How to build a deck

A deck consists of a very substantial sub-frame that you don't see, covered in the decking boards that you do see.

The sub-frame is built to the same standards as in the upstairs floor of your house. It should be very solid to the point that you don't realize at all that you are walking on a structure above the ground.

Specifications

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

6" x 2" (150mm x 50mm) timber main frame at maximum of 500mm (19") centres.

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" rather than "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 colored having only a slight green tinge to it and can easily be stained with exterior wood stain that takes almost exactly its intended color.

Laying out

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.

Foundations

Deck foundations should be laid on either large 4" x 4" (100mm x 100mm) tannalised posts set into at least 1ft (30cm) of concrete in the ground, or onto breeze blocks laid flat and bedded onto a dug-in concrete foundation. The first gives the minimum extra elevation i.e. deck at about 6" (15cm) above ground level, the second gives extra elevation of the breeze block too unless these too are dug into the ground (with concrete beneath them).

Go deeper than 1ft (30cm) for post foundations if the deck is to be elevated up to 2ft (the same in the ground as out of it). If the elevation is over 2ft then consult the professionals. If you don't feel you need to, then you are already beyond the scope of advice on these web pages (or you should be).

It is possible to secure the edge of the deck to the house using a "wall plate", I prefer not to do this if possible so as not to start drilling into the brick work. It is perfectly possible to install a deck right up to the house suspended on free standing fixings.

Fixings

Use large and substantial fixings.

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.

Finishing

Your decking boards are what will be seen, so when you come to placing them take a little extra time to get things right.

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 .

Tools and equipment

Measuring tools

Tape Measure - min 5m
Builder's Line
Chalkline, (also useful for lining up screws if using these to fix decking boards)
Framing Square
Combination Square
Bevel Square
Spirit level - the longer the better.

Hand Tools:

Hammer
Hand Saw
Set square
Adjustable spanner
Socket spanner

General Equipment:

Spade
Wheelbarrow
Extension Cord

Power Tools:

Circular Saw ( a must, there's a lot of large pieces of timber to get through)
Corded Drill
Jig Saw
Miter Saw (optional)
Power Augur (optional, for making post hole - can be hired)