window. The only saving grace, I think, is that the top plate just inches above the window, is a double 2x6. I don't think it is correct. However, the window opening and room have been in existence for about 55 years. Thoughts?? I guess I could reinforce the 2x4 with a steel angle or similar (I really hate to rip out framing at this stage)

Thoughts on any of this mess? Any help would be much appreciated....

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Comments (3)

davidandkasie
16 years ago
can the GDO railings be lowered? if not, why not raise the floor in the room above? maybe sister the 2x10's so that they stick up some instead of down. this would allow them to sit on top of the existing walls. if you sister them to hang below, they would have to be notched out at each end.

2x6 on a 16' span is going to be bouncy. a friend has a deck that is 12 ft and framed with 2x6 and it is bouncy anywhere that is not close to a support.
Brewbeer
16 years ago
For the deflection problem, do you have an option for a steel beam in the center of the 16 foot span supported at either end? Is elevating the floor as recommended by david an option?

For the insulation, I would be inclindes to go the foamboard route. Extruded polystyrene is R-5, spray polyisocyanurate may get you R-6 or 7 per inch. 2x6s are typically 5.5 inches high. A 1.5 inch airspace would leave 4 inches for foamboard (R-20). Firing out with 2x4s laid perpendicular to the rafters would get you another 3.5 inchs (R-17.5), and greatly reduce the thermal bridging through the rafters. Another layer of foamboard and 2x4s would get you to R-55, but R-37 using foam and crossing the firing would probably be pretty darn good.

It's a tough call on the drywall. It is inexpensive, but it is also time consuming.
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mike_g_sdak
Original Author
16 years ago
Thanks for the replies.

First, I will rule out the building the floor up - the bedroom floor has 1.5" subfloor+linoleum tile + 6 year old laminate planks. We would like to keep the laminate floor at this time, plus furring the rafters down will already eat up some valuable headroom, good thought, though.

I don't know too much about GDO rails. I will need to look at that a bit.

As far as the insulation, I am thinking about this - 1). 1 to 1.5" vent channel, probably constructed with 1/2" or 3/4" styrene with spacer blocks to form the channel (about R-4 for the vent chute) 2). 2-R15 High Density fiberglass batts (7" total thickness, R-30 total) This will require about 2.5"-3" of furring strips, laid on top of the rafters & plywood gusset plates to attach on the sides 3).Either 1" or 2" Extruded polystyrene sheets (R5 to R10) over top of the furring strips, with taped seams. Total = R-39 to R-44.

About Number 3, I am undecided. If I use 1", I think I could use about a 2.5-3" drywall screw. With 2", I may have to use 1x2 or 1x3 straps laid transverse to the rafters to attach drywall. Maybe I will need the straps with the 1" foam as well, I don't know.

Thoughts on this?

Rafter and Joist Woes

Comments (3)

davidandkasie

Brewbeer

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Rafter and Joist Woes

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davidandkasie

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