How to find joists through sheetmetal ceiling?

Question

I have an antique sheetmetal ceiling of historical importance in my centenarian vintage home. As I am currently in the last stages of installing an AC system and the parts of the house that have vintage ceiling get exposed spiral pipe as to avoid ugly soffits or other forms of ceiling destruction (looks very, very cool), I actually need to suspend the pipe in one or two places. The ceiling is on the first floor and there is a carpeted bedroom above. From demolishing a patch of such a ceiling in another part of the house, I know (with a high degree of certainty) that between the joists and the sheetmetal is lath and plaster. I suppose I could anchor it into 3/8" lath but I would prefer a joist for extra firmness.

The problem, as you can imagine, is locating ceiling joists through a sheetmetal ceiling. I tried a really nice stud finder (Bosch) but that did not help much as the sheetmetal is blocking the signal. I also tried using it through the carpet upstairs but the experience was similar. Any idea how I can find a joist with a relative degree of certainty, as any mistake will leave a visible hole in the ceiling?

Answer 1

You might be able to pull the carpet a bit in a corner of the room. Perhaps you can find nails holding down the subfloor to the joists. A good place to start would be a closet where it would be easy to hide and tack back down. From this point it might be easier to guess what the typical joist spacing would be.

Typically ceiling joists will be spaced 12", 16", 19.2" or 24" OC. Rather than damage the ceiling it would be easier to pull up some carpet upstairs and try driving a nail or screw at each of these lengths and see which length grabs a joist.

Answer 2

Any of the metal ceilings I removed in old buildings were mounted on a secondary layer of 1X material either connected directly to the bottom of the joists or suspended from the joists by wood supports, tied to the same layer of 1X. Either way, the metal ceilings are typically nailed in given places were the nail becomes part of the look of the ceiling. At least on the ones I took apart... This is usually in the corners since the metal pieces were small, at least in the field, the border were done in longer straight runs. The border was also had a texture to it so when the round headed nails were used, they blend in.

Back at the matter at hand. The increment of nailing, the size of the metal pieces and the spacing of the joists never agreed. The 1X material that was added gave the metal ceiling its nailing. The joists will not be at the back side of the metal ceiling. There will be nailers at the dimension of the ceiling panels in one direction.

You can determine which direction by gently pushing mid span between the nail heads, if there is no nailer, the ceiling will give a little. If the backer is there then it should not give at all. Do not do this where there is a nail, surely there is a nailer there. This may take some careful searching, the ceilings I worked on actually had fake nailheads as part of the pattern in the ceiling.