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How to build a raised garden deck

Build a raised garden deck

Raised decking is not only practical, it's beautiful too.

Bring the garden into your home by creating a fabulous outdoor living space. With a wide range of wood and finishes available, there's a deck to suit every garden.



Sloping sites are difficult to fully utilise, but elevated decking can create a flat area perfect for entertaining, relaxing and for children to play. Why not make a quiet place for contemplation beside a pond, or celebrate the surrounding landscape by designing a deck where you can relax and enjoy the view.


Use different heights in your design as a feature, this will add interest and lift your garden beyond the ordinary. The possibilities are endless and with a sympathetically designed decking area you can extend the summer and enjoy your garden to the full.

If you intend building a deck adjoining the house, make sure the proportions are in keeping with your property. Draw a plan, clearly marking the position, height and dimensions. This will help you visualise the deck and you can refer to it when marking out the foundations.

If you want to build a very large structure then you may need planning permission, so seek advice from your local authority before starting.

  • Take into consideration the height from the ground of the door you will use to access the deck.
  • Check for drain covers, as you may need to include a trap door.
  • Is there an existing path or walkway? If not, you may want to create one.
  • Do you want a single raised area, or would you like to extend to another level in the future?

Try and look at all aspects of the construction before you finalise the design.

Extending a deck

The most straightforward way to enlarge the area of your deck is by extending the sub-frame and boards at the same level. You may need to remove a section of balustrade first.

If your extension changes direction or is a walkway to another section of the deck, you may need or want to change the direction of the boards.

If you are building a deck over rising land you may need to step up part of it to accommodate the change in level. Or you might choose to add a new level as a design feature or to access a door. The higher level is achieved by building a new sub-frame on top of the existing one.

More detailed information on extending your decking can be found in our You Can Do It book, available to buy in store.

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Prepare the site

1 Using a tape measure, mark out the position of the deck. Put a wooden peg in the ground at each corner and stretch string between the pegs. Measure the diagonals to check it's square.


2 Remove the turf making sure you cut a neat edge along the outside of the string line, or if there are stones and debris clear back to firm ground. If part of the area is concrete, leave it intact as you can use metal post supports bolted to the concrete to hold the joist supports in place.

Measuring - get it right!

For any building project it's essential to measure the area accurately. You must set out string lines to make sure the foundations are in exactly the right position. To ensure that the sides of the deck are square, you can use the 3-4-5 triangle method. This technique uses basic geometry to achieve a perfect right angle.


1 Make timber profiles, you could use off-cuts for this. The uprights should be about 600mm high with a cross-piece the same length, nailed in place 50mm from the top of the pegs. You will need two profiles for each corner. Tap them into the ground about 450mm outside your marked line and on either side of each corner. Remove the original pegs and string.


2 Stretch string between the cross bars so you have a tight string outline. Put a line level in the middle of each string line and adjust the height of the profiles until it indicates that the string is level. Now use a spirit level to get each of the cross bars on the profiles level. Lightly tap the pegs until you achieve this.


3 Set out the first corner of the deck. Lay a joist along an edge of the cleared area. Stand a spirit level vertically against the joist in the corner. Slide the string lines along the cross bar on the profile until they cross directly over the corner of the joist. Now you can use the 3-4-5 triangle method to square each corner.

3-4-5 triangle

Measure 900mm from the corner on one string line and wrap a piece of masking tape around. Then measure 1200mm from the corner along the other line, and mark with masking tape.

3-4-5 triangle

Now measure the distance between the two marks, the third side of the triangle. It must measure 1500mm, so adjust the position of the string tied to the profiles until it does and the corner is square. Any multiples of 3,4 and 5 will give you a right angled triangle.

Make quite certain you have the strings square by measuring across the diagonals. Mark on the cross bar with a pencil where the string is positioned. Move the string to one side and make a small notch with a saw. Tie the string securely so that it lies in the notch. Do this on each corner.

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Good foundations are essential.

1 Mark the positions for the joist support posts and dig holes 600mm deep and about 400mm square. Put a piece of old concrete block in the bottom of each hole for extra support. Each corner post must be offset by the width of one post. this is to allow for balustrade posts to slot into the corners.

2 Cut the first post to the correct length and treat the end with end-grain preserver. Set the post in the hole using dry post mix. Use a spirit level to make sure it's vertical, and make a temporary support to hold it in place. Tamp down the mix as you add it, and shape the top so that it slopes away from the post.

3 Use a spirit level to set the height for the rest of the posts using the first as your guide. Leave to set for 24 hours. Dry post mix takes moisture from surrounding soil, you can speed up the process by watering the area around each hole. Finally, lay building membrane over the area and cover with gravel to a depth of about 40mm. Leave to dry for a further 48 hours.

Fixing to a building

If you are building adjacent to a house, then you must attach the sub-frame to the wall. If there are any air bricks in the wall, then you should not build across them or interfere with the damp proof course, raise or lower the height if necessary. Drill holes in the joist to be fixed to the wall, alternate the holes top and bottom at 400mm intervals.

You will need to fit joist hangers to the ends of the joists later, so check where they locate before drilling the holes. Use a spirit level to get the joist level, then mark with a pencil through the drilled holes onto the wall. Remove the joist and drill holes into the wall where you have marked. Fix it to the wall using expanding masonry bolts and washers.

Making the sub-frame

1 Cut the outer joists to the correct length, remembering to allow for the timber overlap at the corners. If you are attaching the frame to a wall, fix each side joist to the one attached to the wall using metal joist hangers.

2 For the other corners use two countersunk coach screws through the joist and into the post. Use two more through the adjoining joist and into the end of the one you are fixing to.

3 Decide which direction the deck boards will lay as the inner joists must run the opposite way. Cut the inner joists to length and fix each one to the frame using two countersunk coach screws. Drill pilot holes first, go through the outer joist and into the end of the inner joist spacing them evenly. Inner joists should be positioned no more than 400mm apart, measured from the centre of each joist. For accuracy, lay a tape measure along the side joist and mark the inner joist positions with a pencil. Do the same on the other side, line each inner joist up to the marks.

Working round existing features

Don't cut down a tree, build your deck around it. All you need to do is include a bit more sub-structure to frame the trunk. Simply block off the area around the tree by positioning joist off-cuts between the inner joists, secured either end with two coach screws. Remember to leave enough space for the tree trunk to thicken and for it to sway in the wind. Cut and fix the deck boards to fit around the tree.

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The balustrade - not just decoration

Railings are an essential safety feature and there's a great range to choose from. You must attach them before laying the deck boards. The handrail should be 1m above the deck and the baserail no more than 75mm above the deck board surface.

1 Work out the positions for the deck posts, they should be no more than 1200mm apart. Cut the handrail and baserail to the correct length. Using a work bench, clamp the baserail, and mark on it the position of the balusters. They should be no more than 100mm apart, measured from the centre. Drill pilot holes right through the rail in the marked positions.

2 Stand the balusters into the slot in the underside of the handrail. One at a time, make a pilot hole diagonally through the baluster and into the rail. Secure with a 50mm deck screw. Use a piece of wood, cut to the right size, to give you the spacing.

3 Fit the baserail by lying it across the balusters, with the slot uppermost, and securing with 50mm deck screws inserted through pilot holes. Use the wooden spacer to maintain accuracy.

4 Fit the deck posts into position. Use a quick release clamp to hold them in place whilst you fix each one to the subframe with two countersunk coach screws.

5 Strengthen the joint by using a joist off-cut fixed into the gap between the post and the next inner joist.

6 Drill two 12mm holes, 38mm deep in the end of both the hand and baserail. Put wood glue on the end of two 12mm dowels and insert them into the hole in the rails. Hold against the post, mark the position of the dowels and drill two 12mm by 38mm holes at the fixing points.

7 Put wood glue on the other end of the dowels and join the hand and baserails to the post by inserting the dowels into the holes.

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Laying the boards

Check you have not left any tools under the frame and that pets cannot get trapped once you start to lay the boards. Lay the deck boards at right angles to the sub-frame inner joists, working from the outer edge of the frame inwards. Boards on the outer edges will need to have notches cut in them where they meet the deck posts.

1 First cut the board to length. Use a clamp to hold it in position against the outside of the frame whilst you mark the outline of the post. Transfer the board to a workbench and cut out the unwanted wood using a power jigsaw.

2 Lay the board in position, check it is level and that the inner edge is flush with the deck post. Use a wood plane to shave a little off the width of the board if it doesn't quite fit. Fix to the sub-frame using two 50mm deck screws into each joist.

3 Fix all the deck boards to the sub-frame using two 50mm deck screws into every joist. Leave a 3mm gap between boards and make sure any joins are located halfway across the joist. Stagger the joins to increase strength and produce a professional finish.

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Making steps

A raised deck may need steps for access into the garden, or to link different heights of decking. The easiest option is to buy ready made risers and treads which can be adapted to fit. Steps should not be less than 760mm wide and a good width is 900mm. If you decide to make them wider than this you would need to add a central step riser.

1 Create a firm base for the steps. Level the ground and add either a deep bed of gravel, paving slabs or a concrete pad.

2 Decide on the width of the steps and cut the treads to this size. Measure the height of the risers against the height of the deck and if necessary shorten them at the bottom edge.

3 Cut two sections of joist timber to the same length as the treads and attach them to the risers, top and bottom, using two coach screws at each point.

4 Put the risers in position against the sub-frame. Check they are level and fix the top rear support panel to the deck sub-frame. Drill four equally spaced holes and attach using countersunk coach screws.

5 Fix the treads to the risers, using two 80mm deck screws at each end. Don't forget to treat all newly cut wood with end-grain preserver.

PEFC & FSC® logo

At B&Q, we work hard to ensure, wherever possible, we only use wood that comes from sources that are backed by schemes which certify that the forests are well-managed. The FSC® or PEFC symbols give you that assurance. Look out for them on our products.

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