How to lay decking on riser pedestals

Why install your decking on adjustable risers?

Whether you plan on laying timber or composite decking, you will usually need to install your boards on supports to raise them off the ground and protect them from damp. But there's no need to lay a concrete foundation – as long as you have decking supports! The idea behind adjustable decking supports is very straightforward: these plastic risers are simply placed on the ground at set distances. As they are equipped with a base, the risers (also known as pedestals) can be fixed to the ground if required or else left as a floating installation. The main advantage of adjustable risers is that they enable you to easily adjust the level of your deck.

Riser pedestal characteristics

Adjustable riser pedestals generally have the following characteristics:

• made of polypropylene;
• frost- and heat-resistant from approx. -40°C to +60°C;
• resistant to chemical cleaning solutions (industrial and household products);
• can be laid on bitumen, gravel, grass etc.;
• adjustable from 25 mm to 260 mm for the highest models. The difference between the minimum and maximum height is about 20 to 50 mm, the most common being a difference of 20 mm;
• support weights of 1000 kg or more.

Steps for installing decking on riser pedestals

1. Preparing the ground
2. Spacing out your risers
3. Putting the riser pedestals in place
4. Laying the joist frame
5. Laying the decking boards
6. Trimming the decking boards
7. Dealing with warped boards
8. Adressing miscalculations
9. Installing decking brackets
10. Finishing and wood treatment

1. Preparing the ground

Adjustable decking supports can be installed on any type of surface: tiles, concrete slab, earth and so on. The most important factor is to ensure the ground is stable and the surface is not likely to shift.

• On natural surfaces such as bare earth, you'll usually have to go for a floating installation meaning that your structure is held down by the weight of the deck.
• If you'd rather fasten down your risers, you can first lay a concrete foundation and then fix them in place with ground anchors.

Creating a level surface

• Your surface must be level. While minor irregularities can be dealt with, your terrain should otherwise be perfectly flat. 
• On well-laid concrete or existing tiling, there shouldn't be any issues but be sure to check everything is level with a spirit level before you start work.

Laying a geotextile membrane

• If you are dealing with bare earth make sure you flatten out the ground in advance or lay down a fine concrete screed to create a level surface.
• If you decide against laying concrete, you will have to put down a geotextile membrane to prevent weed growth.

2. Spacing out the decking risers

You will have to decide on the overall height for your decking whether you want your decking to be flush with your door or install a step.

Your chosen height will dictate which risers and timber frame you choose.

Plotting your riser layout

The first step will be to calculate the number and type of riser pedestals and joists you need. Manufacturers will often indicate the number of risers required per m² so this needn't be complicated!

Number of riser pedestals per m²

Decking board direction

Next, you will have to work out which way you want your boards to face. The direction of your decking boards will affect the overall look of your deck but it will also help you to determine how to space out your supports.

3. Installing the risers

Lay out your risers at the correct intervals (as decided in the previous step). Secure them using the appropriate fixing plugs (if required) and adjust the supports to the desired level. Don't forget to incorporate a slight slope of around 2% to direct rainwater away from your house.

Please note: the spacing of the risers depends on the thickness of your top boards but also the joists underneath which serve to distribute weight evenly across the decking. Make sure you read the manufacturer's instructions and check all diagrams carefully. As a general rule, it is not recommended to leave more than 70 cm between two consecutive decking supports.

4. Laying the joist frame

Screw the joists onto the risers so that they will eventually form a right angle with your decking boards. Take great care not to damage the adjustable mechanism of the risers as you screw the joists down.

It is advisable to leave a gap of at least 10 cm between the ground and the decking boards in order to ensure the wood has adequate ventilation.

5. Laying the decking boards

At this point, you're over the worst of it! All you now need to do is to lay your decking boards one by one and attach them to the timber frame.

Fastening decking boards

There are two possible ways to fasten down your boards:

• screws
• or clips.

Please note that composite decking is almost always secured using clips while screws are the fixing of choice for timber decking.

Screwing down decking boards

It is important to pre-drill holes in your decking boards boards. Handy tip: make yourself a drilling jig so you drill to the same depth every time meaning your screws will all be level. The screw heads should be flush with the boards as drilling further will create a hollow where water can stagnate. Finally, each board should be fastened down using two screws at each end.

6. Trimming the decking boards

If you've positioned your risers well, you shouldn't run into any problems at this stage. For wooden decking, try to hide the least attractive boards at the edges or use them as end pieces which will be cut off. Leave the ends of the boards untrimmed as you go. Once you've finished the whole job, you can then cut them all at the same time using a circular saw.

To trim your decking boards, follow these steps:

1. Take measurements.
2. Mark your cutting line with a measuring square or a rule, if the boards are already installed.
3. Set the cutting depth on the circular saw.
4. Cut along the cutting line.
5. Sand the edges with sandpaper.

Wear safety goggles and gloves when cutting the decking boards with the circular saw.

9. Installing decking brackets

Fix one or more boards along the vertical edge of the decking structure by screwing them into the joists. If needed, strengthen the corners of your decking with brackets.

10. Finishing timber decking

If your wood isn't treated, it's a good idea to apply varnish and an anti-humidity or anti-fungal treatment – this is especially important for timber that has been cut.

How to lay decking on riser pedestals

Required skills

Now a widely used system, anyone with a few tools and some DIY know-how can use adjustable risers to lay their decking. Nonetheless, you will need to be know how to use a tape measure and spirit level, and be able to do simple calculations.

Tools and equipment

• An electric screwdriver (with sufficient battery power);
• Large spirit level;
• Tape measure;
• Chalk line for levelling;
• Jigsaw for cutting boards (or an electric mitre saw for faster cutting);
• Accessories such as a set square, sliding bevel and pencils.

Decking screws

Your decking screws must be appropriate for the decking material. This is an important investment that will ensure a sturdy installation. Ideally, opt for stainless steel decking screws with the right length for your timber frame. If you don't want your fixings to be visible, go for decking clips (these must also be suited to the individual board). Decking spacers should be inserted between boards to create an evenly spaced deck, with a minimum spacing of 3 mm.

Decking supports

Your decking supports, or riser pedestals, must correspond to the desired height of your deck and should be strong enough to withstand heavy loads. The risers must be chosen according to their height and weight which will determine whether they have the right load capacity to support the weight of your decking. Of course, the bigger the risers, the higher the price (usually given per m²). Please note that all riser pedestals must be topped with joist frames which, in turn, serve to support the decking boards. Your joist frame and decking must always be made from a timber with the same durability class rating. Choose a robust, hard-wearing and weather-resistant variety of timber.

Time required

Approx. 8 hours for every 10 square metres.

Number of people required

1 person

Safety wear

• Eye protection
• Gloves
• Mask

More information

For more tips on related accessories, check out our editors' other guides:

• Decking boards buying guide
• Outdoor floor tile buying guide
• Drill buying guide
• Spirit level buying guide
• Jigsaw buying guide
• Mitre saw buying guide
• Fixing plugs buying guide
• Screw buying guide
• Circular saw buying guide
• Wood care products buying guide

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