Build a Simple Square Deck
11 3600mm lengths of 150mmx50mm treated decking joists
33 3600mm lengths of 125mmx35mm treated grooved deck boards
4 2400 lengths of 100mm x 100mm treated fence posts
650 decking screws
4 inch galvanised nails
1 roll weed membrane
1 tonne 20mm aggregate (concreting chips)
1 tonne building sand
6 bags cement
handsaw or compound mitre saw
hole digging spade
Firstly, decide where you want the top height of the deck to be. Remove any grass or weeds from the site and dig down to a minimum depth of 210mm below the top height if you need to. 150mm for the deck joists, 38mm for the boards and 10mm so that the joists don't sit directly onto the soil.
The site should be as level as possible after you have dug out.
Building the Frame
Lay out your decking joists as shown below, with 450mm centres between internal joists and nail together with 4 inch galvanised nails.
Carry the frame into position and level it temporarily using bricks. The frame should be level with the run of the joists and should have a slight run over the joists so that, when you lay your deck boards the rain will run off along the grooves.
Now you need to dig post holes for the supporting posts. You can mark the ground and move the frame away to give you more room or you can dig the holes with the frame still in place. Dig down to a depth of 40cm for a ground level deck or further of you have soft ground or if the deck is being raised. Dig post holes for the arrangement of posts as shown below and make sure you dig holes wide enough for you to fit concrete all the way around the post.
With the frame in place and levelled, mix up some concrete using the sand, cement and concrete chips. Cut the posts to the correct size and set them into the concrete, making sure you put a little concrete in the bottom of the hole, underneath the post. Use a level to level the posts as you set them so that they sit perfectly straight in the holes.
Leave the posts to set for as long as possible ( at least 48 hours ) and then nail the frame to the posts checking the level of the frame as you go. Lay some weed membrane or DPC on the ground, cutting around the posts.
You can lay the remaining gravel over the weed membrane. This will help weigh down the membrane and contribute to good drainage under the deck.
Laying the deck boards.
Now you are ready to lay the deck boards. The first board should go at the front (the most visible) end of the deck and should be laid flush with the front joist. This means that, if you have any cuts to make along the boards, these will be at the back. Check over your deck boards and find the one that is most straight. Often, if boards have been stored for any length of time in the timber merchants, they might warp and bend. Your first board should be perfectly straight because you will use this board against which to set all the others.
Screw the board into place using a power screwdriver and special decking screws and. (These will not rust and spoil your deck). Slightly countersink the screws so that they are not too visible. You will need two screws to every joist.
Lay your next and subsequent boards depending on the boards themselves - if the boards are very dry, you will need an expansion gap of 2 - 3mm. You get an even expansion gap by using 3mm nails between the boards. If the boards are wet, you don't need an expansion gap because you will find that, when they dry, the resulting gap will be too big.
To get bent boards to sit nicely against each other, use an off cut of deck board, about a metre long, and screw this temporarily into the joist beside the board you want to straighten. You can then use this smaller board as a lever to push the board into place.
Continue screwing down all the boards.
Face any visible joists with deck boards to create a nice finish and then seal the boards with decking seal or decking oil. This will help prolong the life of your deck. Be sure to let the boards dry out thoroughly before you apply the seal.