Drywall finishers cover the seams where boards intersect. They use joint compound and paper tape. If they do their job correctly, none of the seams will be visible through paint. If the seams are too wide -- if there's too much space between the boards -- the drywall finishers have difficulty covering the seams. In some cases, finishers have no choice but to fill large gaps with special cement or excessive amounts of joint compound, a waste of time and materials.
To avoid large gaps, drywall installers hang the boards on the ceiling first. They push each board flat against the ceiling. Along the edges of the ceiling, the installers push the boards as close as possible to the walls to help minimize gaps. However, installers often must cut the drywall so it fits properly, which means that some edges where a ceiling meets the wall will be rough and uneven.
After installers hang the ceiling boards, they start on the walls. The long edge of a drywall board is completely straight, so they push that side against the ceiling when they install the board on the wall. This covers any rough and uneven edges on the ceiling boards, leaving a straight, narrow seam that is easy for workers to finish later.
Installing the boards on the ceilings first doesn't always remove the problem of large gaps. Drywall installers must cut ceiling boards carefully, or they risk creating large gaps that the tops of the wall boards cannot cover.