In all cases of garden shed base preparation, the ground must be level. base instructions to prevent condensation. the concrete slab base should be a few inches larger than the base rail size of the shed, e.g. a shed with base rail measurements of 93" x 70" should have a foundation measuring of 96" x 73".. I want to build a concrete base for a garden shed, size approx 5m x 3.5m (16'6 x 11'6). the shed will it on timber beams, and have an insulated wood floor, so no standing (or seeing) directly the. Shutter it up with tanalised timbers fill it in with gravel, whacker it down firm, staple dpc to the bottom of some 3
