A common problem that can befall a drywall garage ceiling is the buildup of mold caused by accumulated moisture. This most often occurs either due to a leak from the outside or washing machine use coupled with poor ventilation. However it happened, repair of the drywall is in order.
The first thing to do is fix any leak in the siding or the roof. You can repair the drywall, but if you don’t attend to the source of the moisture, the same thing will happen again. Moisture accumulation issues because of a washing machine can be trickier and usually require the installation of a vent or fan. That, however, is a separate consideration. Foremost on your mind right now is the repair of the rotted drywall.
Step 1 - Lay Out Drop Cloth
Move any immediate obstacles to the repair job and lay out the drop cloth underneath and around the area, you will be working. If you are dealing with moldy drywall, wear a face mask when removing the tainted piece, for you do not want to breathe in the spores.
Step 2 - Build a Box Around the Rotted Drywall
Using the stud finder, locate the joists to either side of the rotted drywall. If the rot abuts up to a joist, find the next farthest one. Mark the center of the joist in two places. With the straight edge, make a line connecting the two marks made on each respective joist. Use the straight edge again to make two perpendicular lines connecting the first two and completing the box. It should fully contain the moldy drywall and leave at least 5 inches around it on all sides. Note the exact dimensions of this box.
Step 3 - Remove the Section
Along with the drawn box, score the drywall with the utility knife. Run it over the area several times until the drywall is pierced. It should either fall out or be easily pried out. Check your cut. Make sure half of the joists on either side of the section are clearly visible. You will need them to fasten the replacement piece. With the sanding block, smooth out the edges of the intact drywall.
Step 4 - Measure and Cut Replacement Piece
Take the new drywall and mark a box on it the exact dimensions of the piece you removed. Use the T-square to start a 90-degree line. Use your tape measure to mark the distance to the corner and draw the line with a straight edge. It’s easiest if you use two sides of the replacement drywall as two sides of your patch piece. That way you’ll only need to make two cuts. Score the lines with the utility knife. Work the blade through slowly so you do not tear the paper on the opposite side. Once the piece is cut, sand down the cut edges with the sanding block. Hold the piece up to the hole to make sure it will work for the patch.
Step 5 - Fasten the Patch
The patch piece will create up to 4 joints between it and the adjacent drywall. At two of these joints, there should be a joist to which to fasten two sides of the patch. If there is a crosspiece between joists, note its location. The more places you can fasten the drywall the better. Mark the position of the joists on the replacement piece so you fasten the screws accurately. Hold the drywall in place and drive the first screw. If there is a joist running through the middle of the patch, fasten screws there first. Always work from the middle out where possible. Set screws every 8 inches into every available joist or crosspiece
Step 6 - Finish the Joints
With the drywall set in place, you have as many as four joints to patch. Use an all-purpose drywall compound for the task. With the drywall knife, smear enough compound in each crack to fill, smoothing it out as you do. Cover each seam with the appropriate length of the tape. Run the knife over the tape to both adhere to it and squeeze out the excess mud. Let it dry. Cover up any screw head with a quick coating of the compound and smooth it out too. Apply one more coat of mud over the tape to conceal it and to make the joints a little smoother.
There is no need to fully finish the joints in the garage ceiling. Two coats of compound will do just fine.