How to build decking in 5 easy steps

Render of composite decking with garden furniture on a sunny day
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Decking is growing in popularity thanks to new materials which make it last longer and give more colour options. Decking is now a viable long term alternative to a patio.

We install composite decking on a weekly basis in the northwest and thought we would write a guide based on what we have learnt over the past few years of installing decking. 

This guide takes you through building a simple square or rectangle of decking with a wooden frame on soft ground such as a lawn.

The surface can either be composite decking or wood and there will be a simple skirting around the outside.  We always recommend composite decking for it durability ,lifetime cost and various other reasons.

This method would work just as well on a concrete, paving or tarmac base but you would skip the first two preparation steps.

Make sure to read up on the types of decking you are using before planning out a complex design. For simple designs uncapped decking should be fine. For raised or more complex designs you would be best to go for capped decking.

Kit list

At each section in the guide we will tell you what tools you will need but here is a basic list. Click on the links to buy our recommendations through Amazon.

Materials list

  • MOT type 1 hardcore (Available from most builders merchants)
  • Weed Proof membrane
  • Concrete slabs
  • 100x50mm treated timber joists (preferably kiln dried to avoid warping)
  • 80mm screws
  • Limestone gravel
  • Decking boards
  • Fascia boards (If using composite decking)

Step by step guide to building decking in your garden

This method uses no concrete and simply rests on a solid base. This method is absolutely fine for low or ground level decking but should not be used for elevated decking or decking on uneven ground. You can see how to build decking for uneven ground here.

1. Dig out the area

Small area of ground dug out for decking

Mark out the size of are you would like then carefully dig this out to a depth of 100mm, be sure to check there are no underground cables or utilities running through the area.

A 100mm depth will mean the decking ends up around 15cm above the ground, if you want it to be lower then all you need to do is dig the area out a little deeper.

2.Compact hardcore base

Compacted hardcore base for composite decking

Type 1 MOT otherwise known as hardcore is used for roads, paths, patios and other building works for its ability to create a solid and permeable base with little effort.

Hardcore is available from most builders merchants and typically comes in ton bags, as a general rule you will need 2000kg per m3 of space that you are filling.

Fill the area to around 100mm then compact using either a compactor plate or a tarmac tamping square. For small areas a tamping square will be fine.

The compacted hardcore needs to be level and at roughly 80mm depth. Use a spirit level or laser level and measuring tape to check the levels. You will probably need to add more then go over it again to make sure its all correct.

When compacting, make sure to go over the area at least 3 times, this will make sure there is no movement left in the hardcore which would gradually make the decking uneven.

3.Weed membrane and paving slabs

Weed membrance and concrete paving slabs go over the hardcore

The last thing you want is weeds growing through the decking which you can’t get to, weed membrane is an essential to prevent this. Roll it out then either weight or peg it down while you do the next steps

For the paving slabs you can use almost any old slabs as long as they are stable and of uniform thickness, old slabs concrete slabs are perfect for this job.

The decking’s outer frame is the most important as this creates the shape of the decking. Place your slabs at each corner then use the remaining slabs to support the outside edges and finally the middle area.

A rule of thumb is to make sure there is no more than 1 meter between supports.

The more supports the better.

4.Treated timber frame and gravel

Treated timber decking frame set on concrete supports

Use treated timber joists which measure 100mm x 50mm or close to this. For easier installation use kiln dried timber which warps less.

Build the outer frame first and make absolutely sure the angles are correct, its common for decking to look square whilst actually being a bit off. This will make the next stage hard so make sure to take care on this stage

Fit the joists at 350mm apart (anywhere between 300 and 400mm will work). Fix them in place using 80 – 100mm outdoor screws.

You will likely find at this stage that the base is no 100% level and some parts of the frame don’t touch the supports. If any of the decking frame is more than 5mm off a paving slab you will need to use an off cut of wood to create a little leg for that part of the frame.

Make sure to check the level of the surface throughout using a spirit level. Its essential that this stage is right so allow yourself a little more time.

When the frame is in pour 20 – 50mm of gravel onto the weed proof membrane, to weight it down. This will stop wind, rodents and plants being able to move it over time.

5.Decking and fascial boards

Slate grey composite decking boards with fascia boards

The final stage is laying the decking its self

Make sure to start from the outer edge (so that any cuts are at the rear) and fix the first board in line with the edge of the timber frame.

Work backwards ensuring that each board is tight in place before fixing the next one.

Follow your manufacturers instructions for installing your chosen type of decking boards.

When the decking is in you will want to fit the fascia boards. Ensure they line up with the surface of the decking and screw them in place with 50mm screws.

And there you have it. An extremely simple method to build great decking which will last years.

If you would like you decking professionally built please visit our decking page for more information. On the fence about buying composite decking? we have a helpful guide to help you decide.

Render of composite decking with garden furniture on a sunny day

When you decking is built it takes very little maintenance but still need looking after to be the great investment you are hoping for. See our quick guide on how to clean composite decking.

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