Existing Wall Reinforcement – DBL 2×4 or Single 2×6 ???
I have a 15+ year old house that’s two levels with a walkout basement in the back. The back walkout wall is framed with 2×4 studs at varying spaces (14″ – 18″) and not aligned under the floor joists above. The wall provides support for the 2 levels of flooring above and the roof… so there’s a decent amount of load. I’m about to add a new deck off the wall at the floor level directly above this wall and want to reinforce the framing for the additional load. The existing wall has a single 2×4 sill plate, 2×4 studs with 2×4 blocking at mid-height, and a double 2×4 top plate that picks up the floor joists and rim joists for the floor above. I want to either replace the existing framing with new double 2×4 studs at 12″ oc to align with the above floor joists or replace the existing framing with new single 2×6 studs at 12″ oc to align with the above floor joists. My plan is to basically remove and replace each stud one at a time to keep from having to shore everything. I also plan to replace the exterior sheathing with new OSB sheating as I work my way along the wall (due to existing water damage). I’ll replace the studs one at a time and replace the sheathing in 4 foot intervals.
My qustion is which option would be recommended? If I go with the double 2×4 stud option then I can work with the existing 2×4 plates top and bottom as-is. If I go with the single 2×6 stud option then I’ll have to add an additional piece of wood along the existing top and bottom plates to accomodate the wider studs. Any recommendations or advice would be greatly appreciated!
Replies
Wall strength
I'm a mechanical engineer and couldn't help but do a quick analysis. Since the wall is almost completely in compression, I think you are better off doing double 2x4 studs. In the simplest terms, stress = force/area. A double 2x4 has 10.5 square inches of cross sectional area, where a single 2x6 only has 8.25. From another standpoint, if the concern is that the studs will buckle, they will buckle in the weakest direction. A double 2x4 is 3" thick in its weakest direction, where a 2x6 is only 1.5" thick. NOTE: in order for this comparison to be true, you should tie the adjacent 2x4's together with some combinations of screws, nails, and/or adhesive. Another note: since this is an exterior wall, the OSB sheathing will give a lot of strength to each stud in buckling, so I wouldn't worry much about that.
In terms of bending (only wind loads in your case, I think), the strength is proportional to the thickness * width^3. So if a double 2x4 has bending strength of (3")*(3.5")^3 = 128.6, then a single 2x6 has (1.5")*(5.5")^3 = 249.6. This means a single 2x6 is almost twice as strong in bending as a double 2x4. Another argument for the 2x6's would be the insulating value. You can get much more insulation between single 2x6's than double 2x4's, and you will also have less thermal bridging.
I guess the bottom line is, I don't think you can go wrong either way. After considering everything, I think the 2x6's might actually be a little better, but I'm not sure it's worth the hassle. This is all very theoretical, hopefully someone with more experience can give you a better, more practical answer.
Spacing
You don't need 12" centers if your joist are 16" centers. And unless there are rot porblems with the old studs, you need not take them out. IRc reguires that the joist bear within 3" of a stud, so simply adding 2x4 studs under the joist is sufficent. Double them if you like, but it is not neccessary. For 2x6 walls code only require them to be 2' on center, so doubling them and placing on 12" centers is way over kill and a waste of rescoures IMO.
Your new deck will only share half of its' total load with the existing. Most important is the conection of the ledger board to the existing structure. that is ussually where deck failure occure. Check your local codes for the ledger board requirements. You may also want to do a search on the new or impending changes for deck attachments. If you have exposed interior studs and floor joist, with easy access to the rim joist the newest attachments methods are easy to make and is money well spent.