Watertight expansion joint covers: how they keep water out of podiums, roofs and car parks

Where an expansion joint crosses a roof, a podium deck, a car park or a wet area, the cover has to seal as well as move. A joint that leaks there sends water into the space below, and a joint seal is expensive to re-open once the finishes are down. This guide explains how a watertight joint system works and what to check when you choose one.

Why expansion joints leak

A joint is a deliberate break in the slab, and with it a break in the slab’s waterproofing. The cover on top is fixed to two edges that move apart and together, so every line where two parts meet (the insert and the profile, the profile and the slab, one length and the next) is a possible path for water. Rain, washing water and standing water on a podium all find those paths over time.

Two lines of defence

A watertight joint system therefore works in two layers:

• The cover itself sheds most of the water. In Genotek systems such as the Z1000, aluminium support profiles hold an elastomeric insert of EPDM or TPE that closes the gap and takes up the movement.
• A water barrier below the cover catches what gets past the top. On the Z1000 this is an optional clip-in water barrier made of TPE: it holds water during rain or washing and lets it evaporate once the source stops. TPE can be heat-welded, which is how the barrier is joined into seamless intersections.

The barrier only works if it is continuous and connected to the waterproofing of the slab on both sides of the joint. That connection is detailed with the waterproofing contractor and the membrane supplier before the work starts.

Corners and transitions decide the result

Straight runs rarely leak. The weak points are where the joint turns a corner, meets a wall or an upstand, changes level, or ends. Genotek supplies watertight covers with compatible corner pieces, transitions and end dams so that the seal line stays continuous. When you send a deck layout, mark every junction: we mark up the joint runs and the junction details.

Choosing from the watertight range

The watertight range lists 14 systems in recessed and surface-mount versions, each with its gap range, movement, mounting, fixing and traffic on the product page. Some starting points by application, from the product pages:

• Roofs and transitions to wall joint covers, with seismic or multi-directional movement: R2001.
• Flat roofs, parapets and rooftop joints: R2003.
• Podiums, service areas, driveways and other exterior areas with high water use and heavy traffic: WTZ1685.
• Front-of-house areas with a visible tile or carpet finish where seismic movement is expected: WTZ5500.
• Basements, below-grade structures, car parks and tunnels: GENFOAM.

Check the movement and the traffic listed on the product page against your joint, as for any cover; our guide on how to choose an expansion joint cover goes through those checks.

On site

Follow the installation instructions of the model, which are part of its document pack. Two points deserve a check before the finish is laid: the barrier is continuous through every joint between lengths and every junction, and it is connected to the slab waterproofing on both sides. Fixing either one later means taking the finish up.

Send your deck or roof layout through the contact page or to support@genotek.global and we will propose a watertight system for each joint run.