The concrete slab should be a minimum of 6” larger each side and end than the overall unit size. The length of the base rail is always 6" shorter on each end or overall 1' shorter than the roof line of the structure. For example: If you order a 12' x 21' carport, the foot print is 12' x 20' So your concrete would need to be poured at 12' 6" x 20' 6"