Strapping and leveling a ceiling?

[MushCreek]

Anybody have experience with this? I'm in the middle of it right now, and it's not as simple as it looks. My living room is 15 X 16. The trusses are on 24" centers; the strapping runs perpendicular on 16" centers. I tried to use my laser level, but it's not level enough. There are 130 intersections, and in theory, one of them is the lowest. The question is- Which one, and how do you find it? Once I find it, I have to shim the high spots down to match. That's a lot of shims, although I assume there will (hopefully) be intersections that are 'close enough'. How fussy should I get with this? 1/8"? 1/16"? I only have two large rooms where the ceiling is really visible. One other one will be coffered, and another will have a tin ceiling, which will hide minor variations. I'm not worried about closets and bathrooms.

So far, I have been stretching strings to try to find the high/low spots. My trusses were pretty wavy when new, and they were exposed to months of weather before the roof was dried in. The good news, if there is any, is that the trusses should be pretty stable now due to the slow rate of building. I'm trying to speed up this tedious process. I know that the ceiling won't be level, but I'd like to get it sort of flat, or at least not noticeably wavy. Now I know why tract home builders spray popcorn on the ceiling!

[Don_P]

All I can say is screws for the strapping so you can adjust easily, strings, a french dictionary and quit when it's close enough. Happily it's for yourself, you get to be the judge of what close enough is. I've been called back over 1/32 and had others who couldn't care less as long as the roof didn't leak. If there is a light up tight and low angle to the ceiling plane, get mighty close.

[flyingvan]

The cottage's bedroom had some ceiling joists (no trusses) that were off by as much as 3/4".   I just bridged it all with a 2x12 on top with pre-drilled oversized holes and screwed it in to the tops of each, mid span.  It pulled the low ones up and pushed the high ones down.   Bonus is it provided a nice way to traverse the attic without touching the insulation

   best picture I have of the joists, before the 'catwalk' (this note added after Don identified it as a catwalk)

[Don_P]

We call that a catwalk, usually there is a 2x4 or 2x6 nailed to the side of it upright to make it stiffer, they are an excellent idea.

[Alan Gage]

I attached the sheetrock right to my trusses. Then I installed about 2 feet of cellulose and afterwards calculated how much weight that was and realized it was probably a bit more than I should be putting on sheetrock with 2' OC trusses. After a few weeks I put a straight edge along the ceiling and to see if the sheetrock was bowed from the weight. Uhh....yeah.

I don't remember how much but there was considerable droop between each truss. Haven't checked again since then but as far as just living in the house I've never noticed it. I wouldn't sweat it too much.

Alan

[MushCreek]

My ceilings won't be all that conspicuous, because they are relatively tall, so you won't get a low angle look at them, unless you are very tall. It occurred to me that I should strive for perfection mostly around the edges, since that's where it shows the most. I'm using crown moldings, and it's hard to make molding look good against a wavy ceiling.

Alan- The weight of insulation is my main reason for the strapping. I'm going to use 5/8" drywall as well for R-49 cellulose.

I put the strapping up with Torx head screws. I've yet to find Philips bits that don't cam out, especially when backing the screw out and re-driving it. My trusses are southern yellow pine, and the exposure to weather made them very hard to nail or screw.

I messed around in the dining room, stretching strings here and there. I've found up to 1/2" error, and some of them are surprising sharp, which would make a very conspicuous hollow.

I can give some advice: Forget doing overhead electrical work until the strapping is done. There are 8 light fixtures in the kitchen, and every single one of them had to be moved for any kind of logical pattern for the strapping and drywall sheet usage. A fixture attached to a truss may or may not be the correct depth once the strapping is leveled and drywall installed. I have no idea what you would do if a fixture 'had' to be in a specific location. Between the trusses and strapping, there are a lot of places that fixtures CAN'T go.

[MountainDon]

I put the strapping up with Torx head screws. I've yet to find Philips bits that don't cam out,

I've had a love - hate thing with Philips head screw for decades; love them compared to slotted but hate them when they slip. Torx are also so superior when driving screws in awkward positions as Philips require so much more pressure to keep the bit engaged in the head of the screw.  I don't mind having to pay a slight amount more for Torx.