Concrete or fibreglass on mine subsidence land?

Why the obvious answer is the wrong one

Ask anyone which pool copes better with ground movement and you will get the same answer: fibreglass, because it is one flexible piece, where concrete is rigid and cracks.

It is a reasonable intuition and it is not how the NSW guidelines are written. Read across all seven and concrete appears in every guideline that names a material. Fibreglass appears in two. Guidelines 1, 2, 3, 3A and 6 permit concrete pools without naming fibreglass at all. Only Guidelines 4 and 5 list “swimming pools (concrete or fibreglass)” together — and in those two, both materials are capped at 8m.

So on the ground with the highest permitted length — Guideline 6, at 12m in-ground — the material named is concrete.

The reason is about what the approval is assessing. A pool on moving ground is not being judged on whether the shell will crack. It is being judged on whether it remains serviceable through the movement, and on whether it can be designed and certified against stated parameters. A concrete shell can be engineered: reinforcement, articulation, and a structural engineer's certification against a defined event. A moulded fibreglass shell is a manufactured product with fixed properties — you can choose it or not choose it, but you cannot engineer it to a strain figure on someone's drawing.

That is why Guideline 1, covering the highest pothole risk, permits concrete with certification from a qualified structural engineer that it will remain serviceable under a 5m diameter pothole. There is a document that can be produced. For a moulded shell there generally is not.

The two guidelines where fibreglass is on the table

Guidelines 4 and 5 cover land likely to be undermined in the future, and both state the movement the design must tolerate:

• Guideline 4 — maximum horizontal ground strain 5mm/m, maximum tilt 7mm/m, minimum radius of curvature 3km.
• Guideline 5 — maximum horizontal ground strain 3mm/m tensile or compressive, maximum tilt 7mm/m, minimum radius of curvature 5km (hogging and sagging).

Both permit swimming pools in concrete or fibreglass, with a limit of 8m in length unless the parameters detailed in the section are designed for. Both also require that connections to existing structures stay flexible rather than being rigidly attached — the pool must be able to move relative to the house rather than pulling on it.

Eight metres is a real constraint. It is a comfortable family pool and it is not a lap pool. If you want to swim laps on Guideline 4 or 5 land, the 8m cap is not a hard prohibition — it lifts if the parameters are specifically designed for — but “designed for” means an engineer's design against stated strain, tilt and curvature figures, and that is a cost and a lead time rather than a tick box.

Wet edges: the feature that is banned in one place and conditional in two

Infinity and wet edge designs are where these guidelines are most specific, and it is worth knowing before a designer draws one.

Guideline 6: no wet edge designs are permitted. Flat prohibition, on the same guideline that allows the longest pool.

Guidelines 4 and 5: any wet edge or infinity must be adjustable and designed with sufficient freeboard to ensure it remains operational within the movement parameters for that guideline.

The engineering logic is neat once stated. A wet edge works by water flowing evenly over a level lip. Tilt of 7mm/m across an 8m pool is more than 50mm of level change — and a wet edge out of level by 50mm is not a wet edge, it is a waterfall at one end and a dry wall at the other. Hence “adjustable”: the lip has to be capable of being brought back to level after the ground has moved, and the freeboard has to be generous enough that the feature keeps working in the meantime.

Guideline 6 simply declines the problem. If you are set on an infinity edge, the guideline that applies to your land may settle the question before any designer does. What an infinity pool costs covers the general economics of the feature.

Find out which guideline applies before you design anything

Everything on this page turns on one fact about your address, and that fact is free.

The guidelines are not a menu you choose from. Subsidence Advisory NSW assigns them by location, based on the mining history and the ground movement expected there. A property in a Mine Subsidence District sits under one of them, and that one determines your material, your maximum length and whether a wet edge is possible at all.

So the order of operations is: find the guideline, then brief the designer. Doing it the other way round is how people end up paying for a 10m fibreglass lap pool design on Guideline 6 land, where the permitted answer is a concrete pool up to 12m with no wet edge. The design fee is not recoverable and the disappointment is avoidable.

It is also worth raising with every builder you talk to. A builder who works in the Hunter, Lake Macquarie or the Illawarra will know the districts; one from outside may not. Ask each quote which guideline applies to your block and what they have designed to it — the answers will separate them quickly. Comparing several quotes is the fastest way to find the builders who have done it before.

The approval process itself is a separate matter from the material decision, and it has its own steps and timing: building a pool in a mine subsidence district covers how approval works. Whether you need a soil test covers the ordinary ground investigation that applies everywhere.

What this does not mean

Two honest qualifications, because this page inverts a widely held belief and should not overcorrect.

Fibreglass is not prohibited on subsidence land. It is expressly permitted under Guidelines 4 and 5, at up to 8m or longer where designed for the parameters. What is true is narrower: it is named in fewer guidelines than concrete, so on a given block it is more often concrete that is on the list.

“Not named” is not the same as “refused”. Guidelines 1, 2, 3, 3A and 6 specify concrete pools as allowable additions and do not mention fibreglass. What that means for a particular fibreglass proposal on that land is a question for Subsidence Advisory NSW, not for this page — and it is precisely the question to put to them before you commit to a shell.

Outside a Mine Subsidence District none of this applies, and the material decision returns to the ordinary trade-offs of cost, install time, size and finish. Concrete versus fibreglass covers that comparison on normal ground.

Common questions

Sources

• Subsidence Advisory NSW — Surface Development Guideline 6
• Subsidence Advisory NSW — Surface Development Guideline 4
• Subsidence Advisory NSW — Surface Development Guideline 5
• Subsidence Advisory NSW — Surface Development Guideline 1

General information only, current at 2026-09-16. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.