Truss uplift prevention
Hey, I am a construction manager and owner’s rep in the ski town of Steamboat Springs, CO. I am working on a house with long trusses that span over some non-load-bearing interior walls. There is already about a 1-2″ – 3/4″ gap between the bottom truss chord and the top plates of the wall. I know this builder isn’t necessarily going above and beyond on attic insulation and air tightness, so I am anticipating that there is a chance of truss uplift. The builder has in the past shimmed the gap between the bottom chord and the top plates and has even gone so far as to screw at an angle a truss screw from the top plate into the bottom chord to suck it tight and prevent the uplift.
I know that the screwing might not be best practice, but is it really that bad? What about the shimming?
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Replies
Reese,
If the trusses aren't free to move by being fastened to intermediate walls one of two things can happen: The trusses will lift those walls, or the truss chords will distort and can cause them to fail. The truss drawings typically include specifications requiring truss clips to be used. Clips, and keeping fasteners 16" away from the ceiling / wall intersection are the standard way this is dealt with.
Malcolm, I've never seen truss clip details on truss designs here in the northeast--I wish they would do that!
i remember having the conversation with my plaster guy over 20 years ago. How is truss uplift still a mystery?
I'm in an online architects' group and the topic was recently raised there. A majority of commenters either didn't know about it or didn't believe it was an actual issue. The fact that the issue and solutions have been known for 20+ years shows just how slowly construction knowledge spreads in the industry, at least among those who aren't seeking out the latest understanding and best practices.
Malcolm, If you are using something like SIGA Majrex as a ceiling air barrier, all those truss clips are going to be penetrating that air barrier with 2+/- inch wide slits. Bigger problem - you can't nail into the top of the wall with that vapor barrier across the top of the walls. Is there an alternative way to install these? I am thinking of inverting the clips so that the slotted connection is nailed to the wall top plates. But the truss clips are fairly thick, so likely would have to notch the top plates.
Another thought would be to predrill holes in the wall top plate at the truss connections. The drilled holes would be slightly bigger than a nail, and then hand-nail leaving the nails proud of the top plate. Basically the same concept as floating basement walls to account for any slab heave. If the air barrier is sealed with acoustical sealant at each truss around where interior walls attach, then air leakage at the nail holes would be minimal. Thoughts on this?
As an aside, i think Steamboat has fairly consistent year round humidity, so maybe truss uplift is less of a problem than in areas that see larger swings in humidity through the year.
Tracy
Tracy,
The severity of the problem definitely varies depending on climate, length of the truss, and how the attic is insulated.
The common practice here is to frame all the interior walls before adding the ceiling air / vapour barrier. When those walls are framed we run strips of poly between the two top-plates, which get lapped and sealed with the poly on the ceilings, so there is no conflict with the clips.
But I'm not understanding the problem you are describing with a continuous membrane on the underside of the trusses. The sequence would be:
- Stand and brace the trusses.
- Run the ceiling membrane.
- build the interior walls
- Nail or screw the truss clips to the top face of upper top-plate on the walls through the membrane.
- fasten to the side of the bottom chord of the truss through the slot.
To Micheal's point: I should have been clearer. Although truss clips are called out, their use still isn't that common. Lots of builders here just nail the hell out of them to try and stop the movement, and many production builders tack the walls to the trusses using just enough nails to keep them in place until the walls are boarded - relying on drywall to keep the walls where they should be.
Here is an alternative fastener I have seen mentioned.
https://www.fastenmaster.com/products/nlb-connector
Ooooh kyle_r I like those
Reese,
What is unclear is whether the sleeves are an effective air-barrier.
If you put some silicone lubricant on the fastner, that should provide an air seal to the inside of the sleeve while still allowing for movement. Acoustic caulk might work too, but I think that stuff is probably too thick to get into the presumably small gap between the fastener and the sleeve. If the sleeve itself doesn't seal well to the top plate, some "regular" caulk would take care of that -- just smear some into the top of the hole prior to inserting the sleeve.
Now that I think about it, there is such thing as "high vacuum grease" that would probably work great for air sealing the fastener to the sleeve while still allowing for movement, because that grease is intended for this sort of application.
Bill
Thanks Malcolm, Your practice of running strips of membrane over the walls, or between the top plates, makes sense. My sequencing was flawed in thinking of installing the clips to the trusses, covering the whole ceiling with membrane slicing the membrane around the clips, then standing the interior walls. The way it would really be done would be to install the clips from above after the walls are stood, but would involve crawling around the attic to do so.
With this sequence (modified yours slightly), the clips can optionally be installed from the floor level, and the ceiling membrane taped after.
- Stand and brace the trusses. A suggestion i saw was to install all roofing before interior walls to load the trusses down.
- Run the ceiling membrane.
- build the interior walls with strips of membrane at top of wall
- slice ceiling membrane at top of wall to allow access to install clips (or crawl around attic installing clips)
- Nail or screw the truss clips to the top face of upper top-plate on the walls through the membrane.
- fasten to the side of the bottom chord of the truss through the slot.
- tape top of wall strips of membrane to ceiling membrane to ensure air seal at top of wall (and repair access hole).
Tracy,
One small difference in how we do it here is that the strips between the plates get folded up onto the side of the upper top-plate, and the ceiling membrane gets folded down onto the same spot. That gives you solid backing at the joint between them, allowing you to staple as well as use tape or sealant. The drywall then sandwiches them as yet another layer of protection.
See
Strong-Drive® SDPW™ DEFLECTOR Screw - long screw with a plastic sleeve - made for interior walls to trusses
https://www.strongtie.com/strongdrive_interiorwoodscrews/sdpw_screw/p/strong-drive-sdpw-deflector-screw