Window waterproofing: making the junction work

Water needs a planned way out

Imagine rain running down the glass towards the bottom of the frame while wind drives water across the terrace towards the threshold. If water collects there, even a small weakness in the junction can become apparent.

A working detail has two jobs: keep water out of the construction and give it a route away. Waterproofing without drainage faces more demanding conditions; drainage without a sealed connection does not protect the concealed layers.

Look beyond the visible joint beside the frame. Finished external levels, falls, outlets, corners and everything that will eventually sit below the paving matter too.

Three functions that should not be confused

A window junction performs several tasks. A single material cannot necessarily do them all.

• Inside: airtightness and moisture control. The internal seal connects the frame to the appropriate wall or floor layer. It needs continuity at corners as well as along straight edges.
• Within the detail: insulation and support. Insulation limits heat loss, while the designed supports and fixings transfer the assembly's weight. Installation foam alone is neither a load-bearing detail nor complete waterproofing.
• Outside: weather protection and drainage. At a threshold, this protection must connect with the terrace or roof waterproofing, rather than simply cover the visible gap.

A general principle for installation joints is to limit moisture entering from indoors while allowing outward drying. But a lower junction connected to a roof membrane is more complex than an ordinary jamb joint. Its layers and vapour properties need to be assessed for the actual construction; a simple rule about an open outer layer and a tight inner layer is not a substitute for that assessment.

Start with levels, not with a roll of membrane

Before ordering the window, bring the finished internal floor, threshold, external finish, waterproofing and drainage together in one section drawing. The slab level is not the finished terrace level: insulation, drainage and protection layers, supports and paving still need space.

Leave that discussion until after installation and the paving may cover the frame's drainage, the bonding surface may be too small, or the water may have nowhere to go. Changing a drawing is much easier than correcting the detail beneath an installed frame.

Terraces above heated rooms deserve particular attention. A leak can affect the roof build-up and appear below, or some distance away from, the point where water entered.

A low threshold makes drainage more important

A nearly level transition from room to terrace is convenient and visually appealing. Yet reducing the height difference also reduces the available margin before water can threaten the junction. A product described as a low threshold does not mean that any external finish can simply be brought flush with it.

The design needs to coordinate falls in the waterproofing away from the junction, surface drainage, the permitted water level and the required height allowance. A roof terrace also needs its main and emergency drainage arrangements defined.

A channel beside the frame may form part of the solution, but it must have a real discharge route. A grating with no outlet is just a recess in front of the door. The channel and outlet need cleaning access: leaves and construction debris should not be trapped beneath permanent finishes.

There is no universal threshold height for every building. The solution depends on the system, exposure, external layers, drainage and accessibility requirements. Dimensions from a particular overseas detail should not be transferred to another project without checking its conditions.

Waterproofing needs a prepared bonding surface

The connection is only as dependable as the surface it bonds to. That surface needs to be sound, clean and prepared to the chosen system's requirements. Permitted moisture, primers, application temperature, reinforcement and curing times depend on the material.

Bonding widths come from the system documentation and the project detail. A narrow strip on an incidental profile edge does not become a complete junction simply because it looks tidy.

Check compatibility between the membrane, adhesive, liquid-applied waterproofing, profile coating and sealants in advance. Silicone residue, for example, may prevent reliable adhesion for certain systems. This is not a blanket ban on silicone: the point is not to use it where compatibility with the next material has not been established.

A decorative cover is not necessarily a suitable bonding surface either. If it conceals a ventilation or drainage route, sealing it may interfere with the window's operation. The connection point should be defined in the manufacturer's detail or an agreed project drawing.

Give the lower corners and frame joints extra attention

A straight band of waterproofing is easy to show on a flat surface. The transition from threshold to jamb and then to wall is harder. Different planes, materials and possible movements meet in that corner.

The external seal needs continuity through the whole transition. Corner pieces, reinforcement, laps and connections follow the chosen system's instructions. Upper and side layers must connect so that water cannot run behind the lower waterproofing.

Joints between elements of a large assembly need separate consideration. Frames change dimensions with temperature, and adjoining building components may move differently. A joint intended to accommodate movement should not simply be covered with a rigid layer.

Keep the profile's drainage and ventilation openings clear as required by the manufacturer. Membranes, sealants and finished paving must not block them. Nor should completed waterproofing be casually drilled for guides, covers or other fixings: penetrations need a designed and sealed detail.

Agree responsibilities before installation

Choosing materials is only part of the job. The window installer needs to know where the waterproofing will go. The waterproofing contractor needs access to its bonding surface and information about frame drainage. The facade contractor needs to know which areas must remain open for inspection.

A useful coordination sequence is:

1. The designer brings levels, structural support, junctions and drainage together.
2. The window supplier identifies permitted connection surfaces, drainage routes and joint requirements.
3. The waterproofing contractor checks the substrate, access and material compatibility.
4. The parties agree the work sequence and inspections before finishes conceal the detail.
5. Concealed work is photographed, and a suitable method of checking watertightness is agreed for the detail. Uncontrolled flooding is not a substitute for an appropriate test procedure.

The exact sequence may vary. What matters is that no part of the junction falls into a gap between the responsibilities of different trades.

Questions to ask before the detail is covered

• Where does the continuous waterproofing line run on the drawing?
• Where does water from the frame and terrace go, and how can the drainage be cleaned?
• Does the actual finished external level match the agreed design?
• How are the lower corners, frame joints and fixing penetrations treated?
• What confirms that the materials used are compatible?
• Who checks the junction before covers, paving and facade finishes are installed?

A well-made junction is barely visible once the work is finished. Its quality is decided much earlier: in the drawing, during substrate preparation and in the coordination between trades.

To discuss your detail, send BIFORIUM a threshold section, internal and terrace levels, the window system name and photographs of the installation area. If the assembly has not yet been ordered, this is the time to check that there is enough room for support, insulation and drainage.