Making the seat frame
The seat frame structure is similar to that of a ladder, with the cross members being the 'ladder rungs' and the seat frame sides being the 'ladder sides'.
The over all frame width (in this case) is 470mm (18 1/2"). That measurement is worked out by the number of decking boards to go on the seat top and the gaps between the boards.
Make the seat frame (ladder) up on the ground. The cross members should be approx 400mm (16"} apart, but when spacing them out, ensure there will be a cross member in line with each post.
Make the seat frame longer than needed. It can be trimmed after being fixed in place.
Corner fitting: Sit the (over length) seat frames on top of the posts, temporary in position. The frames will overlap one another at the corners. It will then become obvious where to mark and cut where the seat frames overlap.
Fix the seat frame in place by nailing the cross members to the posts and then securing with two metal galvanized straps (see picture next page). This will stop any tilting movement. (Note: Some type of stainless steel straps or fittings should replace the galvanized straps, if the seat is close to salt water, i.e. the sea).
Trim the ends of the seat frames to the required length ensuring there is a cross member or end frame piece at each end of the frame.