Mark out a rectangle or square deck with pegs and strings. Check the corners are at a 90-degree angle using a builder’s square. Mow any grass very short.
Lay a weed-proof membrane, and overlap joints by 18 inches. Roll it back when setting the outer posts, but cut through it for internal supports.
You will need a post in each corner and the rest spaced in a grid pattern, one every 4 feet. Dig holes 15 inches deep, and fill 3 inches deep with gravel.
Fill the hole with water to dampen the gravel, and allow to drain. Insert the post, and pour in post-mix concrete, using a level to check if it is vertical. Leave it to set.
Cut edging boards to length, and drill bolt holes through them and the posts. Attach one end of the first edging board to a post using nuts and bolts.
Level the first board, and fix it in place with nuts and bolts. Trim the ends flush. Repeat these steps until all three sides are fixed, then tighten the bolts firmly.
Pre-drill holes, and use nails to fix internal joists between the edging boards across the shortest span, spacing them 16 inches apart. Make sure they are all level.
To hold joists rigid, cut short lengths of lumber and insert them between the joists at 4 feet intervals. Secure them using long nails or metal brackets.
Cut a decking board to length. Pre-drill holes in the board, then attach it to each joist using two corrosion-resistant decking nails or countersunk screws.
Cut remaining boards to size, and attach them to the deck, making any joints above the joists. Space the boards 1/4-inch apart, using spacers, to allow for expansion.