Use the same decking materials to construct a fixed deck seat, as used on the deck itself.
In this example, 90mm x 18mm (3/4" x 3 1/2") hardwood is used. If your deck is constructed out of some other material, make any necessary measurement adjustments to compensate for any material size differences exampled in the following plans.
Note: refer to the comments and photos section to see how users from USA have compensated with a different size lumber.
Marking the post holes in the deck
It will be necessary to cut square holes in the deck for the seat posts to slot into.
The distances between seat posts will vary depending on the existing deck's joist spacings, but should never be more than 1500mm (5ft) apart.
Position the holes next to the joists (see drawing), or in the case where that is not possible or where there are no joists, blocking needs to be fitted.
Cutting the post holes in the deck
Mark the holes central to the width of the seat, and ensure the holes do not span more than 1500mm (5ft). Use the end of a post as a marking template. Cut the holes out using a jig saw, drilling a 10mm (3/8") hole in one corner first, to enable the jigsaw to get started.