How to lay decking

Learn how to lay decking in your garden using our handy step-by-step guide and videos

How to lay decking
(Image credit: Cuprinol)

If you're thinking of learning how to lay decking this weekend, then you've come to the right place. We've put together a step-by-step guide and two videos to show you how to do it. Decking is a great way to improve your garden and helps create a link between indoors and out. It's a time consuming job, but that makes it the perfect task to take on at the weekend.

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Here, we are installing a composite decking product in Midnight by Ecoscape, but the same technique will apply for standard timber decking too. These decking planks measure W15 x L290cm.

As well as your chosen decking boards, you will need:

Building the deck's framework

This is the part of the job that will take the longest to complete, and you'll need an extra pair of hands to complete it.

1. Decide which way you want to lay your decking

Decking boards look best when laid perpendicular to the building line; however, this is down to preference. Calculate the number of board rows you'll need by dividing the width of your space by the width of your boards, plus a 5mm spacing per board. Assuming your decking boards are 150mm wide, that means allowing 155mm for each one.

2. Mark the footings

Your first mark will be 1m from the rear wall and 1m from the edge of the decking area. Continue to make marks 1m apart, until you reach the front edge of your decking area. The footings should be 30cm square and around 50cm deep. Dig these out, and cut the correct number of timber supports to around 1m. The wood should be pressure-treated softwood, although some systems can utilise composite or aluminium battens.

3. Position the first corner support

Set a string line across the front edge of the decking area at the exact depth of the decking edge from the rear wall, minus the thickness of your timber joists. Set a length of timber frame along the left-hand edge of the decking area. Now position the front left support in its footing hole, and, using a spirit level, ensure it's vertical and absolutely square. 

4. Fill the footing with concrete

Mix and add water as specified on the product details. Ensure the mixture is stirred thoroughly. The support should remain upright and in the exact corner position once the first support is set. Repeat the process for the front right corner using a length of timber and the string line in the same way. 

5. Set the remaining front supports

Now the corner supports are set you can set the remaining supports along the front edge of the decking. They should all touch the string line without pushing it out of position. Add the concrete mix in the same way. Now position the remaining supports, starting at the back end, and ensuring they're positioned in a perfectly straight line, row-by-row. Wait for the concrete to set firm before moving on to the next step.

6. Cut the horizontal joists

Measure the distance from the rear wall to the front of the corner support and cut your first length of timber to size. Position the timber at the correct height, taking into account the additional height of the decking boards. Using a spirit level, make sure the timber is almost horizontal, with a slight fall of one to two degrees to allow rain water to drain away from the property off the decking area. Then loosely attach to the corner supports with a wood screw, or use a nail gun. Then secure with masonry screws.

Repeat these steps along the rear and opposing walls of the building, ensuring the joists are one to two degrees from horizontal and the same height as the initial joist.

7. Trim the joists and lay weed membrane

Using a hand saw, trim the edges of the joists. If your building is square, the joist should be positioned on the inside of the corner. Now add the final outer joist across the front of the decking supports. Now you can add your weed membrane, stretching the membrane over the supports and ensuring there are no gaps to prevent any plants growing through. Lay the membrane across the entire decking space. Make sure it's weighed down.

8. Create the lower joists

To create the lower joists, measure the distance from the rear to the front edge once more and cut a length of timber to size, plus 50mm. Attach it with wood screws to the underside of the outer frame and to each of the upright supports. Repeat this across the decking area.

9. Add the upper joists

You're now ready to add the upper joists. Measure the distance across the decking area from the inside edge of the surrounding frame and cut your timber to size. Lay these across the framework, attaching to the upright supports as you go. Double up the joists with a 20mm gap. Use wooden wedges to ensure this gap is consistent

10. Complete the decking frame

Mark along the outer frame where your remaining joists are going to sit. Cut your timbers to size and lay across the decking area, securing with wood screws. Now you can trim off the excess height of the upright supports with a handsaw. You should now have a solid flat frame on which you can install your decking.

Installing the decking boards

You’ve built your frame to the correct size and height and ensured the ground beneath it is covered with a weed membrane. Your joists are running side-to-side, so your decking boards are going to run front to back.

1. Fix your first decking board

Start by positioning the first starter clip in place; align the clip to the end and centre of the first joist. It should sit flush with the edge of the decking frame. Screw it into place. Slide a decking board into position against the starter clip and use it as a guide to fix the rest of the starter clips along the first edge. 

Measure your first board: it should sit flush with the front of the decking, and the rear edge of the board should sit over the gap between the doubled-up joists. This ensures that water doesn't sit on the joist, preventing any damage to the board or the subframe. 

Move the first board into position with the starter clips sitting in the groove in the side of the decking board. Slide a T clip into position at one end of the board and ensure that the board is tight against the starter clip. Half-screw the T clip into place; install further T clips on every joist that you set up. These allow for a 2mm expansion gap between boards, and a place for water to drain.

The join between the boards should sit over the gap between the double joist to allow good surface runoff.

2. Fix the second board into position

Moving on to the second run of boards, stagger the joint like brickwork. Slide the adjoining board into position, snug against the existing T clips, and install further T clips along the opposite length, half screwing in each one.

3. Finish laying decking boards

Measure and cut your remaining decking boards, ensuring they finish flush with the front edge, and that the joint is over the doubled-up joist. Slide the final board into position, with the T clips half screwed. You can now fully tighten the previous row of T clips. Repeat this process across the whole deck area.

4. Fit the fascia

Once your decking is down, you can fit the fascia to the front and sides of the structure. Start away from the corner and at the top, working down. Flip your board over on its back and drill pilot holes into the board top and bottom, around 40mm from the edge, at 400mm gaps. Now turn the board over and countersink the pilot holes. The board can now be attached with powder-coated colour-coded stainless steel fixing screws.

Measure the remaining space left to cover and cut your board to size, remembering to mitre the end of the board 45 degrees to fit the corner. Work around the corner and fit the fascia in the same way. Now work down until your fascia boards meet the ground.

5. Clean the decking

Clean off your decking area with some water and a broom and there you have it: a fully fitted decked area ideal for relaxing outdoors. 

Explore our outdoor living space design ideas and find out how to create an outdoor dining area to see how you could be using your new deck. 

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