They were joined by horizontal beams and crossed with ceiling joists. Small offcuts are used as shims to level floor joists. Medium density coconut timber can be used for walls, ceiling joists and horizontal studs. It can be provided by roof joists or beam web stiffeners. Ceramic knobs were cylindrical and generally nailed directly into the wall studs or floor joists. Kahn's reinforcing system was built in concrete beams, joists, and columns. This had two rows of uprights on which the ceiling joists rested. The floor boards were fitted between the joists. Strutting or blocking between floor joists adds strength to the floor system. Blankets can cover joists and studs as well as the space between them. Subfloors are at least thick, the thickness depending on the distance between floor joists. A large beam in the ceiling of a room carrying joists is a summer beam. Wedges are used to lock the joists in position. Shortened joists are said to be "crippled".