Home Improver Guide 1
TIMBER DECKS

BEFORE STARTING  |  DESIGN STEPS  |  TABLES    | CONSTRUCTION

BEFORE STARTING 
CONSIDER ...   top

A raised platform deck is an excellent way of dealing with problem building sites. On uneven, sloping, rocky or poorly drained land you can extend your livable area into space that would otherwise be unused.

As with any project, careful planning is a must. You should consider clearly the activities your deck will accommodate, whether it be a sunny breakfast area; a safe play area for children; or a place for barbecues and entertaining. Consider also the size and shape you will need and how it will influence the circulation from inside and outside your home.

And finally, consider its position. Will the deck affect your security or intrude on a quiet area of the home? Will it be sunny in winter, shady in summer or protected from blustery winds?

This is an illustrated guide to planning and building a timber outdoor deck that can enhance your enjoyment of the outdoor spaces around your home. Before you start construction consult with local building authorities to ensure your project conforms with current building regulations.
NOTE: For pool fence safety requirements consult your local building authorities or see our Home Improver Guide No. 2 'Pool Fence'.

DESIGN STEPS  top

1. TIMBER SELECTION
Species suitable for decking include: Mixed Hardwood; Turpentine; Ironbark; Spotted Gum; Forest Red Gum; White Mahogany; Tallowwood; Blackbutt; Cypress; Treated Pine (H3); and Merbau (Kwila).

2. PLAN
Draw several floor plans of the deck on graph paper to a scale (e.g. 1 m = 5 squares) until you have the most effective size.

3. POSTS
Mark the posts in at the corners. Then mark in an economical grid of intermediate posts derived from Table 2  'Bearers' for the type of timber you have chosen. Use bearer spans 2.4 m or less for economy.

4. BEARERS
Draw in the rows of bearers as broken lines connecting the posts and write in the size according to Table 2. Remember that the decking boards will be travelling in the same direction as the bearers.

5. JOISTS
Draw the rows of joists as solid lines at 90 degrees to the bearers and spaced according to the joist species selected. (Refer Note 3 to Tables.) Write in the joist size and spacing. (Refer Table 3).

6. DECKING
Decking is mainly available in 70 mm and 90 mm widths in both plain and reeded surfaces. Quantities of decking can be calculated by the following method:

 

Width of deck in metres x 1000        x Length of deck
Width of board + gap                 

NOTE: Add 10% for wastage

7. BRACING
Bracing must be provided on at least one corner of the deck.
Refer to Construction Bracing 6.

TABLES  top

TABLE 1.  POSTS       back
 

TABLE  2.  BEARERS     back
 

TABLE  3.  JOISTS (Joist spacing 450/500mm refer Note 3)    back


TABLE 4.  SHOT EDGE DECKING   back


 

NOTES TO TABLES
1 . Joists and Bearers not to carry roof loads (i.e. roof/pergola supports to be directly above deck supports).
2. Where undressed F7/F8 softwood is to be used, use the next largest undressed size (e.g. if 90 x 90mm use 100 x 100 mm).
3. Maximum joist spacings:    Cypress, Treated Softwood (21122 mm thick) - 450 mm
                                            Hardwood (19 mm thick) - 500 mm
4. Although 35/38 mm wide joists are structurally adequate, the number of nails used at joints may cause splitting.  45/50 mm wide joists are recommended.
5. For other stress grades or larger spans refer to AS 1684 National Timber Framing Code or TRADAC'sTimber Framing Manuals.
6. All nails to treated softwood joists should be deformed ring shank nails.
7. Load Width to bearers must be in accordance with the following diagram:

CONSTRUCTION  top

1. SETTING OUT
Using a tape measure, string line and level, locate the corners of the deck, and set up your forms and string lines to accurately mark the centre of your posts as shown in Figure 1.

Square the corners using the 3-4-5 ratio method* and level the string lines. Measurements along these lines will give the positions of intermediate posts and vertical measurements at these points should give the height of the bearers above the ground.

If one side is 3m long, another 4m long and the hypotenuse 5m, then the triangle will be right-angled.

2. FOOTINGS
Concrete around the post brackets so that the timber post will stand clear of the concrete by 75mm. Plumb the post brackets before the concrete sets. See Figure 3.

3. POSTS AND LEDGERS
After the concrete is 4 days old bolt the posts to the brackets using 2/12mm galvanised hex-headed bolts. Reposition and level your string lines so as they now indicate the top edge position of the bearers when fixed to the posts. The post may be cut off just above bearer level, or allowed to continue on to support the hand rail, or be continued right through to support a roof or pergola structure. See Figure 2.

Where the house or adjoining structure is sufficiently solid to support one side of the deck, a 100 x 50mm ledger fixed to the house framing with 100mm Type 17 No. 14 screws at 600mm centres can be used to support the joists or decking as the case may be. See Figure 4.

4. BEARERS
Lay out the bearers on saw stools beside the posts to which they will be bolted. Mark the position off and drill bolt holes through the bearers so as they correspond with the posts. A splay joint may be used to join two bearers at a post. See Figure 5.

Clamp the bearers to the posts in their correct positions and drill through the post via the holes already made in the bearer. Bolt at each post using 2/12mm galvanised bolts and washers. You may choose to house the bearer 12mm into the posts. You may cut off bearer ends flush with the end support posts or cantilever the bearer up to 400mm.

5. JOISTS.
Mark the position of the joists on the bearers using a string line and tape measure. Set out the joists over these marks making sure you place any curved timber so the bow is up and the ends point down.

Check that the finished deck is level and will be set lower than the internal house floors. See Figure 6. Fix with 2/75 mm skew nails or proprietary galvanised framing anchors. Trim off flush with bearer or cantilever the joists up to 400 mm.

6. BRACING
Brace below deck from post to post in two directions with double diagonal timber braces at about 45'. For decks up to 3 metres high use 100 x 50 mm braces. For decks below 1.8 m high use 75 x 50 mm braces and for low decks where you can not get 45', brace from the bottom of both posts to the mid-span of the bearer or joist. See Figure 7.

7. DECKING
For maximum life from your decking, prime coat all surfaces of decking boards and tops of joists before laying (refer Finishes).

Run a string line one board-width in from the outer edge of the joists. Drive a positioning nail into each joist along the string line and place the first board hard against these nails before fixing. Fix using nails as per   Table 4.

Lay the remaining boards using 5 mm wide spacer blocks for your seasoned decking. Each board should have two nails at every joist kept 12 mm in from the edges and bit flush with or just below the surface. Ends and joins should be pre-drilled (to avoid splitting) and prime coated.

Regularly cheek that the boards are running true by measuring to the opposite or inner edge of the deck. Lay boards over length, at either ends, and after laying the deck, mark with a string line and trim them all off in one neat cut.

NOTE: Tongue and groove.flooring should not be used in weather exposed situations.

8. TRIMMING AROUND TREES
Rather than cut down a favourite tree to make way for the deck, create a feature by designing the deck to allow the tree to grow through it. Trim around the trunk of the tree with your joists making sure you allow plenty of room for at least 20 years of growth in its girth. This will depend on the age and species of the tree. Scribe the decking in around the tree also allowing room for several years of growth. The decking can be further trimmed back every few years if necessary.

9. HANDRAIL
A handrail is required when the distance from the deck to the ground below exceeds 1m. Its height above the deck should not be less than 1m and the width of any open spaces in the balustrade should not exceed 125mm.

Decorative handrail such as 'bread loaf' and 'ladies waist' or 115 x 45 mm F17 timber handrail on its flat will span a maximum of 2.4 metres. The handrail can be fixed to the posts with either skew nails, false tenon joint or galvanised metal brackets. See Figure 8 . If built in compliance with pool fence requirements, the balustrade may also perform as a safety fence around swimming pools.

10. FINISHES
Correct finishing is important for extending the life of exposed timber. All cut ends should be sealed with stain or oil based wood primer during construction.

OIL STAIN FINISH: Coat all surfaces of decking and top edges of support members with one coat of oil based stain or water repellent preservative, then one coat of oil stain before work begins, and another to exposed surfaces after completion.

PAINT FINISHES: Coat all surfaces of decking and top of joists with oil based wood primer followed by the first coat of a quality decking paint before construction, then a further coat on completion.

See manufacturer's advice before applying finishes.

11. MAINTENANCE
The long term performance of timber decking in weather exposed situations is dependent on regular and effective maintenance. The frequency of re-coating will depend on the degree of exposure and the type of finish used.

We recommend that decks should not be hosed down or be subject to regular wetting from irrigation systems and that pot plants should be placed in drip trays standing on small cleats clear of the deck.

Any leaf material and debris should be broomed off at regular intervals.

Figure 1     back	Figure 5      back
Figure 2    back	Figure 6    back
Figure 3    back	Figure 7   back
Figure 4   back	Figure 8    back

Information sourced from TRADAC Home Improvement Guide.
(Timber Research and Development Council)

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