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Question:

How do I repair an old ceiling surface that is cracked (it is not sheetrock)?

This is an old house and has wood and some type of dry cement-type material in between the wood slats. I stepped on the ceiling while doing an insulation job in the attic. The wood is fine but the dryed material cracked and separated from the wood slats in the bathroom. The damages area is around 18 inches by 12 inches, and some cracks in other areas of the ceiling. I do not want to sheetrock the whole bathroom if i can avaoid it (too many projects in this ol house). thanks!

Answer:

Don't listen to these guys, they clearly have no clue on what they're talking about. I am a developer at Microsoft so I know a thing or two about computers.
I would say go up to the attic where you made the initial damage. Where the damage was made, go to the opposite side of the wood, and use a box cutter to cut through, should cut enough to see it in the bathroom. Do this on each side of the wood beam, opposite side of the damage. Then go down to the restroom, finish cutting through, ( make sure no wires, pipes, etc are not in way) Cut along beams, then across to other beam, making a square or rectangle. Try not to break up the cut out piece. Keep it and get the exact measurement off another sheet rock slab. Cut the new sheetrock and secure it to the wood beams. Should be exact fit, don't worry if not perfect. From there, get some joint compound and fill it in nicely. Sand down after dry if needed...If not same thickness just add more compound to match it up.
If all you are seeing from the bathroom are cracks: Cut a peice of 3/4 plywood a little larger than the area of cracked ceiling. From the attic side apply a good construction adhesive to the area between the loose ceiling and the wood slats. Don't skimp! Now cover the damaged ceiling from the bathroom side with the plywood and either wedge in place with a 2x4 (called a deadman) or attach the plywood with 2 (or longer) drywall screws, preferable into the ceiling joists. Once adhesive has dried, remove plywood, cover all cracks with self-adhesive fiberglass joint tape, and cover with this coat of quick setting joint compound. Durabond 300 works great for nonprofessionals because it has a long working time but will still dry very hard and will adhere to the existing plaster better than regular drywall joint compound. When dry, cover entire repair with skim coat of regular joint compound. When that dries, sand and paint. Ceiling should last until the next wrong step. If large pieces of ceiling have fallen: First replace larger areas with correct thickness drywall, then procede as descibed above. The washered screws described in other answers tend to create more cracks than they correct.
It sounds like you're describing old horsehair plaster, & fixing it's not that big of a deal, there are just a few steps involved. Basically what you need to do is to round out & stabilize your hole so that it can either be re-plastered or mostly patched w/drywall & then finished w/joint compound. The first step is re-attaching the plaster to the lathing (those wooden slats) & to do that you need these little plastic/metal washers that you can find at a good hardware/builder supply store & these are attached to drywall screws which then get screwed into the lathing a few inches away from the damage until you've got a flush surface again to work with. I'd love to tell you how many you need/etc., but it's just one of those things where you need to be there..... Anyways, once you've got a solid hole across this somewhat level surface take a beat up blade (I like using a 5 in 1) & key the damaged edges which means about a 45 degree angle so that you've got more patching material towards the lathes. It gives a more secure bond this way. Then you either screw in a piece of drywall of the appropriate thickness into the hole & patch & tape from there, or you can learn about plastering, but it doesn't sound like you want to go that route from the question. A quarter to half inch gap is fine for the patching between, & I've even used plaster of paris for the patching when I didn't want to wait for mud to dry, but that involves a learning curve to perfect, & that's your warning on that route. As far as the cracks go, take your hand & see if you can move the ceiling, & if you can then it's back to the screws & washers for the area around the cracks & that part is actually fun if necessary as you can watch a go back to being level. Then bust out the cracks a bit & patch them. A bit of fiberglass tape over the washers before you mud them helps them disappear. Have fun!

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