People spend a long time choosing deck boards and very little thinking about the frame under them. Yet the frame is what carries the load, sits closest to the damp ground and is the part termites go for. In South East Queensland, with our wet summers, reactive clay and active termites, the frame is where a deck lasts or fails.
We build both steel and timber frames. Here is how they compare on the things that matter most in our climate.
Termites
Subterranean termites live across the region and travel from the soil into timber, often out of sight. A timber deck frame is food. That does not rule timber out, but it needs the right treatment for where each piece sits, and the frame has to be built so termite activity can be seen.
A steel frame is not food for termites. That is the single biggest reason people choose it for a low deck, a deck close to garden beds or a deck on a site that has had termites before.
The house’s termite protection still counts
A deck attached to the house can create a hidden path past the house’s termite barrier, whatever the deck is made of. Your certifier checks that the termite management for the house still works once the deck goes on, and that there is room to inspect where the deck meets the house. We design the junction with that in mind.
Moisture and ground clearance
- Timber in the ground or near it rots unless it is treated to the right hazard level. In-ground posts need a higher treatment level than joists above the ground.
- Cut ends of treated timber lose protection at the cut and need sealing.
- Steel rusts where it sits in wet soil, mulch or standing water, so steel posts sit on stirrups or base plates set into concrete, clear of the ground.
- Both need air flow under the deck. A low deck with no gap to the ground stays damp, and damp is what shortens the life of any frame.
- Good drainage away from the footings protects both, and stops reactive clay from moving the posts.
Spans: where the two differ
A timber frame is sized from Timber Queensland’s span tables for the timber species, grade and spacing, so the joists and bearers are chosen from a table. Those tables are for timber only.
A steel frame is engineered for the job, using the steel maker’s data and the engineer’s design for your wind rating and soil. Steel members can often span further than timber of a similar depth, which means fewer posts and footings, and more open space under a high deck. We never size a steel frame from a timber table, and the engineering goes to the certifier with the approval.
Straight and stable
Timber shrinks and can twist as it dries, which shows up as uneven boards or squeaks over time. A galvanised steel frame stays straight, so the deck surface stays flat. That matters most under composite boards, which show any unevenness in the frame.
Storms and wind
Both frames are tied down for the wind rating of your site, which in most of South East Queensland is N2 or N3 and higher on exposed or hilltop lots. A roofed deck takes far more uplift than an open one, so if a roof is likely later, the frame, posts and footings should be designed for it from the start.
Bushfire areas
Where a lot carries a bushfire attack level, the frame has to suit the rating as well as the boards. Steel framing handles the higher ratings well. Timber can be used at some levels with the right species or protection. We check the bushfire mapping for your lot before we design.
When we suggest timber
- A higher deck in an open, dry spot where the frame is easy to inspect.
- Where the look of exposed timber posts and beams is part of the design.
- Small repairs or extensions where matching the existing frame is simpler.
When we suggest steel
- Low decks with little clearance to the ground.
- Decks next to gardens, lawns or anywhere the soil stays damp.
- Sites with a history of termites.
- High decks where longer spans mean fewer posts underneath.
- Decks that will carry a patio roof now or later.
Looking after either one
Keep soil, mulch and stored timber away from the posts. Do not stack firewood under the deck. Keep downpipes and garden sprinklers from wetting the frame. Have the house termite inspection include the deck. These simple habits add years to a frame of either kind.
