How to seal expansion joints and repair cracks with active leaks

Not all cracks are repaired the same way, and confusing a structural expansion joint with a simple finishing joint is the mistake that lets water through right along the line where the building moves most.

Polyurethane sealant: for structural joints, not cosmetic ones

Polyurethane sealant is an elastic sealer for the building's structural expansion joints, which absorbs the real movement of the construction. Don't confuse it with a simple finishing joint: a structural expansion joint badly sealed lets water through continuously exactly along the line where the building moves most.

Waterstop: fitted before pouring concrete, not after

PVC waterstop tape for construction joints is embedded in the concrete during the pour itself, in construction joints, so that water cannot pass along that line of discontinuity. It must end up centred and well held before pouring the concrete on both sides of the joint — if it shifts during the pour, it stops doing its job even if the concrete itself comes out perfect. It is bought with a 10% margin for overlaps, in widths of 150, 200 or 320 mm.

Resin injection: for existing cracks

Resin injection repairs existing cracks with active leaks, introducing the product under pressure into the crack itself:

• Expanding polyurethane: the usual choice with water actively coming through the crack — it reacts with the water and expands, sealing instantly.
• Rigid epoxy: used on dry cracks where the aim is also to recover the structural strength of the element, not just seal it.

How much a waterproofer's hour costs

The average price of a skilled waterproofer's hour in Spain is €25–45/h depending on the area and the system used. Hot-applied systems (torch, sprayed polyurea) usually need specific qualifications and push up the hourly rate.

Frequently asked questions

Can I fit the waterstop after pouring the concrete? No: it must be embedded in the concrete itself during the pour, centred and well held on both sides of the joint.

Which resin do I use if the crack has water actively coming out? Expanding polyurethane: it reacts with the water and expands, sealing instantly, unlike rigid epoxy, which is meant for dry cracks.

Why does a waterproofer's hourly rate vary so much depending on the system? Because hot-applied systems (torch, sprayed polyurea) require specific qualifications, which pushes the rate above cold-applied systems.

Conclusion

Telling a structural expansion joint from a finishing joint, and choosing the type of resin according to whether the crack has active water or is dry, is what decides whether the repair lasts or fails again. Always documenting the exact system applied —product, thickness, number of coats— in the quote is the first thing checked in the event of a later leak.