In deciding the slope of a roof, the need to make it weatherproof is the first consideration. When, however, the roof is covered with overlapping small units such as slates, tiles or corrugated sheeting, then the slope or pitch must be sufficient to take the water away with some rapidity before, assisted by wind and capillary action, it can seep back into the building below. In deciding on the actual roof structure, it is common practice, as recommended in most textbooks, to relate this closely to the roof span. Domestic pitched roofs are designed to take small roofing units, clay or concrete tiles or slates. Collar tie roof construction is generally limited to buildings in which for economy reasons the ceiling level is kept above that of the wall plates. In normal roof construction there is considerable compression in the foot of the rafter and tension in the ceiling joist.