The foundation is the most critical stage of construction. Any errors here will affect all later work, so take your time and double check all measurements and setting out.
Dig corner stump holes 400mm deep x 300mm square. Dig them so that when the stumps are centred in them, their outside edges are flush with the string line. Place a timber sole plate in the bottom of each hole and stand a stump on it. Or use a 50mm layer of concrete instead of the timber sole plate. Measure and mark 70mm up from the ground on the outside of the stump located at the highest point of ground level. Transfer this mark to all other stumps by using a 2I00mm length of straight timber and a spirit level (Fig.2). Mark all four sides of each post with a carpenter’s square, remove the posts and cut to the marked lengths. Replace in their holes when cut to size.
Now cut two 100 x 100mm bearers to 1800mm long. Place them on top of the stumps, making sure the outside surface of each stump is flush with the end of the bearer and skew nail the bearer to each stump with two 75mm galvanised nails.
Cut five 100 x 50mm joists to 1800mm long. Skew nail a joist to each end of the bearers with two 75mm galvanised nails per fixing point. Set out the three other joists 387mm apart on the bearers and fix them to it in the same way. Now check that the floor frame is square by measuring diagonally from corner to corner as in Step 1. The frame can be pushed and shoved at this stage until both diagonal measurements are the same. Finally, replace the soil in the stump holes and tamp down hard.
Complete the frame by cutting the sheet flooring in half so you have two 900mm wide x 1800mm long pieces. Lay one sheet on the joists with the plastic tongue in the centre and fix to each joist with galvanised 60mm flat head nails at 200mm spacings. Remove the plastic tongue from the remaining piece, lay it on the frame and push it into the tongue of the sheet already fixed. Nail it to the joists as before.