Summary:
Timber Overview and Characteristics
Hygroscopic Behavior
Wood is a hygroscopic material. This means that it will release or absorb moisture from the air until its MC (Moisture Content) has stabilized in relation to the moisture content and temperature of the surrounding air.
CELLULAR
/ Seljula
/ Adjective: cellular
Relating to or consisting of living cells.
"Cellular proliferation"
Expansion and contraction - sun and rain - humidity changes:
Timber will absorb and release moisture.
DECKING OPTIONS & SPECIFICATIONS SUMMARY
Pine
Other Names: Pinus Elliotti, Pinus Patula, Pinus Radiata
Average Density: 450kg/m³
Durability: Non-durable
Uses: Structural Timber, Furniture, Plywoods, Flooring, Ceiling, Laminated Beams, Mouldings
Meranti
Other Names: Light Red/ Red Seraya
Average Density: 385kg – 755kg/m³
Durability: Non-durable
Uses: Joinery, Veneer & Plywoods, Counter Tops, Cabin Fittings, Decorative Work
Balau
Other Names: Selangan Batu
Average Density: 850kg – 1155kg/m³
Durability: Very Durable
Uses: Heavy Construction, Bridges, Wharves, Railway Sleepers, Framework of Boats, Heavy-Duty Flooring
Saligna
Other Names: Eucalyptus Grandis
Average Density: 640kg/m³
Durability: Non-Durable
Uses: Joinery, Counter Tops, Laminated Beams, Furniture
Natural Durability Grouping
Very Durable - Exceeding 10 Years
Durable - 5-10 Years
Moderately Durable - 2-5 Years
Non-Durable - 0-2 Years
The placement of any timber in any of these four groups is based on its performance in "graveyard" testing. This method of testing entails the monitoring of test-sticks measuring 50x50x600mm buried in test grounds. The number of years that the test-sticks can last is the basis for the grouping.
Great care should be exercised when considering durability. It must be emphasised that the number of years, shown in the table above, is relevant only to the test-sticks, which measure only 50x50 in cross-section, and that the tests have been conducted under extremely drastic environmental conditions where the test-sticks are subjected not only to weather extremities but also to attack by a very wide range of wood-attacking organisms.
The number of years shown above is therefore only a yardstick for the grouping exercise and the actual service life of timber can be expected to be much better under the more docile conditions for common usage, especially in temperate countries where climatic and other conditions are less conducive to the activity of bio deteriorating organisms.
SUB STRUCTURE
H2 Internal (Low Hazard)
This is for interior use only and timber treated under this classification should be roof trusses, laminated beams, internally used structural timber, ceiling boards, flooring, panelling, doors, cupboards, skirting, window frames and plywood. Chemicals used here would be mainly CCA, TBTNP and Boron.
H3 Exterior Above Ground (Moderate Hazard)
CCA and Creosote are mostly used for this and higher H class treatments. H3 covers balustrades, fencing bearers and slats, outdoor decking and beams, garden furniture, laminated beams, weather boards, steps, cladding, stairs, log homes, gates, fascia boards and plywood. Spacers and cross arms used with electrical distribution, telephone and light poles are treated to H3.
H4 Ground Contact (High Hazard)
This level of treatment helps prevent agricultural posts and landscaping structures from rotting and termite attack. Also recommended for treatment in this hazard class are playground structures, fencing, pergolas, carports, flower boxes, decking, bridges, and stakes, as well as electrical distribution, telephone and lighting poles.
H5 Freshwater (High Hazard)
Timber which falls into this category, is timber exposed to continual wetting or where the timber is planted in wet soil. Timber which will fall into this category could be jetties, drains, walkways, retaining walls and slipways.
H6 Marine (High Hazard)
Only timber treated with a combination of CCA and Creosote is recommended for severe marine applications. Timber treated with both preservatives provides complete protection against marine borers and is suitable for structures such as jetties, slipways, retaining walls, walkways, and other installations falling within this exposure category.
HOW TO PLANT A POLE
If a treated pole is planted in the ground, it is essential that you allow for drainage of rainwater through the timber.
Stand | |
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If you plant the pole on concrete at the bottom of the hole, let the concrete set before planting the pole. |  |
Collar | |
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If you need to use concrete, then let the concrete form a collar around the pole with the end of the pole protruding through the concrete. |  |
Cup | |
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Do not enclose the planted end of the pole in the concrete, as it restricts drainage through the pole. |  |
TIMBER STORAGE
Structural timber stored on site must be stacked on level ground on suitable bearers to keep it off the soil. The timber should be covered with a waterproof material to protect it from direct rain and sun, while still allowing adequate air circulation through the stack. Bundles of battens must remain strapped until the moment they are installed to prevent unnecessary warping or twisting.
Substructure
Most deck structures will be constructed using CCA Treated H3 SA-Pine.
Suspended deck structures are the most frequently used configuration.
Standard SA Pine sections used:
38X38
38X76
38X114
38X152
38X228
50X76
50X228
These sizes cover joists, bearers, and structural members typically required for deck framing.
Legislation governing Timber Deck construction:
SANS 10163 – Governs structural use of timber
SANS 10082 – Timber Frame Building
SANS 10043 – Solid Wood Decking
A well‑installed, level, ventilated sub‑structure prevents moisture trapping and uneven loading; use treated joists/bearers and ensure drainage and airflow beneath the deck.
Decks which are more than 1.5m off the ground, should be designed by an engineer with experience in timber construction.
Recommended Sub-structure spacing (+90mm decking widths):
Pine (CCA Treated): Maximum 500 mm, Recommended 428 mm
Most deck structures will be constructed using CCA Treated H3 SA-Pine.
Reason: 428 mm divides evenly into 3.00 m (7 times), reducing waste.
Meranti Decking: Maximum 450 mm.
Recommended: spacing to suit available board lengths
Balau Decking: Maximum 450 mm
Recommended: spacing to suit available board lengths
Saligna Decking: Maximum: 450 mm
Recommended: spacing to suit available board lengths
Keruing Decking: Maximum 450 mm
Recommended: spacing to suit available board lengths
Important Cost Consideration
The sub‑structure is far cheaper than the decking boards. It is more economical to reduce joist spacing to suit decking lengths than to waste expensive decking through large offcuts.
Maximum Recommended Safe Spans (Unsupported) (CCA H3 SA Pine)
Section: Max safe span, unsupported
38X114 CCA H3 1.400m
38X152 CCA H3 2.200m
38X228 CCA H3 3.300m
*Note: These are guidelines only. Always confirm spans with your engineer or truss manufacturer.
WEATHERING
Outdoor weathering is a surface‑level process caused by the combined action of sunlight (UV), heat, and moisture, which gradually alters the colour and texture of timber. UV radiation breaks down lignin in the outermost wood cells — typically only 0.075–0.08 mm deep — and rain washes these degraded fragments away, exposing pale cellulose beneath. This results in a visible colour change and the formation of a silver‑grey layer about 0.075–0.5 mm thick, which develops on all timber species exposed to sun and moisture. Although weathering causes a gradual loss of surface wood fibres, the amount is small: for softwoods such as pine, only ±6 mm of thickness is lost over roughly 100 years. Moisture absorption and drying cycles also create mechanical effects — unfinished smooth timber becomes rough as the grain raises, and natural moisture‑content fluctuations lead to surface checks that may enlarge into larger cracks over time. These changes are normal, predictable, and primarily aesthetic; they occur on the surface only and do not significantly affect structural integrity unless biological decay is also present.
SUMMARY
Critical Principles for Timber Deck Installations
Timber Selection
Decking timber must withstand long-term sun, UV and water exposure.
Heavy, durable hardwoods (density >800 kg/m³) are ideal for marine and high‑exposure environments.
These hardwoods are naturally resistant to mould and termites and do not require long-term preservative treatment.
More affordable alternatives include Saligna and CCA‑treated Pine, which perform well when correctly treated and maintained.
Treatment & Finishing
Never use varnish on exterior timber — it does not penetrate, cracks under UV, and accelerates failure.
Use oil‑based penetrating preservatives only.
Maintenance is easier with oils because they nourish the timber instead of forming a brittle surface film.
Untreated hardwood will naturally weather: colour may deepen or turn silvery-grey. This is normal and not a sign of reduced durability. However, unfinished smooth timber becomes rough as the grain raises, and natural moisture‑content fluctuations lead to surface checks that may enlarge into larger cracks over time.
Sub‑Structure Requirements
The deck’s lifespan depends heavily on the quality and treatment of the sub‑structure.
Use CCA, Creosote or equivalent treated timber for bearers, joists and poles.
Cut ends expose untreated inner timber — these must be immediately re‑treated to prevent rot and termite attack.
Sub‑structure design must meet load, span and safety requirements.
Timber Movement
All timber moves with changes in moisture and temperature.
Heavy hardwoods that twist or warp are almost impossible to correct, making correct installation essential.
Correct Fixing of Decking Boards (Top 5 Rules)
Use smooth‑shank, non‑rusting screws
Eureka Ruspert Deck Screws or Stainless Steel recommended.
Correct screw length
Minimum 70 mm screws for 19 mm decking boards.
At least 50 mm must penetrate into the bearer.
Pre‑drill correctly
Drill holes large enough for the shank to pass through without thread binding.
This ensures the screw pulls the board down firmly.
Pre‑drilling reduces splitting, especially at board ends.
Two screws per bearer per board
Prevents cupping, lifting and twisting.
Correct bearer spacing
Maximum 450 mm centre‑to‑centre, ideally 400 mm.
Reduces twist, warp and bounce between bearers.
Board Spacing
Allow a maximum 5 mm gap between slats.
A 100 mm wire nail (4 mm diameter) works perfectly as a spacer.
Final Reminder
Small cost differences in timber, screws, spacing and treatment result in major differences in deck lifespan and maintenance cost. Correct installation is not optional — it determines whether the deck lasts 5 years or 25 years.