Composite Decking Resources

Deck Preparation

Our Guide to getting your deck down. 

1. Planning your deck

Designing and building a deck can be a fun and a rewarding experience. You probably have given some thought as to what you want in a deck, now is the time to really visualise it. What will your deck be used for, relaxing, entertaining, will you put garden seating, BBQ, tables with a parasol on it and how many people might you have on your deck? Will there be children and elderly people using it?
Questions to review before beginning
  1. Where will I install the deck?
  2. What size do I want the deck? 
  3. Which deck board do I like best?
  4. Which direction should the boards run?
  5. Which substructure will I use: Hardwood, *Steel or Aluminium joists?

*Steel / Aluminium joists >2mm: A hole must be pre drilled in the joist and specific 

Steel Joist Installation Kits must be used.


2. Location and Deck Size

When deciding the size of your deck, look around at the space you have, your house, the size of your garden and what proportion of deck will enhance and improve the look of it. Your deck should have a southerly aspect. Is the ground level or falling? How close to the house would I like the deck? When deciding on the size of deck you need to take into account that the deck boards are 3600mm in length. You can make a deck any length, but you want to avoid having a deck with very short end boards. Therefore it’s important to plan your exact deck layout.

3. Tools Required

Whiteriver Decks can be installed using the same tools that you would you use for fitting any timber deck. • Tape Measure • Electric Saw • Level • Square • Cordless Drill • drill bits • Building Line • Safety Goggles

4. General Information

Composite decking has a composition of 60% timber, 30% HDPE Plastic and 10% Resins / Pigments etc. While the timber element is very stable (it is kiln dried at very high temperatures to remove the cell structure), the HDPE expands and contracts on the length of the board in line with changes in temperature and humidity. It is necessary to leave a perimeter gap of 10mm around where the deck meets fixed objects/obstructions and also a 5mm spacing must be left at each short board end to allow for normal seasonal movements.

5. Design

Once you have made a decision on the above, now you need to decide on the deck design. A deck that is well designed can do amazing things to your home. First decide on the direction you would like the boards to run please note boards must drain along the length of the board, the minimum fall required is 1.66% (1:60 fall) to allow water drain off the boards e.g. a 5 metre deck should have a fall of 83mm across the deck. Do not install the decking flat. Will there be steps? Would you like handrails around it? What colour will suit your garden and house best?

6. Ventilation & Site Conditions

There should be good drainage under the deck and all topsoil should be removed and replaced with clean stone, unless the deck is at least 1000mm above ground level with open ventilation under the deck. No soil should meet the deck boards or under structure. The ground/substructure should be properly supported – please consult with an engineer if you are unsure. Whiteriver composite decking products CANNOT be directly installed onto a flat surface. It must be installed onto a substructure, so there is adequate and unobstructed air flow under the decking to prevent excessive water absorption.
Good ventilation under your deck is key to it performing well in the long term.

For non screed and screed surfaces, plan a minimum of 100mm (4 inches) of continuous net free area under the decking surface. This is required to allow for adequate ventilation on all deck types so air can circulate freely between adjacent joist members to promote drainage and drying. Air must have an entry point and exit point to the sub construction.

Joist should be built up on two criss cross layers or the joist should be supported with plastic pedestals to allow for air movement. For small balcony areas, less than 10m2, it is possible to have a lower clearance provided sufficient drainage and air movement can be provided. For balcony projects, we recommend getting the design reviewed by an engineer.

Please note areas that are walled in on all sides are not suitable for deck installation as there will not be enough air movement under the deck, unless ventilation through the walls is provided for the undercarriage. If there is any dampness under the deck, it can lead to mould build up underneath the deck and excessive swelling, expansion and contraction in the boards. In summary, it is vital that the area underneath the deck is free draining and per above, adequate ventilation is provided for.

If you require any technical advice, please contact our sales office on Email:

WRG Resources

Composite Decking Guide 

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Installation - Do and Don't Avoid Installation errors