Looking for a recent discussion of sheathing a ceiling before roof trusses
Vishnej
| Posted in General Discussion on
I vaguely recall either listening to the podcast or reading an article sometime in the last year with a discussion about how to avoid the hysterically dangerous activity ‘walking the top plate’ to set trusses, which is presently nearly always done sans the required safety gear. I think that the idea they hit on was to frame in and sheathe a lightweight floor platform that’s just strong enough to walk on, then lay down the trusses on top of that.
It might have also mentioned something about a ceiling service cavity and/or the air barriers being feasible even with recessed can lights.
The search function is not fruitful.
Anyone recall something like this?
Replies
What you propose is called "scaffolding".
I understand that scaffolding can suffice here, but the concept I'm trying to dig up involved this osb/ply-sheathed floor diaphragm being a permanent presence between the top plate and the bottom flange of the roof truss.
I finally found something. The first few comments attached to this post - https://www.greenbuildingadvisor.com/article/lstibureks-rules-for-venting-roofs propose the idea. I swear I heard it mentioned on FHB podcast as well at some point, but no luck so far.
I don't know how this would serve as a floor diaphragm, even for an attic floor, as it would be on the bottom of the trusses.
Advantage: you could stand on it while you stack the roof.
Disadvantages: It involves a lot of plywood. It involves a lot of framing lumber at a stage in construction when there shouldn't be much left over. It involves a lot of labor. Once the roof is stacked it would have to be screwed to the bottom of the trusses. All this sheathing would have to be alligned very accurately in order to make do this. All work on the trusses at the top plate would have to be done from above or from the outside. Once the roof is sheathed all plumbing, electrical, mechanical, and insulation would have to be done by crawlling in the attic. Probably a whole lot more.
(Brainstorming, stay with me; I like to poke these ideas with a stick until they yield their merits and weaknesses)
Assume that you do it like this: After top plates of the upper floor are up and doubled, you roll out a truck with preassembled panels featuring 2x8" sawed lumber sistered together (offset to deal with joints) at 24" OC, supporting a layer of plywood on top (every other panel has a gap missing, to be applied later). The bottom comes pre-strapped with 1x3 furring strips for drywall and to stiffen the panel. The cavity space is uninsulated, leaving a service cavity to run wiring or ducts. You frame this as if it was another floor, with a proper rim joist, and complete the plywood laydown, tying everything together as a structural floor membrane. After that's done, you tape every single seam on that layer: This is your resilient air barrier. On top of this, you lay down your roof trusses, which get stood up and braced against the ply as walls would be. Access to the roof is via lift equipment butted up against the last part of the roof to be trussed, and just before this is laid down, you use the open space as an easy access point from which you can sheath the roof.
All plumbing, electrical, and mechanical work is done from below the ply. Insulation is primarily via lots of loose-fill cellulose blown in above the ply at your leisure later in construction.
Trusses are screwed into the sistered joists from above with with Trusslok structural screws. Unclear if messing with the sequencing here bears benefits or not.
2x8's are not sufficient to free-span large upper-floor rooms *sufficiently well for storage or cocktail parties*, but deflection is proportional to load, during framing load can be minimal, and midway through framing you will be attaching them to the trusses.
Drywall is attached to the 1x3's, below the 2x8 joists, and (speculatively) truss uplift may pose less of an issue to Airtight Drywall Approach style airsealing as a result of those joists. If you don't choose to go that route, you may elect to skip things like a room's wall-ceiling edge transition during drywalling, and just cover it with trim.
Mostly, the ply dramatically improves safety and the capability to do airsealing even under the oppressive fad of recessed-can-lights-everywhere interior design. It does give the freedom to change ceiling fixtures easily at a later date. Blowing the cellulose is made into an easy operation with zero expensive/time-consuming airsealing boxes to fabricate, and the building can be dried-in and climate-controlled regardless of the sequencing of electrical work, mechanical work, drywall work, etc.
Would this approach (and some fastening details) help to improve wind resistance? That membrane will structurally transmit wind forces at the top of the exterior walls throughout the building, will keep the top of the gable end wall significantly braced against bending inwards just like a floor will.
A 40x50 house adds 2000 linear feet of 2x8 and 2000 square feet of plywood with this approach, adding perhaps $4000 to lumber materials cost, plus whatever fasteners run you.
How might this idea weaken a house, how might it strengthen a house?
How might it complicate or simplify construction? It seems like we might subtract from the number of lifts & scaffolding involved, at least during the truss-setting operation, relative to an OSHA-approved roofing crew (rare as a unicorn at present).
Room-to-room sound insulation may be one of the downsides here. Perhaps the portion of that 8" which isn't used by trades could be filled with more cellulose as acoustic insulation after drywalling, or installed with some Rockwool+board blocking before sealing it in with drywall.
You could also pre-insulate the panels with cellulose + Insulweb and tell trades to dig a path out of it as they want, like one of those custom-cut foam toolboxes.
I am curious if this poses fire-resistance benefits; Dense-pack borated cellulose is asserted to be a fire-stop all on its own, and having the air barrier above a layer of it stops upwards convection before you end up impacting the delicate trusses and their heat-sensitive steel truss plates.
If any of the trades want to chew on those 2x8's to fit their stuff through, once the trusses are locked in they can go ahead, because the trusses are a redundant load path (provided their cuts are offset a little bit joist to joist to avoid hinging action it shouldn't even impact the structure significantly because of the ply). So long as they stay on this side of the plywood (and they have 8" actual to work with) they have almost free reign.
I like the idea of nail-laminating sistered sawed lumber because it eliminates hinging at the joints better than a truss plate to make indefinite-span joists (while not introducing a steel thermal bridge), and because you can use garbage lumber by comparison to the standards for one board. If you prefer, you can replace that detail with a long wood I-joist.