If you have a swinging shower door that won't close at the bottom, the problem is usually that the door is sagging. The fix typically isn't difficult, but the actual procedure depends on the kind of door. It could be a framed door with a metal frame all around it, a semiframeless door with a metal strip on the hinge side, or a frameless door with no frame at all.
Of these, the partially framed door is the most likely to sag, and the reason has to do with how the glass is held in place. Instead of being directly attached to the hinges like a frameless door, it's held in place by a rubber gasket inside the metal strip on the hinge side. The gasket wears out over time, and while you could replace it, there's an easier way to stop the door from sagging.
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Shower Door Adjustment: Door Hinges
Before you do anything else, have a look at the door hinges because if the screws are loose, the door will sag whether it's framed or frameless. Open the door and look for space behind the top hinge and the wall. If you see any, tighten the screws with a drill driver, and that may be all you need to do to fix the problem.
If one or more screws just spin in place without tightening, remove them and replace them with longer ones. It's possible the original screws were too short to bite into the wall stud, or part of the stud deteriorated and let go of them, and longer screws will solve both problems. You might also consider taking down the door and removing the wall anchors (if there are any) and replacing them with new ones.
Fixing a Sagging Semiframeless Door
If your shower door has a metal strip on the hinge side and the door is sagging, you can sometimes fix the problem by loosening the screws on the strip, pushing the door back in place, and tightening the screws. This is probably only a temporary fix, however, as the door is sagging because the gasket inside the strip is slipping, and it will probably keep slipping no matter how tight you make the screws.
A better solution is to loosen the screws and remove the door from the strip. Squirt a bead of clear silicone caulk into the groove into which the door fits and then replace the door, tighten the screws, and wait for the caulk to set before opening or closing the door. The caulk will make a permanent bond, and you should never have to worry about the door sagging again.
Fixing a Sagging Framed Shower Door
When a framed shower door sags and the outside bottom corner won't clear the door frame, the shower door latch won't work. If the hinges are tight and aren't causing the sagging, the problem is usually that the frame has gone out of alignment. You can usually fix this yourself, but proceed with caution to avoid damaging the door.
Open the door and loosen all the screws in the frame on the latch side of the door with a screwdriver. Place a block of wood underneath the corner that's tall enough to leave a small gap and then push a cedar shim into the gap until it makes contact with the bottom of the door. Tap the shim with a hammer to push that side of the frame back into alignment. You may also have to tap the bottom corner upward using a block of wood and a hammer. Once the frame is realigned, tighten down the screws and check the door operation. Repeat if necessary.