Project Spotlight: Blackbutt Pool Deck with Sleeper Protection — Mosman

Location: Mosman, 2088 NSW
Year: 2025
Materials: 140mm Blackbutt decking, extreme joist subframe, timber sleepers

The Site Challenge

This Mosman pool deck project came with an unusual problem: soil washing. The garden bed adjacent to the pool was allowing water to carry soil underneath the deck area during rain events — a problem that, over time, causes the subframe to sit in wet, contaminated conditions and fail prematurely.

The previous owner of this property had a basic pine subframe that was already showing signs of distress from this soil movement. Our job was to rebuild the deck properly and solve the drainage problem at the same time.

140mm Blackbutt Decking

We specified 140mm wide Blackbutt boards for this pool surround. Blackbutt is one of the most popular hardwood decking timbers on the Northern Beaches and North Shore for good reason:

• Durability Class 1 above ground — the highest rating available
• Janka hardness of 9.1kN — hard enough to resist poolside foot traffic and furniture
• Naturally slip-resistant grain texture — important for wet pool surrounds
• Beautiful pale blonde colour that oiled takes on a warm golden tone
• Widely available in NSW, which keeps pricing competitive

The 140mm board width gives the deck a clean, contemporary look with fewer visible joins compared to 90mm boards.

Low Profile Frame with Extreme Joists

Half of this deck sits very close to ground level — too close for standard pine joists to perform reliably over the long term. Ground proximity means moisture, and moisture means rot risk for untreated or poorly treated timber.

We specified extreme joists for the low-profile sections. Extreme joist is an engineered composite joist product (similar to Trex or DecksPlus) that:

• Will not rot, warp, or delaminate even in permanent ground contact
• Is termite proof — no food source for pests
• Carries the same span ratings as equivalent structural timber
• Requires no ongoing treatment — install it and forget it

The higher up sections used a standard treated pine frame, since they had adequate ventilation and clearance.

Timber Sleepers to Solve the Soil Washing

To address the soil washing issue, we installed timber sleepers at the perimeter of the frame where ground contact was occurring. These sleepers:

• Create a physical barrier preventing soil from washing under the deck
• Lift the structural members off direct ground contact
• Channel water away from the subframe and toward the garden perimeter

It's a practical, low-cost solution that extends the life of the subframe by eliminating the primary cause of failure.

The Result

A clean, contemporary Blackbutt pool surround that looks great, performs safely around water, and has a subframe engineered to last. The soil washing issue is solved at the source, and the extreme joist sections mean the low-profile areas will perform as well as the elevated sections for decades.

This is the kind of thinking-ahead that separates a well-built deck from one you'll need to replace in 10 years.

Considering a Pool Deck in Mosman or the North Shore?

If you've got a pool surround that needs replacing — or you're planning a new one — we'd love to take a look.

Call us on 0468 093 569 or get a free quote.

MJ Leckie Carpentry — Licensed Carpenters, Lic 353438C