Installing GRP and Polyester Pools: Delivery, Base and Backfilling
A prefabricated GRP pool is in the ground in a single day – provided the access route, foundation and backfilling are right beforehand. This build report guides you from the crane lift, via the bearing surface, through to connecting the fittings, and shows where most projects stall.
GRP or Polyester – and Why Logistics Is the Deciding Factor
GRP pools (glass-fibre reinforced plastic) and polyester pools essentially describe the same construction: a swimming pool shell made of polyester resin with glass-fibre reinforcement, usually with a coloured gelcoat surface. The pool arrives on site as a finished, dimensionally stable monolith – saving you formwork, liner and rendering. The trade-off is a single component that has to be transported whole and is decidedly bulky.
The real effort therefore shifts from building the pool itself to preparation and logistics. A properly planned foundation, a suitable crane and a controlled backfill determine the result. Mistakes at these stages can hardly be corrected afterwards, because by then the pool is already in the ground.
Crane Logistics and Site Access: The Underestimated Sticking Point
The most commonly underestimated point is not the pool, but the route to get it there. A GRP pool five to seven metres long is an oversized load that has to be lifted clear over fences, hedges or even the house. Before you order, clarify whether a mobile crane can be set up and what boom length it needs for the planned lifting distance and the pool weight.
Check these points together with the crane and pool-building companies:
- Access and set-up area: width, load-bearing capacity of the ground and turning circle for the low-loader and crane.
- Clear lifting path: overhead lines, trees, roof overhangs and neighbouring buildings within the slewing range.
- Weight and reach: the pool weight plus the lifting gear determine the crane size required – the reach decreases as the weight increases.
- Clearances: safety clearances to embankments, cellar walls and the open excavation pit.
Rigging and lifting are a job for qualified personnel. Build in a buffer: once the crane is set up, every missing piece of preparation becomes expensive.
Preparing the Foundation and Bearing Surface
The pool needs a load-bearing, level and horizontal bearing surface. The usual solution is a reinforced concrete base slab on a compacted layer of crushed stone and blinding; the specific design and thickness are specified by the manufacturer’s installation instructions and – on difficult ground – by a structural engineer. Stick to these requirements, as they safely carry the load and prevent later settling.
The excavation is dug larger than the pool all round, so that a working space remains for backfilling. Depending on the system, a thin levelling layer of sand or grit is applied to the finished slab, so that the pool floor rests over its full area and free of point loads. Check the levelness carefully – unevenness or stones under the floor lead to stresses in the laminate.
Take the height of the plant room and of the pipe penetrations relative to the water level into account now, so that the later pipework can be laid to a fall and can be drained.
Backfilling in Step with Filling
The most important principle when backfilling is: the water level and the backfill height rise together. You fill the pool with water step by step and backfill the annular space on the outside only ever to just below the current water level. This balances the internal and external pressure, and the pool walls are not pushed in.
Backfill in layers and evenly all around the pool, so that it does not shift sideways. Depending on the manufacturer’s specification, different options can be considered as material:
| Backfill material | Property | Note |
|---|---|---|
| Lean concrete | high stability, rigid bond | often prescribed by the manufacturer; as flowing concrete largely self-levelling, whereas earth-moist concrete must be compacted in layers |
| Round-grain gravel (rounded) | free-draining, easy to backfill | use rounded grain and place in layers; sharp-edged grit is to be avoided against the pool body |
| Sand | easy to place | turns to slurry when wet; only if expressly permitted |
Align the pool exactly horizontal in the first few centimetres of water and keep checking the position throughout the entire backfilling. Once the pool is half embedded, the alignment can no longer be corrected.
Uplift Protection and Groundwater
An empty GRP pool is a large hollow body and behaves in the ground like a ship’s hull: if ground or perched water is present in the annular space, it can float up. This is not a marginal risk, but the reason why an emptied pool can be seriously damaged or lifted out of its bearing surface when the water level is high.
Structural protection against uplift – for example via the base slab, a rigid, load-bearing concrete backfill or a drainage system with a sump – is a structural and hydraulic-engineering question. If in doubt, have the design determined by a specialist firm or a structural engineer; this applies especially with a high or fluctuating groundwater level. As the operator, remember the two practical rules that follow from it:
- Never fully empty the pool for overwintering; only lower the water level (observe the manufacturer’s specification).
- Before every draining, check whether groundwater is present – if in doubt, leave the water in the pool.
Connecting and Piping Up the Fittings
On GRP pools, the skimmer, inlet nozzles and – where fitted – the main drain are usually set at the factory or installed at prepared positions. Your task is the pressure-tight pipework from the pool to the plant room. Use PVC-U pressure pipe in d63 (DN50) throughout for the circulation lines; shorter skimmer and nozzle connections are run in d50 (DN40) depending on the design.
Make the solvent-cemented joints cleanly: degrease the pipe and fitting with cleaner, apply PVC cement (solvent welding agent) thinly and evenly, push home to the stop, and observe the joining time and the curing time before applying pressure. At the pump and filter, fit unions so that the units remain removable, and one ball valve per line for shutting off.
Lay the lines to a slight fall and so they can be drained, so that the residual water runs out of the pipes before winter. Check the entire circuit for leak-tightness before the working space is finally closed – a joint that is cast in concrete and later leaks can only be reached with great effort.
Installing a GRP/polyester pool – step-by-step procedure
- Plan access and crane operation Determine the access, set-up area, clear lifting path and the crane reach required, based on the pool weight and lifting distance, together with the crane and pool-building companies.
- Excavate the pit Excavate all round with working space, assess the ground and, where there is groundwater, provide a drainage system with a sump.
- Create the foundation Produce a reinforced concrete base slab on a compacted layer of crushed stone to the manufacturer's specification, horizontal and level; apply a levelling layer of sand or grit.
- Lift in and align the pool Have qualified personnel rig and lift in the pool, set it down on the bearing surface and align it exactly horizontal.
- Fill in the first water level Let in a few centimetres of water, check the position again and fine-tune the alignment while it is still possible.
- Backfill and fill in step Raise the water level and the backfill height together; backfill the annular space in layers and evenly all around the pool to just below the water level.
- Pipe up the fittings Pipe the skimmer, nozzles and main drain to the plant room with PVC-U pipe (d63/d50), join cleanly with cleaner and cement, fit unions and ball valves, and maintain the fall.
- Check leak-tightness and commission Check the entire circuit for leak-tightness, only then finally close the working space and commission the system.
Bill of materials
Example – please verify with a specialist. Parts and quantities depend on the installation; sizes shown are examples.
| Component | Size | Art. No. | KLN No. | Qty |
|---|---|---|---|---|
| PVC-U pressure pipe with socket | d63 | 76911 | 51515805 | as needed |
| PVC-U pressure pipe with socket | d50 | 76910 | 51515804 | as needed |
| 90° Bend, solvent socket (PVC-U) | d63 | 70980 | 00000641 | as needed |
| 90° Elbow, solvent socket (PVC-U) | d63 | 70507 | 51513083 | as needed |
| Socket, solvent socket (PVC-U) | d63 | 70740 | 51513257 | as needed |
| 2-way ball valve M1, solvent socket (PVC-U) | d63 | 128815A | 51511560 | 1 per line |
| Union Ocean, solvent socket / solvent socket | d63 | 170403 | 51512465 | on pump and filter |
| Griffon cleaner / degreaser | – | as needed | ||
| Griffon UNI-100 PVC-U adhesive | – | 20023 | 51512566 | as needed |
| GRP/polyester prefabricated pool | – | – | – | Pool body made to size; the manufacturer's installation instructions govern |
| Mobile crane | – | – | – | Size according to pool weight and lifting distance; positioning and lifting by qualified personnel |
| Concrete base slab / lean concrete | – | – | – | Foundation and, if applicable, force-fit backfill per structural specification |
| Filtration system | – | – | – | Sand or glass filter with circulation pump in the plant room |
FAQ
What does the crane operation for a GRP pool cost?
That depends on the pool size, the lifting distance and the set-up situation, and is calculated individually by the crane company. Clarify access, reach and a clear lifting path before ordering, so that the right crane size can be planned in.
Why do I have to backfill and fill at the same time?
This keeps the internal and external pressure in balance and prevents the pool walls from being pushed in. Always backfill the annular space only to just below the current water level and work evenly all around the pool.
May I completely empty my GRP pool for overwintering?
No. An empty pool can float up when groundwater is present. Only lower the water level, as specified in the manufacturer's installation instructions.
Which pipe diameter do I use for the connection?
For the circulation lines, d63 (DN50) is standard; shorter skimmer and nozzle connections are run in d50 (DN40) depending on the design. The decisive factors are the throughput of your pump and the connection spigots provided.
What is the difference between a GRP and a polyester pool?
Practically none: both terms describe a pool body made of polyester resin with glass-fibre reinforcement, usually with a gelcoat surface. The installation is the same in both cases.
This guide provides general technical orientation and does not replace professional planning. Safety-, electrical- and permit-relevant work must be carried out by qualified specialists.
