How to build a deck

A great deck comes together in the planning stages. To start, check with your local council to see if you need a permit – every local planning division is different, so it’s always worth asking before getting started. For example, in Auckland, a deck that’s up to 1.5m high doesn’t need building consent but this varies depending on what part of the region you live in. No matter where you are, if your deck is over 1m high, it must have a handrail or safety barrier.

You also need to assess the ground you’re building on. The surface – be it grass, soil or concrete – will determine what materials you need. Always check at beforeudig.co.nz to see if there are any cables, water or gas pipes that you need to be aware of.

Decking timber comes in a wide variety of styles and durability. Choose a material that’s suitable for your area’s conditions and your skill level. For example, if you live in a coastal environment, a durable hardwood such as kwila, would work, but for beginners, softwoods, such as macrocarpa and radiata pine, can be easier to use. Quality stainless steel or galvanised decking screws are a must to withstand rust.

Before you begin construction, set out your deck using profiles – a series of pegs, stakes or survey markers and horizontal pieces of timber – to show you the perimeter and level line. Use marking paint to draw the deck dimensions on the ground – this helps you place the profiles and visualise the finished deck. Put a profile in each corner of the deck – to give yourself room to move, hammer the profiles just outside your paint line.

Next use a string line to create the shape of your deck. To do this, hammer a nail into one side of the profile, attach the string and extend it to the other side, making sure the string is taut.

There are two ways to check your deck measurements are square:

• The diagonal method: Measure from one corner to the other and repeat the steps on the other corners. Your measurements should be the same.
• The 3:4:5 rule: Measure 3m from the corner in one direction, then measure 4m from the corner in the other direction. Connect those two points to complete a triangle shape. The length of this should be the longest side and should equal 5m.

Decks built over grass and soil need posts (also known as piles). Before you install the posts, you need to check the height of the deck to make sure you have enough clearance between the ground and the bearer. Bearers are the main load-bearing beams of a deck and connect directly to the posts. If you don’t have enough clearance, you may need to dig out some soil. Clearance heights vary based on what part of the country you live in. Check with your local council or building authority guidelines before you begin construction.

To work out where to put the posts and to ensure your deck has the correct load bearing capacity, refer to timber span tables. Use marking spray paint to indicate where the posts will go.

The depth of the holes needed for the posts may vary depending on your site and council requirements, however, footings below cleared ground level must have a minimum depth of 200mm for ordinary posts, 450mm for braced posts and 900mm for anchor posts. A good rule of thumb is to make sure that ordinary post holes are 600mm deep and around 300x300mm in size.

Before you mix the concrete, cut your posts to roughly the height of your deck then mix enough concrete to fill each hole. You will need a minimum of 150mm of concrete beneath the post for support. Place the posts into the holes in line with the string line. Fill the holes with concrete. Ensure each post is plumb and backfilled with soil and leave to dry overnight. Once set, cut the posts to the right height. Use this equation to work out the correct height for the posts: the overall deck height minus the height of joists, bearers and decking boards.

Bearers carry the weight of the deck, so it’s important to install them correctly. To work out the correct length for the bearer, measure the distance from your house to the string line, then cut the timber to size. If the deck is going to be long and wide, you may need to fix two pieces of timber together to provide sufficient strength. Do this with a nail gun.

Use framing nails to nail both sides of the bearer and tie it together with strapping (wear gloves to protect your hands from the sharp edges). Secure the strapping with at least six nails on the bearer and six nails on the post.

Joists are smaller timbers that sit on top of the bearer crossways to make up the deck frame and support the decking. For maximum support, joists should be no more than 450mm apart – divide the width of the deck into even sections to work out how many joists you’ll need. Attach them to the bearers using a nail gun or hammer. If you need to extend the joists, join two pieces of timber together, overlapping the timber at least 1m on either side.

Nogs – also called dwangs or noggins – are short lengths of timber that sit between every second joist for extra rigidity. Alternatively, you can use brackets and screws to provide further support. If your deck has a drop of more than 800mm, you’ll need a balustrade, which you can create with CPC brackets for extra support.

To connect your deck to your backyard and improve access, you may need to add a step. This needs to be done before you start laying the decking boards. To work out the height of the step, measure from the top of the joist to the ground, then divide this by the number of steps. The height of a step must be a minimum of 140mm to a maximum of 190mm.

Use this calculation to work out the length of the post: the top of the joist to the ground divided by two, minus the height of the subfloor, equals the post height. To create a space for the step in the post, measure and mark the thickness of the timber you’re using for the step. Set your saw to the thickness of the timber and cut the post at that thickness.

Pre-build a subfloor for your steps using joists and nogs. Keep the spacing of the nogs in line with the joists to ensure the screw lines are straight. Position the subfloor in the cutout in the post and screw in place, checking the step is level as you go.

The surface of your deck is the most visible part, so it’s important to get it right, this is where running measurements will help. They are incremental measurements taken on a single line – measure out the same distance on every board at regular intervals, then use a mitre saw to cut to size, keeping an eye on the running measures.

To position the boards, place every fifth board down first to make sure your deck stays parallel. To avoid splitting the timber, pre-drill holes into the boards, then drill a screw into each hole.

Once every fifth board is placed, stagger the rest of the boards ensuring both ends are on a joist. Make sure the joins of each board are staggered and are on top of a joist. Leave some extra length of timber overhanging the edges of the deck so you can cut the boards down to size later. The New Zealand Building Code requires a minimum separation of 12mm between the wall cladding and a timber slat deck so that water will not pool.  

To make sure the screws will line up, use a chalk line to mark out where they’ll go. Place wedges in between every board to help space them evenly, then pre-drill holes and add the remaining screws.

Cut the ends of the boards so the edges of the deck line up – use a plunge saw on a track or a straight edge and skill saw. Finish your deck with capped edges and attach screening to the sides of the deck. To do this, cut screening boards to size and work your way from the top to the bottom, placing wedges between each board.

Measure the length of your steps and cut decking boards down to size to fit on the step subfloor. To arrange the boards, start with the outside board first and then work your way inwards and upwards. Use a piece of decking off-cut to gauge how much overhang you need on each step to meet the side screening.

Attach the first piece of screening on the board closest to the horizontal step board and set in position. You will need to install balustrades and handrails if your drop or stairs are more than 800mm.