Performance halls should be designed and built with sprung floors in mind. A depth of at least should be allowed for the floor. This can be a major constraint when laying a sprung floor in a hall not designed for it. Most can accommodate a maximum of , and some sprung floors designed for refurbishments are as low as . Ramps for wheelchairs will be needed at the doors. If the ramp is outside the hall, the doors will need the bottoms trimmed off (easing) and their height will therefore be reduced. Ramps can have a 1:12 incline at most, and they may also need a safety zone around them. Thus if the floor is 5 cm deep, the ramp should be long or more. The underfloor needs to be made flat either with levelling cement, very careful trowelling, or by using shims and a layer of masonite. Any new cement must be allowed to dry for at least a month. A membrane vapour barrier should be used to prevent moisture from the ground.