Damaged Concrete Ceiling


Old 01-17-08, 02:04 PM
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Join Date: Jan 2008
Location: New Jersey
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Damaged Concrete Ceiling

I recently had central AC installed. To run the ducts to the first floor, the contractor needed to cut through 10 inches of concrete. He was quite surprised to find our first floor ceiling is 10 inches of solid reinforced concrete. In any case, obviously the holes for the ducts were not exact and they now require me to repair the holes around the ducts. I have had 2 suggestions.

1. Fill the excess space with concrete.

2. Stuff the holes with something (ie newspaper, wood, and filler) and then just plaster the holes from the 1st floor (for ceiling) and repair the wood floor from above.

Please give me your suggestions. I have included a graphic of the problem below

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Old 01-17-08, 02:39 PM
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Join Date: Mar 2005
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Welcome to the forums!

While I've painted a number of concrete ceilings, I've never had to make any repairs other than minor cosmetic issues.
What I would do would be to start with mortar mix. If I stuffed the hole with anything [besides mortar] it would be screen wire or hardware cloth - something like wire lath. After I got the majority of the hole done, I'd switch to a drywall setting compound [like durabond] to do the finishing touches - on the ceiling side.
Old 01-19-08, 09:57 PM
Join Date: Dec 2000
Posts: 1,019
Sorry, but both of you have overlooked a few things. (1) Fire & smoke safety. (2) Unequal expansion & contraction of the materials. (3) Cement shrinks as is cures (4) If the duct is metal (22 gage maybe) it will rust through if encased in cement or mortar. If the duct is duct board the moisture from the mix may cause mold to grow.

Your goal is isolate the duct from the slab; seal against smoke & fumes in a fire, and fill the hole structurally.

For a ten inch thick slab, glue 7” or 8” wide strips of “mineral wool” insulation (not fiberglass) to the duct at mid point (fire resistant isolation material). Adjacent to the duct on all four sides create a 1/8” wide channel (1” to 1-1/2”deep). Use 1/8” thick wood or plastic. Tape it to th duct. The channel will be filled with a non sagging “NS” fire resistive gray caulk later. (The 1” to 1-1/2” depth will allow installation of a collar if someone wanted to add one later). (The caulk seals against passage of smoke & fumes in a fire and moisture otherwise).

The easiest way (not the only way) to repair the hole is from above. This means shoring from below. It's not a bridge or overpass. You don't need steel shoring or heavy wood members, just some common sense. Spring 1 x 4s between board stock or plywood.

The 1 x 4s are cut maybe 1/2” longer than required for a plumb installation. Plywood spans the hole. Plywood is laid on the floor (padding under the plywood to protect the flooring). 1 x 4s are sprung (bowed & kicked into position) between the two. Mix some concrete (not too fluid) and gently fill the hole. Allow to cure for 1 to 2 days then remove the shoring & the shim stock for the channel. After seven days fill the channel with fire resistive caulk.

Tip: if you want a smoother finish on the lower floor ceiling place a piece of cardboard covered with tin foil on the plywood or board stock before positioning it on the ceiling. This creates a indentation in the ceiling at the patch. When the shoring is removed, mist patch with water. Mix sand (3 parts) & cement (1 part) together into a stiff mixture. Apply with firm pressure for adhesion. After 30 minutes, mist with water and float smooth.

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