How to replace rotten deck boards

🪚 Trade Guide: How to Replace Rotten Deck Boards

What is involved in replacing rotten deck boards?

Replacing rotten deck boards is a straightforward repair when the damage is limited to the decking surface and the subframe underneath is still sound. The key is to identify the full extent of the rot, remove the damaged boards cleanly without harming adjacent timber, and install matching replacement boards with the correct fixings, spacing and finish for Australian outdoor conditions.

For Australian homes, harsh UV, trapped moisture, leaf litter and failed coatings are the usual causes of board failure. If the rot has spread into joists, bearers, posts or the ledger, the job moves beyond a simple board replacement and should be assessed against AS 1684 Residential Timber-Framed Construction and local council or state requirements.

What skill level is required to replace rotten deck boards?

Intermediate — Estimated time: 3 to 6 hours for 1-3 boards, plus drying/finishing time

What tools are required to replace rotten deck boards?

What materials are needed for replacing rotten deck boards?

Safety Requirements

Wear safety glasses, hearing protection, cut-resistant gloves, long sleeves and a P2 dust mask when cutting, sanding or handling deteriorated timber. Rotten boards can fail underfoot, so do not stand on visibly compromised sections while working.

Check for concealed services before cutting, especially where lighting cables, pool equipment feeds or irrigation lines may run under the deck. If the deck is elevated, use fall protection and comply with relevant state workplace and domestic safety expectations. For structural concerns involving joists, bearers, posts or balustrades, repairs should align with AS 1684 and any applicable local building requirements.

If the deck is part of a strata property, apartment, bushfire-prone area, or pool barrier zone, additional rules may apply. Pool fence and barrier compliance must not be compromised during repairs.

Step-by-Step Guide

Step 1: 1️⃣ Inspect the deck and confirm the rot is only in the boards

Start by checking whether the problem is isolated to the decking boards or extends into the subframe. Walk the area carefully and mark all soft, splintered, cupped or discoloured boards with pencil or painter’s tape.

Use a screwdriver or bradawl to probe the timber. If the tool sinks easily into the board but the joist below remains firm, dry and structurally sound, you are likely dealing with a straightforward board replacement. If the joist is soft, blackened, cracked or holds moisture, the repair becomes a structural carpentry job.

💡 If more than 20-25% of boards in one area are failing, replacing a full section often gives a better result than patching individual boards.

Step 2: 2️⃣ Measure and source matching replacement boards

Measure the existing board width, thickness and profile accurately. Most Australian decks use 90x19 mm or 140x19 mm decking boards, but older decks may vary slightly depending on whether the timber is DAR, pencil-round or ribbed profile.

Try to match:

If you cannot match the species exactly, replace all boards in a visually grouped section rather than a single standout piece. For example, replacing three adjacent 90x19 mm merbau boards will look better than inserting one mismatched board into a weathered field.

💡 New merbau can bleed tannins. Pre-weathering or washing before oiling helps reduce staining on paving and walls.

Step 3: 3️⃣ Remove the old fixings carefully

Locate the joists under the rotten board and identify where each fixing lands. If the board is screwed down, back out the screws with a drill or impact driver. If the screw heads are rusted, stripped or snapped, use locking pliers, a screw extractor, or cut the board around the fixing points and remove sections in pieces.

If the board is nailed, punch the nail heads through or lever the board up gently with a pry bar. Work slowly to avoid damaging adjacent boards, especially on hardwood decks where edge splitting is common.

Where only part of a board is rotten, plan to cut the board so the new joint lands over the centreline of a joist. Never leave a board end unsupported.

Step 4: 4️⃣ Cut out the rotten board cleanly

Set the circular saw blade depth just slightly deeper than the board thickness, usually around 20-22 mm for a 19 mm deck board. Make a relief cut down the centre of the damaged board if needed, then pry each half up separately. This reduces stress on neighbouring boards.

Use an oscillating multi-tool at each end where the circular saw cannot finish safely. If replacing part of a board only, cut square across the board directly over the joist so both old and new board ends are fully supported.

Once removed, clean out debris, leaf litter and any trapped moisture from the cavity. This is a common spot where rot begins.

💡 If the joist top has been staying wet, install joist protection tape before refitting the new board. It is not mandatory on every repair, but it is a very good upgrade.

Step 5: 5️⃣ Inspect and repair the joist if required

With the board removed, inspect the joist top thoroughly. Probe it for softness, check for splits around old fixings and look for moisture damage where debris has been trapped.

If the joist is still sound, brush it clean and treat the top surface and any exposed cuts with exterior timber preservative. If only a small section is slightly weathered but still structurally solid, you can continue.

If the joist is rotten, do not simply screw a new board over it. Repair or sister the joist using suitable treated timber such as 90x45 mm H3 treated pine or 140x45 mm H3 treated pine, fixed properly to the existing frame. If the deck is elevated or load-bearing components are affected, this should be assessed carefully in line with AS 1684 and may require a licensed carpenter.

Step 6: 6️⃣ Cut and seal the replacement board

Measure the opening in at least two places, because deck gaps and board alignment can vary slightly. Cut the new board to length, dry-fit it, and confirm it sits flush with the existing run.

Before installation, seal all cut ends with end-grain sealer or timber preservative. End grain is where moisture enters fastest, particularly in hardwoods like merbau and spotted gum.

For hardwood boards, pre-drill and countersink at every fixing point. Place fixings consistently, usually two per joist, set in from each board edge evenly.

Typical layout:

💡 Hardwood decking should almost always be pre-drilled. Skipping this step is one of the main causes of split board ends and mushroomed screw heads.

Step 7: 7️⃣ Install the new board with the correct spacing and fixings

Position the replacement board and use spacers so the gap matches the adjacent boards. Fix the board down to each joist using stainless steel decking screws. For most 19 mm boards into timber joists, 10g x 50 mm screws are standard; use 316 stainless steel in coastal or poolside environments.

Do not overdrive screws. The head should finish neatly flush or slightly below the surface without crushing the timber fibres excessively. Check that the new board sits level with the old deck and does not rock or crown.

If you are replacing multiple boards, stagger the work logically and keep a consistent fixing line. Uneven spacing or wandering screw lines will stand out immediately on a finished deck.

Step 8: 8️⃣ Sand, clean and refinish the repaired area

Lightly sand the new board and feather the edges where needed so it blends with the surrounding deck. Use 80 grit first if required, then 120 grit for a smoother finish. Do not oversand weathered hardwood too aggressively or you will create obvious colour differences.

Sweep and wash the area, then allow it to dry properly before applying oil, stain or sealer. Follow the coating manufacturer’s spread rate and recoat window.

If the existing deck is faded, patch coating one new board may still look fresh compared with the old surface. In many cases, refinishing the full deck section gives the best visual result and provides more even protection against sun and rain.

💡 After coating, keep furniture and foot traffic off the repaired area for the product’s recommended cure period.

Common Mistakes to Avoid

Pro Tips

Recommended Tools

If you are buying tools for repeated property maintenance or trade use, check current ATO EOFY instant asset write-off rules and eligibility, particularly for tool purchases over $300.