One of the great joys of decking is its flexibility. As your needs change, you might start thinking your initial deck is too small and feel that you want to extend it. This is simply a matter of adding on another sub-frame. If your extension runs over a downward slope, you can accommodate the change of level by adjusting the length of the joist support posts - and maybe installing a set of steps to lead down to the garden. To extend your deck up a slope, you can start the new sub-frame on top of the last.
The most straightforward way to enlarge the area of your deck is by extending the sub-frame and boards at the same level. You might need to remove a section of balustrade first to do this.
Unscrew the boards along the edge you want to extend to expose two joists. To enlarge the sub-frame, you'll need to put up additional joist support posts and fix three sub-frame joists to them. These will define the area you want to extend.
Fix new deck boards across the exposed joists - you'll need to stagger the joins between the boards. Remember to position the joins half-way across a joist and screw both boards into it. When you reach the edge of the extension, saw off the excess deck boards in line with the sub-frame.
If your extension changes direction or acts as a walkway to another section of the deck, you might want to change the direction of the boards.
First, attach a new joist to the sub-frame with coach screws. Then mark out the area of your extension, and put up joist support posts and the remaining sub-frame joists. Next, nail the internal joists to the connecting joist using metal joist hangers. If your deck boards are going to lay diagonally to the joists, the latter need to be spaced 300mm centre-to-centre. If they're horizontal, 400mm centres will be enough.
Screw each deck board down with two screws into every joist. If they lay diagonally, you'll need to clamp each one onto a workbench. Use a builder's square to mark the ends at a 45 degree angle and cut them neatly with a circular saw or handsaw. When they're all screwed down, you can saw the edges in line with the sub-frame.
If you really want to make your deck stand out, an angled corner is a striking feature that's quite straightforward to achieve. You'll need to cut the outer and inner joists of the sub-frame to the required angle, then fit and cut the deck boards to match.
Use a straightedge or long spirit level to mark the angle across the outer and inner joists with a pencil. Then cut the joists along the line you've made.
Measure a length of joist, and cut its ends in line with the next joists of the sub-frame. Next, screw it to each inner and outer joist using two countersunk coach screws.
Lay the deck boards that overhang the angled outer joist and screw them down. Then clamp a spare board to the deck to mark the line of the angled corner. Use a circular saw or handsaw to cut the ends of the overhanging boards in line with the sub-frame. You'll need to coat all the sawn ends with end-grain preserver.