Swimming pool, spa and timber deck structural design in Templestowe Lower

The brief

Our client in Templestowe Lower, in the City of Manningham, was planning a new concrete swimming pool with a spa at the shallow end, a spillway and two built-in planter boxes, and a timber deck on steel stumps beside it. They needed structural drawings for the pool and spa shell, the planters and the deck, to be read with the architectural plans.

Why it needed a structural engineer

In Victoria, you need a building permit before installing a pool, spa or safety barrier, unless the work is exempt. That is the guidance from the Building and Plumbing Commission (BPC), the regulator that took on the Victorian Building Authority's functions on 1 July 2025. The owner appoints a registered building surveyor, private or municipal, to issue the permit. The application must include detailed drawings and specifications of the pool, the spa, the safety barrier and the water recirculation and filtration systems, showing how the design complies with the National Construction Code (NCC) and the relevant standards, including AS 1926 Parts 1, 2 and 3.

On this project the pool and spa walls and floor are reinforced concrete, the planters are concrete boxes next to the pool, and the deck is fixed to the pool walls. Each of them needed engineered sizes, reinforcement, footings or connections before the builder could start. The drawings' general notes also say they are subject to approval by the relevant authority before construction, and that only plans stamped and approved by the building surveyor or relevant authority are to be used to build.

What we designed

We prepared a seven-sheet structural drawing set: a cover sheet, general notes, a pool foundation plan, a deck framing plan, two sections through the pool and spa, two sections through the planters, and typical pool details. Our swimming pool engineering service covers concrete pool shells like this one.

The pool and spa shell

• Concrete. The general notes specify 32 MPa concrete to AS 3600-2018, with a maximum slump of 75 mm and a maximum aggregate size of 20 mm, unless noted otherwise. Cover to the reinforcement in slabs and walls is 20 mm where sheltered, 30 mm where exposed and 65 mm where cast without formwork.
• Walls and floor. Sections A-A and B-B show 250 mm thick walls reinforced with N12 bars at 150 mm and 200 mm centres, and a 200 mm floor with N12 bars at 125 mm and 150 mm centres. Each wall-to-floor junction has haunch bars (N12 at 150 mm centres) with two extra N12 bars.
• Depth. The plans mark the pool 1200 mm deep at the shallow end and 1800 mm deep at the deep end, and section A-A shows the floor falling between the two.
• Spa and spillway. The spa, with a footwell, has its own 250 mm reinforced concrete walls, and N12 bars at 200 mm centres in the raised section beside the footwell. The spillway is 300 mm deep, with N12 bars at 200 mm centres in its floor.
• Under the shell. The floor sits on a continuous vapour-proof and damp-proof membrane over 50 mm of sand bedding, on well-compacted natural ground.
• Skimmer and main drain. Where wall bars are cut for the skimmer box, they are cut back to leave 50 mm clearance to the skimmer body. N12 U-bars at 150 mm centres (with 40 mm cover), four N12 vertical bars, 900 mm N12 diagonals on both sides of the skimmer throat and an extra 900 mm N12 horizontal bar for each bar cut are added. At the main drain, the floor bars are cut to keep 50 mm cover, and four 900 mm N12 bars are laid at 45° both ways, tied under the main reinforcement layer.
• Corners and top edge. Wall corners are spliced with L-bars matching the horizontal wall bars, with 400 mm legs. An alternative top-edge detail adds an N12 bar and N12 L-bars at 150 mm centres under the coping.

The planters

Sections C-C and D-D detail the two planters as reinforced concrete boxes: 250 mm walls either side of a 1400 mm wide bed, on a 200 mm base on well-compacted natural ground. The walls are reinforced with N12 bars at 150 mm and 200 mm centres, and the base with N12 bars at 200 mm centres. Their height follows the architectural drawings.

The deck

• Ground. The foundation design is based on a Class M site, as specified in the site's geotechnical report, with an allowable bearing capacity of 150 kPa. The ABCB's Housing Provisions describe Class M as moderately reactive clay or silt sites which can experience moderate ground movement from moisture changes.
• Footings. There are eight P1 piers, 450 mm in diameter and in mass concrete, at least 900 mm deep or down to hard strata. Whatever the specified depth, every footing must go into stiff natural soil or rock with an allowable bearing capacity of at least 150 kPa. The notes add that ground conditions can vary significantly over a short distance, and that founding depths are to vary with them. The top of each pier should be rounded at the base of the stump so water doesn't pond.
• Stumps. Each pier carries a 100 x 100 x 5 mm hot-dip galvanised SHS adjustable-head steel stump with an L bracket and an ant cap. In the fixing detail, the stump is welded to a 150 x 150 x 6 mm plate set 700 mm below natural ground level inside the pier.
• Frame. All framing is MGP10 timber. FB1 beams are two 240 x 45 mm members, continuous, and FB3 beams are two 140 x 45 mm members. FJ1 joists are 240 x 45 mm and FJ2 joists 190 x 45 mm, both at 450 mm centres, with double joists where the plan marks them. Each beam is bolted to its stump head with two M12 bolts.
• Fixing to the pool. FB2 is a 240 x 45 mm MGP10 waling plate, fixed to the pool structure with two M12 chemical anchor bolts at 400 mm centres. Where a timber beam meets a concrete wall, it sits between two 100 x 100 x 10 mm steel angles, 200 mm long, fixed to the wall with three M16 chemical anchor bolts on each side and through the beam with three M16 bolts.
• Bracing. The B1 bracing is 30 x 0.8 mm galvanised steel strap, run across the top or underside of the joists and fixed once at each joist (or equivalent framing member) and at the waling plate.
• Loads and durability. The general notes set a design live load of 2.0 kPa for external floors. The framing notes call for H3-treated timber above ground and exposed to the weather, and H4 for timber in ground contact. Structural timber must comply with AS 1720.1 for design and AS 1684 for residential construction, and the builder is to confirm whether the timber needs termite protection.

Element	Mark	From the drawings
Pool and spa walls	–	250 mm reinforced concrete, N12 at 150 mm and 200 mm centres
Pool floor	–	200 mm reinforced concrete, N12 at 125 mm and 150 mm centres, on a membrane over 50 mm of sand bedding
Planters	–	250 mm walls and a 200 mm base; walls N12 at 150 mm and 200 mm centres, base N12 at 200 mm centres
Pier	P1	450 mm diameter, mass concrete, 900 mm deep or down to hard strata
Steel stump	UP	100 x 100 x 5 mm SHS, hot-dip galvanised, adjustable head, L bracket and ant cap
Beam	FB1	Two 240 x 45 mm MGP10, continuous
Waling plate	FB2	240 x 45 mm MGP10, two M12 chemical anchor bolts at 400 mm centres into the pool structure
Beam	FB3	Two 140 x 45 mm MGP10
Joists	FJ1	240 x 45 mm MGP10 at 450 mm centres
Joists	FJ2	190 x 45 mm MGP10 at 450 mm centres
Bracing	B1	30 x 0.8 mm galvanised steel strap

What has to be confirmed on site

The drawings leave some checks to the site:

• The client or their representative is to engage a geotechnical consultant to evaluate and verify the foundation soil conditions before construction starts.
• The builder is to confirm all dimensions, existing conditions, obstructions and limitations before ordering or delivering materials, and report any discrepancy to the engineer.
• If groundwater is found at the design depth of a beam, footing or pier, the engineer is to be contacted straight away.
• The design assumes the existing structure can carry the loads from the new work. The contractor is to verify this before starting, or get a structural assessment from a specialist structural engineer.
• Any inspections required during construction are to be booked 72 hours in advance.

The result

Our client received a seven-sheet structural drawing set for the pool, spa, planters and deck, issued for approval. It gives the building surveyor the structural drawings for the permit application, and gives the builder the wall, floor and footing sizes, the reinforcement, the frame members and the connections to build from.

Once the pool and spa are built, the BPC says they must be registered with the council, which for Templestowe Lower is Manningham City Council. The safety barrier must also be inspected, and a certificate of barrier compliance lodged with the council.

Planning a pool or deck in Templestowe Lower?

Templestowe Lower is in Melbourne's eastern suburbs, one of our service areas. For a pool, send us your pool builder's layout or plans, a site plan showing the distances to the house and boundaries, a few photos with an idea of the slope, any soil report, and anything your council or building surveyor has asked for. Decks that need engineering are part of our residential structural design service. Request a fixed-price quote or call 03 7048 4006.