Once you have the posts and joists installed, protect them from rot and insect damage prior to laying the decking. Even though the lumber for the joists should be pressure-treated, the pressure treatment may not have fully penetrated the wood. This is often true with lumber made from fir. The nails or screws that attach the deck boards will slightly split the top of the joists, and this allows moisture to enter deep into the wood.
In order to protect the joists from this potential moisture damage, cut plastic film into long, narrow strips. The strips should be wide enough to cover the top of the joists. Three inches wide is a good target. Staple these strips over the top of the joists before installing the decking boards.
For the best appearance, select the proper length decking boards for the specific deck. If the deck is short enough so that one decking board can span it, use single boards that are at least one inch longer than the deck. This will provide some overhang for a smooth trim-cut edge.
In your case, with a large deck, you'll need several boards to span the entire length. Don’t just buy the longest boards possible and then use shorter pieces to finish the span. Select a variety of board lengths to create a random pattern of joints, and don’t make any of them too short. Just use your judgment as to what combination of lengths will look good.
You have two options of where to start laying the boards: either at the house wall or the deck rim. Starting at the rim is easier because you don’t have to plan the location of each board. If there’s space for only a narrow board at the house, it will be supported well and won’t look bad.
On the other hand, if you start at the house, plan the spacing of the boards. You don’t want to attach all the boards and then find you have room for only a very narrow board at the outer rim. This board will be difficult to attach securely, and it will look bad.
At the rim edge, snap a chalk line across the joists at the location of the edge of the first board plus 1/4 inch. This extra quarter inch allows you to see the line when you are laying the first row of boards. Attach them at every fifth joist and then check to make sure everything is straight and even.
When you’re satisfied, screw them in with two screws to every joist. Space the screws the same distance from the edges of each board for a symmetric fastener pattern. Use spacer blocks for uniform spacing between the boards.
