Delta’s documented solution for a pre-formed concrete swimming pool is a negative-side waterproofing assembly: clean and jet-wash the concrete, repair imperfections, apply KÖSTER Polysil TG 500, install KÖSTER NB 1 crystalline slurry, then apply KÖSTER NB Elastic with reinforced joint details. It is a project example, not a universal specification for every pool.
The right design depends on how the shell is engineered, which face can be accessed, groundwater and drainage conditions, expected movement, joints and penetrations, and the finish that will cover the waterproofing. A coating cannot correct an under-designed or poorly detailed concrete structure.
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What Delta’s documented pool system includes
Delta’s official case study describes waterproofing applied from the pool interior, which it calls the negative side. The external face is the positive side. The sequence below was specified for one pre-formed concrete pool and should be treated as a configured system with interfaces and details, rather than a product-by-product recipe that can be transferred to another shell without design review.
| Stage | Documented material or work | Purpose and qualification |
|---|---|---|
| 1. Substrate preparation | Clean and jet-wash new concrete | Remove contamination and expose a suitable surface before repair and coating. |
| 2. Concrete repairs | KÖSTER Repair Mortar Plus | Fill imperfections; the repair material does not replace structural crack or joint design. |
| 3. Primer/anti-lime treatment | KÖSTER Polysil TG 500 | Applied according to the case study as an anti-lime coating. |
| 4. Mineral waterproofing | KÖSTER NB 1 crystalline slurry | Installed as the negative-side waterproofing layer after preparation. |
| 5. Flexible waterproofing | KÖSTER NB Elastic | Applied after NB 1 has cured; construction joints receive additional detailing before the second application. |
| 6. Joint reinforcement | Coved fillets and fiberglass reinforcing mesh | Used at construction joints to accommodate local geometry and reinforce the flexible layer. |
The case study does not state a universal thickness, coverage rate, curing time, water-retention test or guarantee for other pools. Those values must come from the current product data sheets, project specification and the installer’s method statement.
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Why pressure direction and structural movement control the design
Positive and negative water pressure
A pool can experience water pressure from the inside when filled and from the surrounding ground when groundwater rises outside an empty or partly empty shell. Delta’s method statement therefore calls for a system that can resist both positive and negative pressure where the project requires it. The accessible face and the direction of pressure determine whether an internal coating, an external membrane, a structural integral solution or a combination is appropriate.
Filling, emptying and seasonal movement
Concrete, reinforcement, finishes and waterproofing respond to changes in moisture, temperature and load. Filling, emptying, expansion and contraction can flex a shell and stress joints or penetrations. Delta’s older pool guidance recommends emptying and refilling over several days to reduce sudden structural shock. That is guidance from Delta, not a substitute for the pool designer’s operating instructions for the specific shell.
Crack bridging is a requirement, not a promise of immunity
Delta’s NB Elastic Grey product page states a maximum crack-bridging capability of 2 mm. This is a manufacturer product claim under its stated test and application conditions; it does not mean an unspecified pool can tolerate a 2 mm structural crack. Active movement, leaking joints, inadequate reinforcement or a crack wider than the system’s qualified capacity requires structural assessment and a designed repair.
How Type A, Type B and Type C waterproofing differ
Delta describes the three protection types referenced by BS 8102:2022. The classification helps frame the design conversation; it does not select a system without site and structural information.
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| Type | Basic concept | Questions to resolve for a pool |
|---|---|---|
| Type A | Barrier protection applied to the structure | Can the external face be reached and kept continuous around joints, penetrations and corners? Will the barrier tolerate the site’s groundwater and movement? |
| Type B | Structurally integral protection | Are concrete thickness, reinforcement, mix, joints and placement controlled so the structure itself provides the required resistance? |
| Type C | Drained protection, using a cavity or channel to collect and remove water | Is drainage accessible for inspection, cleaning and pumping, and is there a reliable discharge route and backup for pump failure? |
Type A, B and C can be used alone or as a designed combination. In Delta’s separate below-ground pool-facility case, a Type A external barrier was combined with Type C internal cavity drainage because the site had clay soil and variable perched groundwater. That project-specific arrangement is evidence that combined protection can be appropriate, not a default prescription for every domestic pool.
Joints, penetrations and corners need their own details
Construction joints, movement joints, pipe entries, drains, lights and tie-bolt holes are common leakage paths. A continuous coating over a poorly prepared joint does not create a reliable seal. Delta’s pool method statement includes fillets, reinforcement and joint sealant as separate components, and its case study places coved fillets and fiberglass mesh at construction joints before the second NB Elastic application.
Construction and movement joints
Confirm the joint type, expected movement and whether a waterbar or sealant is required. Form a sound coved fillet where the wall-to-floor or wall-to-wall geometry needs a gradual transition. Reinforcing mesh should be embedded in the compatible flexible layer at the locations and widths specified by the system designer, with laps and terminations installed according to the current instructions.
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Remove laitance and loose material, repair voids and use a detailing product compatible with the selected waterproofing. Delta lists KÖSTER Waterstop for concrete and screed repairs, dry-pack and construction-joint detailing, tie-bolt holes and service penetrations. It is a repair/detailing material, not a complete pool waterproofing system; its use, primer requirements and curing limits must be checked against current manufacturer instructions.
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Step-by-step installation logic for the documented negative-side approach
- Verify the design before coating. Confirm structural calculations, concrete strength and thickness, reinforcement, joint layout, groundwater assumptions, access to both faces and the intended finish. Resolve whether the shell requires Type A, B, C or a combined design.
- Inspect and prepare the concrete. Remove dirt, laitance, curing residues and weak material. Delta’s case study specifies cleaning and jet washing. Allow the substrate to reach the moisture and surface condition required by the current products.
- Repair imperfections. Fill honeycombing, voids and surface defects with the specified repair mortar. Investigate cracks: distinguish shrinkage or dormant cracks from active structural movement before covering them.
- Detail joints and penetrations. Install coved fillets, reinforcement, waterstops, sealants or repair plugs where the design calls for them. Check that each detail is chemically and mechanically compatible with the adjacent layers.
- Apply KÖSTER Polysil TG 500. Use the coverage, dilution and recoat instructions in the current data sheet; the case study identifies it as an anti-lime coating.
- Apply KÖSTER NB 1. Install the crystalline mineral slurry to the specified coverage and cure it as directed. The NB 1 Grey product page claims sealing against pressurized water up to 13 bar. That figure is a manufacturer claim, not an independent performance statistic or permission to ignore the project’s pressure calculation.
- Apply the flexible layer after curing. Once NB 1 has reached the required condition, apply KÖSTER NB Elastic. Treat corners, joints and penetrations exactly as the system detail requires rather than relying on a uniform field coat.
- Install the second reinforced application where specified. The case study calls for coved fillets and fiberglass mesh at construction joints before a second NB Elastic application. Maintain the specified overlaps, embedment and curing intervals.
- Protect and test the finished assembly. Prevent damage from trades, standing water or premature tiling. Carry out the inspection, flood or water-retention test and sign-off required by the project specification before covering the work.
What Delta’s broader pool method statement adds
The separate method statement presents a larger assembly rather than a single coating. Its component list includes substrate preparation, KÖSTER Polysil TG 500 primer, NB 1 negative-side waterproofing, a WP Mortar fillet, NB Elastic or NB 4000 positive-side waterproofing, glass-fiber mesh, flexible tile adhesive, tiles, primer, backing rod and joint sealant.
The alternatives in that list matter. NB Elastic and NB 4000 are not interchangeable merely because both appear in the diagram; the selected product, side of application and layer sequence must match the approved design. Likewise, tile adhesive, primer, backing rod and sealant are interfaces that can determine whether a sound waterproofing layer remains watertight after finishing.
Finishes must be compatible with the waterproofing
Render, screed and tiles add their own moisture, adhesion and movement requirements. Delta warns that hydrophobic coatings can interfere with bonding and discusses the need for suitable bonding agents. Before ordering tile adhesive, verify that it is approved over the cured waterproofing, that the primer is compatible, and that the adhesive and grout tolerate continuous immersion and the pool’s water-treatment regime.
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Structural-integral option: KÖSTER BDM
KÖSTER BDM is a different route from a post-applied coating. Delta describes it as a liquid additive incorporated into reinforced-concrete mix designs and lists swimming pools among water-retaining applications. It must be considered at structural-concrete design and batching stage, with mix compatibility, dosage, placement, curing and quality control documented by the concrete and waterproofing designers. It cannot retrofit a completed shell and cannot replace structural design, joint detailing or an appropriate external and internal strategy.
Groundwater, drainage and below-ground pool decisions
For a pool below ground, establish the soil profile, water table, perched water, drainage route and consequences of a blocked or failed pump. Clay soil and variable perched groundwater, as recorded in Delta’s facility case, can create pressure that is absent from a pool on free-draining ground.
- Use a Type A barrier where the external face can be prepared and protected continuously.
- Use Type C drainage only when channels, sumps, pumps, power, alarms and maintenance access are practical for the building’s life.
- Consider a combined design when the consequence of leakage is high or groundwater is difficult to predict, subject to the waterproofing designer’s calculations.
- Do not assume an internal negative-side coating will stop groundwater pressure from reaching the concrete or relieve hydrostatic uplift.
How to compare proposals from installers or designers
Ask each proposal to answer the same technical questions. This prevents a low-price single-coat allowance from being compared with a fully detailed assembly.
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- Which face is waterproofed, and how will the inaccessible face and pressure direction be addressed?
- What concrete thickness, reinforcement, crack assumptions and joint details does the design rely on?
- How are construction joints, movement joints, penetrations, corners, drains, steps and tie holes sealed?
- What crack-bridging and pressure capabilities are documented for the complete assembly, not just one product?
- What preparation, moisture condition, coverage, curing, protection and inspection records are required?
- How will tile adhesive, primer, render, screed, grout and sealant bond to and move with the waterproofing?
- If drainage is included, where are the channels and pumps, how are they accessed, and what happens during power failure?
- Which current manufacturer data sheets and installation instructions govern the work, and who approves substitutions?
Common failure modes and practical responses
Coating over contaminated or weak concrete
Dust, laitance, curing compounds and friable mortar reduce adhesion. Stop, remove the contamination, repair defects and confirm the surface condition before restarting.
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Treating an active crack as a cosmetic defect
A flexible coating may bridge limited movement, but it does not stabilize a moving shell. Have the engineer identify the cause, monitor movement where needed and specify a structural or joint repair.
Ignoring the outside of an empty pool
Groundwater can impose pressure on the positive side while the interior is dry. Review external access, drainage and temporary operating limits before emptying the pool.
Using an incompatible tile system
Loss of bond can expose or puncture the waterproofing. Obtain written compatibility confirmation for primer, adhesive, grout and sealant over the exact cured coating.
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Assuming a product figure is a project guarantee
The 13-bar NB 1 description and 2 mm NB Elastic crack-bridging description are product-page claims. They do not establish the performance of a complete, installed pool assembly under different substrates, temperatures, workmanship or pressure conditions.
Evidence limits and approvals to obtain
Delta’s case study, method-statement component list, product descriptions and separate below-ground facility example provide the available information. They do not provide an independent statistical success rate, a universal bill of quantities, current regional pricing or a design for a particular site. The pool design page is older, even though its page listing indicates a recent crawl, so the current edition of BS 8102:2022 references, product data sheets and application instructions should be checked before construction.
Before work begins, obtain the structural engineer’s drawings, the waterproofing designer’s written specification, current manufacturer instructions, substrate and weather limits, detailing drawings, inspection checkpoints, water-testing requirements and a repair procedure for defects found during testing. Keep batch numbers, coverage records, photographs of concealed details and approval of any substitution in the handover file.
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