How to Build a Slatted Deck
This type of decking is suitable for level or gently sloping ground. It is relatively easy to build; all you need to do is create a layer of supporting joists and add pressure-treated or hardwood deck boards.
Preparing the Foundations
- Remove any plant growth from the area, including weeds and tree roots.
- Mark out the area with pegs and string, setting the string at the final surface level.
- Dig out the area to a depth of about 20 cm (8 in). This will allow for a 10 cm (4 in) layer of hardcore plus a 10 cm (4 in) thickness of concrete. Peat and heavy clay soils can shrink in dry weather, reduce the effect this may have on the concrete surface by excavating a further 5 cm (2 in) and increasing the depth of the hardcore to 15 cm (6 in).
- Tamp the base of the site down with a plate compactor.
Creating the Framework
You will need to make a frame to keep the concrete in place whilst it sets.
Insert 5x5 cm (2x2 in) pegs into the base every 1 m (3 ft) along the perimeter of the site, so that the tops are level with your string guides. Use a plank and spirit level to ensure that they are absolutely level.
Line planks of wood up against the inside faces of the pegs, making sure that they are level with the top of the peg. Nail them into place to form a framework to confine the concrete until it sets. The planks should be 2.5 cm (1 in) thick, and at least the depth of the concrete (10 cm or 4 in).
Frameworks for curves may be made in much the same manner, although you will need to set the wooden pegs closer together. Smooth curves may be made by soaking lengths of softwood in water to make them pliable and then bending and nailing them to the pegs. Alternatively, you could use several thicknesses of thin hardboard, which is easy to bend and shape.
Preparing the Sub-Base
Spread a 10 cm (4 in) layer of hardcore over the area and compact it with a roller or stout timber. Lay a damp-proof membrane (dpm) over the area, making sure that it extends all the way up the frame walls. Any joints should overlap by 35 cm and preferably be taped. The dpm will prevent the concrete from drying out too quickly (causing surface cracks), and will also protect the underside of the slab from attack by dampness and chemicals in the ground beneath.
Laying Concrete
Use an in situ (poured) concrete mix as follows:
- 1 part cement
- 2 parts sharp sand
- 4 parts 20 mm (3/4 in) aggregate
Alternatively, use 6 parts combined aggregate ('ballast') to 1 part cement, and omit the sand.
Start by adding about half a part of water to one part of dry ingredients to produce a very stiff mix. Gradually add more water until you reach the consistency you need.
Spread the concrete so that it is level and just proud of the formwork, working it well into the edges. Tamp down the concrete with a length of wood that spans the width of the frame, compacting it with a downward chopping motion. Slide the beam from side to side to level the surface. You may need to fill in any hollows that appear. Level again using the wooden beam.
Lay a waterproof covering such as plastic sheeting over the concrete until it dries.
Deck Board Pattern
Decide what pattern you want to lay your deck boards. This must be done before you start to construct your joists as the joists and the boards must normally lie at right angles to each other. For example, if you want your deck boards to run north to south, your joists must run east to west. Some deck board styles may affect the spacing and number of joists, for instance, a double joist will be required for a chevron style layout. Horizontal or vertical joists may be used for diagonal deck boards.
Laying the Supporting Bricks
Lay a row of bricks along the concrete base in the direction you want your joists to run (usually at right angles to the intended direction of the decking). Space this line of bricks so that they have a gap of about one brick's width in between each other. Mortar them into place, and then add further rows of bricks to act as supports for the remaining joists. These rows should be 40 cm (16 in) apart and the gaps within the rows staggered, that is, a gap between bricks in one row should lie opposite a brick in the two adjacent rows. Keep checking that the rows of bricks are level.
Installing the Joists
Lay 75 x 50 mm (3 x 2 in) lengths of timber along the rows of bricks to act as joists, inserting a layer of plastic or other damp-proof material between the two. If you need to join a length of timber, make sure that the join rests on one of the supporting bricks. Screw a plain bracket across the join. If the deck is such a length that you need to make a join in every joist, make sure that you stagger the joins across the deck; do not allow them to run in a straight line. Check levels between joists accurately with a spirit level and a straight edge.
Fixing the Deck Boards
Once the joists are installed, you can lay the 250 x 25 mm (10 x 1 in) deck boards at right angles to them. Allow approximately 5 mm in between each plank to allow for drainage and movement in the timber. Stagger the joints in the decking from row to row, making sure that all joins are positioned over a supporting joist. Fix the boards with countersunk rust-proof screws, and then fill the holes with a wood filler that matches the colour of the wood.
Creating a Finished Edge
Create a neat finish to the deck by screwing or nailing a length of facing timber along the cut edges. Alternatively, build a low brick edging and cut the deck boards so that they overlap the edging by 5 cm (2 in).