These are usually in the form of a

These are usually in the form of a rebated lap joint, the upper panel being lapped over the top edge of the lower panel. As with the vertical joints, the provision of an adequate back seal to prevent air movement through the joint is of paramount importance. The seal must also perform the function of a compression joint:therefore the sealing strip is of a compressible material such as bituminised foamed polyurethane or a preformed cellular rubber strip.The profile of the joint is such that any water entering the gap by flowing from the panel face or by being blown in by the wind is encouraged to drain back onto the face of the lower panel. The depth of joint overlap is usually determined by the degree of exposure, and ranges from 50 mm for normal exposure to 100 mm for severe exposure. Note that the effective overlap of a horizontal joint is measured from the bottom edge of the baffle in the vertical joint to the seal, and not from the rebated edge of the lower panel.