Wood Works No. 3 – Top tips when building a deck

Here are some top tips to consider when planning, building and maintaining your deck to ensure it performs well for many years to come

These tips are based on industry good practice, but also on personal experience of planning and building my own deck 15 years ago. As an ‘experiment’, I did not apply any finish to the deck and, frankly, did very little maintaining during this time in order to assess its performance (nothing to do with laziness on my part you understand!).  

Planning a deck

  • Consider planning consent if the deck is to be more than 300mm from the ground; visible within 20m of a highway; likely to exceed 50% of the garden or if there are conservation restrictions. Also, balustrading may be required to comply with Building Regulations depending on the height of the deck.
  • Choice of materials: hardwood, softwood, or wood plastic composite? Each has advantages and disadvantages so do your research online and find out which is the right one for you. I chose a softwood, Redwood (Pinus sylvestris), as this was the preferred choice of many deck designers at the time, it can be supplied pre-treated with preservative (see below) and is the most affordable.
  • Plain or grooved boards? When I was closely involved with decking during the ‘Ground Force’ era, where every other garden makeover programme featured decking, we were flooded with faxes of profiles from suppliers wanting approval for their decking before they went into production. Most of these were, and still are, of the grooved design. Why? Well, this was largely due to the limited experience of decking in the UK, being mostly used as walkways in marine environments. These boards are typically short length, arranged across the walkway, and laid at a distinct angle to allow water to run-off into the surrounding water; does that sound like your garden? No, mine neither which is why I went with plain boards.
I planned the deck with Grecian columns
My modest deck in 2005, incorporating Grecian columns
(from www.hoppings.co.uk)

The perception remains that grooved boards provide a degree of slip resistance. But in my experience, they provide a trap for debris to collect and mould to thrive which encourages a slippery surface, particularly in shaded locations, and thus require regular maintenance. During trials (watching from my window with a coffee after a period of rainfall) I watched the plain timber boards drying quickly before my eyes after a spring shower. I can’t remember there ever being an issue with slipperiness, other than when I was wearing very worn footwear and took the deck at speed (old Crocs – lethal. Oops street cred’ gone with mention of Crocs, but I’m still a fan!). And if you do slip, as you can on any wet surface, the benefit of decking is that you bounce! (To quote Alan Titchmarsh from Gardeners World at the time).
I didn’t realise I was a plain board campaigner until just now, but of course the majority of boards on the market are still grooved. Some are dual profile: plain on one side and grooved on the other, which gives you the option to mix and match in your design, or if you can’t make your mind up!

  • Use timber that is supplied as Use Class 4 for joists, posts and bearers in ground contact and Use Class 3 or Use Class 4 for deck boards. If using preservative treated softwoods, request evidence of treatment at the time of ordering the timber to ensure it has been treated to the correct Use Class. (See Wood Works No. 2 for info on Use Classes).
  • If using softwood, consider maximum joist spans but elect for C24 timber even if C16 is strong enough as this will contain fewer knots, and worth any extra marginal cost (See Wood Works No. 2).
  • Consider the ground below the deck. This of course needs to be suitable, and some ground preparation works such as clearing vegetation are likely to be needed, even under low level decks. Lay a weed-suppressing sheet blinded with a layer of gravel to prevent vegetation growth in the void, or worse, appearing up through the deck!
  • Also, provide for ventilation to the sub-structure to allow the timbers to dry following wetting and thereby limit the risk of decay.

Building a deck

  • Assemble the correct tools for the job: Usual tools for measuring, checking squareness and level, but a good quality cordless drill (18 or 20-volt) or two (one for pilot holes, and one for fixing to save continually changing bits) and a mitre, or chop saw are essential. Also, worth taking a few minutes to knock up a jig to ensure screw holes are in the correct position in the boards each time. Looking down a line of fixings and seeing them all over the place bothers me, especially when it is just as easy to do it properly, sorry.
  • Use correct deck screws that are designed for the job which are at least 2 to 2.5 times the depth of the deck board. There is a bewildering array of screws to choose from, but it is worth the research and extra expense to use ones that are suitable for the board that you are using. For example, self-countersinking screws cut the fibres at the surface of the board rather than compressing them. By providing a neat fixing, with the screw head flush with the surface, this avoids a potential water trap and avoids weakening the surface fibres which can lead to premature failure in service.
  • Drill pilot holes and countersink before screw fixing, no closer than 15mm from board ends (25mm for hardwoods) and at quarter points across the board width. Pre-drilling is particularly important when installing hardwoods. Providing a pilot hole that is 2mm oversize allows for seasonal movement whilst limiting the risk of failure of the fixings and splitting of the timber, both of which can occur if the boards are fixed too tightly.
Check moisture content of deck boards before building the deck
Checking moisture content of deck boards before installing
  • Consider size of gaps between boards. We know timber moves with changes in moisture content (see Wood Works No. 1 – moisture content) so the deck boards will noticeably expand and shrink across their width with seasonal changes in the weather. The rule of thumb is to allow between 6mm and 8mm between the boards. However, if you have access to a moisture meter (See Wood Works No. 1) and the timber is around 18 – 20% or higher, then its movement will be predominantly shrinkage from here on in, so consider leaving a gap of less than the standard recommendation to prevent ending up with over-size gaps. This is especially the case where timber has only been air-dried or supplied in a ‘green’ condition. Check with the supplier in the first instance.
  • Leave a corresponding gap between board ends and any posts or upstands; it looks neater, allows for drainage, prevents build-up of debris, and allows you to paint the board ends during maintenance.
  • If using treated softwood, have a pot of end-grain preserver to hand and liberally apply to all cut ends. Better still, leave cut ends in a bucket overnight to absorb the preservative. The aim is to re-instate the ‘protective envelope’ of preservative that is breached when the boards are machined.

Finally, some top tips when finishing and maintaining a deck

  • Some timber preservatives contain a wax emulsion treatment to increase the water repellency of the timber. Check with the supplier if this is the case as it will require regular re-application to maintain the coating, or otherwise allow the timber to weather for a few months before applying an alternative surface finish if desired.
  • Avoid clear, film forming varnish-type finishes as they tend to break down prematurely, crack, flake and trap moisture. Instead, consider a lightly pigmented modern deck stain which are available in a range of colours, penetrate the surface so have good abrasion resistance, and are vapour permeable, allowing the timber to dry following wetting. They can be re-applied as necessary, perhaps every 3 years or so, to maintain the appearance of your deck.
  • Regular maintenance. Decks will benefit from a spring clean to remove dirt and debris, and there are a range of deck cleaning agents and finishes on the market which will help to maintain its appearance and prevent it from becoming slippery. A pressure washer can also be used to remove mould and surface staining, but use with caution by not getting too close to the board surface and maintain a sweeping action so as not to scour the timber.
Maintain the deck by not placing plant 
pots directly on the deck boards which can lead to fungal decay
Fungal decay in deck boards due to plant pot location, 2019

Remember, in the interests of scientific research, I did not treat my deck, though it would still be functioning perfectly well today if (a) my wife hadn’t wanted a new kitchen extension and (b) we hadn’t displayed plant pots on it. By doing so, we provided an ideal environment in the deck boards under the pots for decay fungi to thrive – adequate moisture and plenty of nutrients! Once the fungi took hold, it spread silently through the untreated core of the boards that had supported the pots – happy fungi!

So, do as I say, not as I do! Avoid placing plant pots directly on to the deck (and house extensions!). Instead, place on trays supported on battens, or similar, to provide some ventilation under the pots. Also, move pots, furniture, and fixings around the deck from time to time to inspect the deck and prevent any build-up of debris and check for deterioration.

  • Check on the fixings to ensure they remain flush with the surface, preferably before the warm weather when the bare feet come out!
  • Whilst conducting routine maintenance, keep an eye on the condition of the sub-structure. If necessary, lift the occasional board to enable you to clear away any build-up of leaves and debris to maintain ventilation and promote drying of the sub-structure following wetting.

For further information:

The Timber Decking and Cladding Association – www.tdca.org.uk
The Wood Shop Consultancy – www.timberconsultancy.co.uk

Posted in

Roger

Scroll to Top