How to build a deck

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• Framework of 150mm x 50mm (6" x 2") *tannalised timber at 500mm (19") centres

• Ground supports at maximum of 1.5m ( 5ft) centers, i.e. no more than this span between supports.

• This gives a main frame where there are spaces no more than 1500mm x 500mm, these spaces edged by 150mm x 50mm (6" x 2") timber

• These spaces should be filled in cross-ways using at least 100mm x 50mm (4" x 2") giving a maximum gap size of 500mm x 500mm (19" x 19") .

• All cut ends of decking boards and tannalised timber should be treated with timber preservative (not cheap "fence coat")

• Area under the deck should be stripped of turf and weeds and have a landscaping membrane laid with gravel over the top to keep it in place and prevent weed growth.

Plan of a 3.1m x 3.1m (10ft x 10ft) deck.

Minimum 4" x 2" (100mm x 50mm) infill timber frame. maximum unsupported area 500mm x 500mm (19" x 19"). The gaps are shown for clarity - there shouldn't really be any gap!

Ground supports at maximum of 1.5m (5ft) centres.

These sizes are the limits of what should be used, if the deck is 4m x 4m for instance, an extra set of ground supports should be used, rather than "stretching it", likewise extra infill should be used rather than make the unsupported gaps even larger.

Aim to "over-engineer"  NOT  get-away-with-it.

*Tannalised timber

This is timber that has been treated with a long-lasting preservative. A copper-based stabilizer compound is forced deep into the timber under pressure. This results in a finished product that is impregnated with a long lasting preservative far better than you could ever manage just painting it on. It should be used for all outdoor woodwork as it will last far longer than untreated timber or any timber that you treat yourself. It also means that you don't need to worry about timber that is out of site and can't be re-treated.

Tannalisation results in timber that looks very much like it does in the raw state. It is not strongly coloured having only a slight green tinge to it and can easily be stained with exterior wood stain that takes almost exactly its intended colour.

• Use a straight edge from which to measure your deck - usually the house.  

• Mark out the outline of the deck to begin with using builders line.

• Right angles can be fixed using a "framing square" - a large set square about 16" x 24". Alternatively three pieces of wood nailed together with the edges in the ratio 3:4:5 will give a right angle - measure carefully and make it large, a 1mm error over 1m is less important than over 10cm.

• Dig foundations first for the supports and place the supports very carefully and accurately.

• Don't rely on the timber you are using to be as straight as it should be. Exterior grade tannalised timber and decking boards can be somewhat warped, especially over the lengths used in deck building. Rely on your builders line for accuracy, not the pieces of timber.

• On the main frame use at least 90mm x 6mm screws, three off at each corner joint. Pre-drill a 4mm hole so as not to split the timber near to the edge. Alternatively use coach screws 100 mm long into pre-drilled holes. Coach screws give a protruding head whereas normal wood screws can be driven flush with the timber (you may need to counter-sink).

• Suspend internal frame members using joist hangers. These can be nailed in place or better still use screws power-driven to save time and effort and give greater accuracy.

• Smaller internal frame members can be held in place with 6" nails, make sure you buy galvanized nails and take care when hammering in place so they are not bent over - use a large hammer! Pre-drilling a hole about 2/3rds of the nail diameter for about half the length of the nail will help.

• Decking boards can be held in place with decking clips, decking screws or nails. Decking screws leave the most visible head, but are probably the most secure fixing and also the most time consuming. If using nails hire a "Paslode" nail gun from your local tool hire firm, these run on gas and each nail is fired into the wood by a small explosion. Always fix with at least two and preferably three fixings at each point to prevent "dishing" of decking boards as they weather. Fix decking boards whenever they cross a frame member.

• Only join cut decking boards above a frame member so that the ends of both boards are suspended.

• Use a fine-toothed saw to cut the ends of decking boards and avoid splitting of the wood, either a circular off-cut saw or a jigsaw.

• Seal all cut ends of decking boards with timber preservative.

• Use pieces of 5mm thick wood as spacers when fixing decking boards against each other, fix the ends first and then the middles - not all decking boards are as straight as they should be and this will "iron-out" the discrepancies.

• The edges of your deck will have visible rough cut 6" x 2" (150mm x 50mm) timber, face this with decking boards for a better finish.

• If your deck is not laid at perfect right angles due to surrounding areas then lay the decking boards at 45°  this covers up a multitude of sins.