Questions About Overhead Slabs
I’ve done several suspended slabs and had a few concerns about the article “Pouring a Structural Slab” (7/12). I noticed that none of the structural steel was primed or painted. Given the job’s proximity to water, I would assume that the more humid air would quickly rust the steel and that Massachusetts code would require the exposed steel to be painted. During temperature swings or humid weather, moisture in the air will quickly condense on the unpainted steel in the enclosed storage area and continuously drip onto the concrete floor, causing staining and wetting the owners’ belongings. At a minimum, the pans should have been painted with a rust-inhibiting primer and top-coated with enamel before they were set in place.
I was also surprised the builders used 6x6 mesh instead of a rebar grid. Because this is a structural suspended slab, a 12-by-12-inch grid of 1/2-inch rebar should have been used. Normally 6x6 mesh is used on slabs-on-grade, where it keeps the slab surfaces more or less in alignment if the slab cracks. But unless great care is taken to pull the mesh up into the middle of the concrete during the pour, it serves little purpose. This was obviously not a low-end project, so I would think the additional cost of the rebar would have been justified.
Kudos to the author for framing temporary support walls under the steel pans “just in case.” Contrary to the engineer’s opinion, these supports are necessary, in my experience. I typically use courses of pipe staging with adjustable feet, then place 2-by-8-foot concrete wall forms on edge across the top. It’s easy to adjust the height of the staging with the adjustable feet to provide firm bearing. I leave the supports in place for the 28-day cure cycle when possible — and always for at least a couple of weeks.
Doug Amsbary,Sugar Hill, N.H.
Author Mike Rockwell responds: Thank you for your comments. First, there is no code requirement in Massachusetts for priming or painting structural steel for residential applications. Having had a series of inspections from our town building department, I’m fairly certain this would have been brought to my attention (trust me, our inspectors are not shy when something doesn’t meet their approval). So far, condensation has not been a problem, even though we’ve had a lot of humid days with temperature swings of 20 degrees between night and day. I believe the steel doesn’t sweat because, being in the ceiling, it’s well-ventilated and stays at ambient temperature. (I wish I could say the same for the painted concrete floor below, which has been getting fairly slick from condensation — to the point where the owners plan to try a dehumidifier for the worst part of the summer.)
Would rebar mesh be stronger than 6x6 wire? Certainly, but again I believe it would have been overkill. I can’t give you the safety factor that the engineer calculated, but I’m sure it was there. None of us would want to risk our professional reputations by underbuilding something as critical as a suspended slab. (Overbuilding we’ve been accused of; never underbuilding!) As I stated in the article, both the engineer and the forms contractor have experience with this type of construction, and I’ve worked with both of them for more than 10 years.