Stringers below ceiling joists; also adding plywood plates on top of walls for air barrier attachment
I believe I’ve read in GBA the idea of putting 3/4″ plywood strips on top of both exterior and interior walls for attachment and sealing of interior air barriers. It is definitely discussed in FHB ( https://www.finehomebuilding.com/2014/03/05/a-practical-air-sealing-sequence )
My question is what is the nailing schedule for such a modification? The trusses or rafters are typically nailed to the top plate. However adding a piece of 3/4″ plywood and using the same nails as before assumes that (1) the plywood is fastened with enough appropriately sized nails that it is “one with the top plate,” and (2) the plywood grips the nails the same as the typical top plate. If these assumptions are false, then probably longer nails should be used. But none of this is spelled out in the code (for example, 2015 IRC Table 602.3(1) ). Is everybody just nailing down the plywood similar to a top plate and calling it good? Or has anyone actually engineered this connection?
A similar question — apparently in New England it is common to attach 1-by stringers to ceiling joists. How wide are these stringers and how are they fastened? Are they 1×3 or what? A couple nails at each joist intersection? Again I don’t see this as part of the prescriptive code, so are people just doing whatever and hoping the building inspector OKs it?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I'm curious why one wouldn't just seal the exterior sheathing to the top plate and seal the top plate directly to the drywall. Preferably with gaskets.
Longer nails make sense to me (otherwise you are relying on the top plate to plywood adhesive).
Using a plywood top plate as part of the air barrier is more common with thick walls - double stud walls, Larsen trusses, even thick foam if the air barrier is going on the outside. The plywood plate extends from well inside the top plate to the outside of whatever your wall system is. Also in designs where the air barrier transitions from the outside of exterior walls to interior ceilings. The plywood plate is very rugged and will survive the various insults dealt to it during the framing process, and it provides a nice wide surface to attach the interior and exterior membrane air barriers. But it does change the framing and finishing inside and it's always seemed to be a fussy detail to me.
There are a number of options for making the connection from truss/rafter/joists to plywood plate to top plate. The plywood can be glued and nailed and treated as another top plate. It would be prudent to use longer nails if toenailing the rafters to the top plate. With longer nails, the fastening schedule for the plywood isn't really important. You don't find a nailing schedule in the IRC because the additional plywood top plate is not a recognized construction method in the code. Technically, this means that the connections are supposed to be engineered, but the local code official will have the authority to make the final call on this.
In high wind and/or seismic areas, this connection must be engineered as is customary in these areas. In these areas, steel strapping is needed to connect the roof to the walls. These would be a PIA with the plywood top plate extending to both sides and I would think a different air barrier design would be required, with the plate extending either to the inside or outside but not both.
As far as interior stringers, they would certainly be compatible with a 3/4" top plate and they would reduce thermal bridging through the ceiling to a degree. Since they are just hanging the drywall, engineering that connection is not really an issue. The only exception would be where the drywall is being actively used as a shear diaphragm, and then engineering would be required. Diagonal shear bracing could be added to the attic side of the joists if necessary.
A decent engineer/architect should be able to specify all of these details if necessary for your particular design. If your DIY'ing your own house, your local code official will generally let you know if you are crossing the lines for your local conditions.
Peter, Thanks for your thoughtful answer. We are DIYers building a double stud home in Pennsylvania Climate Zone 5.
Good point about stringers (or strapping) providing additional thermal break. But I'm still curious about what size members are used and how they are nailed. Drywall weighs a lot and I can understand that IRC Table 602.3(1) calls out fasteners every 7" at all edges and supports which amounts to about 50 fasteners per 4x8 sheet. But if stringers are nailed at 16" spacing to roof trusses spaced at 2 feet, that's only about 30 fastening points to hold up the drywall and the 17" of cellulose above it. OK, so as I'm writing this I think I've answered my own question. If I use 2-by-3s and 2 nails at every fastening point, that'd be about 60 nails, more than the code for drywall. I think that is a fair argument that stringers attached as I described would be suitable, even if not prescriptive.
Dennis,
Nail the plywood to the top-plates using two 2 1/2" common nails over each stud. Resist the temptation to put nails in-between where they can interfere with holes for plumbing and electrical. The connection to the trusses should be run by your truss supplier. The typical hardware (like truss-clips) won't work. Probably the best alternative are truss screws. https://www.fastenmaster.com/products/timberlok-heavy-duty-wood-screw.html
The number of nails required for drywall is determined by a number of factors not present when attaching strapping to the ceiling. The poor pull-out strength of gypsum, the poor strength of the fasteners used, the desire for flat surfaces, and to allow the drywall to provide some shear to the wall or ceiling. A 4' x 8' sheet of drywall weighs around 50 lbs. the pull out strength of a 2 1/2" nail embedded 1 3/4" into the trusses is roughly 60 lbs. So given each nail can on its own support a sheet, you don't have to worry about the connection even with a substantial amount of cellulose above.
Dennis, stringer is the name for the framing that supports stairs. The word you're looking for is strapping. You're correct that it's common in New England. Typically 1x3 (3/4" x 2 1/2") spruce, but sometimes white pine or other softwood, sometimes 1x4, and occasionally thicker than 1x. In high performance homes we often strap with 2x material or double up 1x strapping to allow space to run wires and install low-profile LED fixtures below a fabric air barrier/vapor retarder at the ceiling.
These days, strapping is typically installed with 8d ring shank nails, two per joist, with the strapping 16" o.c.. (24" would require 5/8" drywall to reduce sagging). Some builders screw the strapping into place, but that's pretty rare. Some use smooth-shank nails installed at a slight angle, for a dovetailed effect that resists pullout. I've never heard of a failure with ring-shank-nailed strapping.
Strapping was originally a way to create a level surface on uneven framing. My 1830, timber-framed house has beams notched for full 1-inch strapping. But with dimensional lumber, that's rarely necessary. It has other advantages, though.
Michael,
At the FHB conference there should be a workshop where delegations of builders from both coasts agree on where to draw a north-south line of a map beyond which strapping is either allowed or not.
Great idea, Malcolm. Maybe I'll work it into my Pretty Good House sessions...
Screws for attaching the trusses thru the top plates & plywood with engineering load info
Strong-Drive® SDWC TRUSS Screw
https://www.strongtie.com/strongdrive_interiorwoodscrews/sdwc_screw/p/strong-drive-sdwc-truss-screw