Why Should Swimming Pools Be Filled and Drained Slowly? 25 mm (1 Inch) per Hour

One of the most common recommendations encountered during the commissioning of swimming pools is that the pool should be filled and drained at a rate of approximately 25 mm (1 inch) per hour. This guidance is frequently included in project specifications, waterproofing system recommendations, and pool construction manuals.

 

Why Should Swimming Pools Be Filled Slowly?

Although there is no mandatory EN requirement, gradual filling is considered good engineering practice for several important reasons.

1. Gradual Application of Hydrostatic Pressure

As the water level rises, hydrostatic pressure increases continuously.

If the pool is filled too rapidly, this pressure is applied suddenly to:

·        Reinforced concrete

·        Cementitious levelling mortar

·        Waterproofing membrane

·        Tile adhesive

·        Ceramic or glass mosaic tiles

Gradual filling allows the structure and finishes to accommodate these loads progressively.

 2. Reducing Differential Moisture Movement

Concrete and cement-based materials absorb water when submerged.

Rapid filling can create steep moisture gradients between saturated and unsaturated areas, resulting in:

·        Differential expansion

·        Internal stresses

·        Localized cracking

A controlled filling rate allows moisture to penetrate more uniformly throughout the structure.

3. Protecting Waterproofing Membranes

Liquid-applied waterproofing membranes are designed to withstand hydrostatic pressure after full curing.

Rapid filling may:

·        Overstress weak or improperly cured areas

·        Expose application defects

·        Increase the likelihood of localized debonding

Gradual filling provides a more controlled loading condition during the initial commissioning of the pool.

4. Protecting Tile Installation Systems

Swimming pool tile systems depend on several bonded layers working together:

·        Levelling mortar

·        Waterproofing membrane

·        Tile adhesive

·        Tile finish

·        Grout

If the pool is filled too quickly, hydrostatic pressure increases rapidly.

Where defects exist—such as:

·        Poor adhesive coverage

·        Inadequate tile beating

·        Air voids beneath the tiles

·        Weak substrate preparation

Rapid water loading may accelerate tile debonding or reveal latent installation defects.

5. Allowing Continuous Inspection

One of the greatest advantages of slow filling is that engineers can monitor the pool throughout the process.

During filling, inspections can identify:

·        Water leakage

·        Crack development

·        Movement joint performance

·        Waterproofing integrity

·        Tile system behaviour

If problems are detected early, filling can be stopped before the pool reaches full capacity.

 

Why Should Swimming Pools Be Drained Slowly?

Draining a swimming pool can be even more critical than filling.

When water is removed rapidly:

·        The internal hydrostatic pressure decreases suddenly.

·        External groundwater pressure may exceed the internal pressure.

·        Differential stresses develop within the concrete shell.

·        Waterproofing systems experience stress reversal.

·        Bonded tile systems may become vulnerable if underlying defects exist.

In extreme cases, excessive groundwater pressure can even lead to uplift (flotation) of the pool structure if adequate hydrostatic relief measures are not in place.

For these reasons, controlled draining is equally important.

 

Where Does the 25 mm (1 Inch) per Hour Recommendation Come From?

The recommendation is based primarily on practical engineering experience and manufacturer guidance.

It is commonly referenced in:

·        Waterproofing manufacturers' installation manuals

·        Tile installation guidelines

·        Structural engineering recommendations

·        Pool commissioning procedures

·        Project-specific technical specifications

The exact filling or draining rate may vary depending on:

·        Pool dimensions

·        Water depth

·        Structural design

·        Groundwater conditions

·        Waterproofing system

·        Tile installation system

·        Engineer's recommendations

Some projects permit faster filling, while others specify much slower rates for large or critical structures.

 

Is the 25 mm per Hour Rule Mandatory? - No.

• EN 15288-1 – Swimming Pools for Public Use – Safety Requirements for Design
• EN 15288-2 – Swimming Pools for Public Use – Safety Requirements for Operation
• EN 16582 – Domestic Swimming Pools

These standards address the design, safety, operation, and performance of swimming pools. However, they do not specify a mandatory filling or draining rate.

The commonly quoted 25 mm (1 inch) per hour filling or draining rate is not a mandatory requirement under European Standards.

Instead, it should be regarded as good engineering practice intended to:

·        Minimize hydrostatic shock.

·        Reduce differential structural stresses.

·        Protect waterproofing membranes.

·        Safeguard bonded tile systems.

·        Allow progressive inspection during commissioning.

Ultimately, the filling and draining rate should be determined by the project specification, structural engineer, and system manufacturers, taking into account the specific characteristics of the swimming pool.

 

Conclusion

While European Standards do not mandate a filling or draining rate of 25 mm (1 inch) per hour, the recommendation remains a widely accepted engineering practice. Slow and controlled filling and draining help reduce structural stresses, protect waterproofing and tile systems, and provide valuable opportunities for inspection during commissioning.

Every swimming pool is unique. Therefore, the appropriate filling and draining procedure should always be established based on engineering judgement, manufacturer recommendations, and project-specific requirements, rather than relying solely on a generalized rule.