A water leak can stain drywall.
With professional installation, drywall panels provide a seamless finish for walls and ceilings. Drywall, unfortunately, is not impervious to damage. If cracks appear in your once-smooth wall, there could be a number of problems, but moisture might not be the culprit. Cracks in drywall are usually due to movement of the structure. Cracks are likely to reappear in the same place after you make repairs unless you correct the underlying problem. Moisture also damages drywall, sometimes extensively.
Moisture Damage
Moisture compromises drywall's structural integrity. Due to its porous nature, drywall, which is composed of gypsum particles, can absorb water from a leaky pipe or flooding. Once saturated, the tightly compressed gypsum particles can swell, leaving lumps or areas where the drywall crumbles away. The extent of the damage depends on the amount of moisture and the length of time the drywall is wet. If you fix a leaking pipe before the drywall swells, you may end up with a brown stain on the drywall that you can paint. Once drywall swells, you must remove and replace the damaged sections.
Causes of Cracks
Drywall panels are large, usually 4 feet wide and 8 feet long, although they can be as long as 16 feet. When installed, imperceptible shifting of your home's structure can stress the panels, causing them to crack. Cracks will occur at the weakest point of the wall, which is usually along a seam. Improper drywall installation, where a panel seam lines up with the top of a window or doorway, is likely to be the first place a crack appears. Ceiling cracks in new construction can be the result of movement of engineered roof trusses as they dry and contract.
When It Only Looks Like a Crack
Not all cracks are really cracks. When the drywall contractor fills the seams between the panels with joint compound to form a smooth surface, he applies paper drywall tape over the seams. If the tape does not adhere securely to the wet joint compound, it can eventually pull away from the wall, resembling a crack. Problems with drywall tape are the simplest and least expensive to repair.
Drywall Repair
Small cracks, less than 1/4 inch wide, require only filling with joint compound or spackling and then sanding before painting to match the rest of the wall. Brown water stains will bleed through a new coat of paint and require one or two coats of a stain-blocking sealer before repainting. You must replace swollen or crumbling water-damaged drywall, because it is no longer structurally sound. Before repairing moisture-damaged drywall, fix the plumbing or leaky roof that caused the problem.
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