A façade can contain well-made panels, tested windows and carefully selected membranes, yet still leak where those elements meet. The interface is where separate products must become one working enclosure. It is also where different designers, trades and installation sequences overlap.
Consider a window inserted into a rainscreen wall. Evidence for the window addresses a defined product configuration. The completed opening also depends on the surrounding wall, perimeter connections and drainage arrangement. Reviewing the window alone leaves an important part of the design unresolved.
Trace what each layer actually does
Begin an interface review by identifying the rain-shedding surface, the drainage route and the air seal. Where relevant, trace the insulation and vapour-control strategy too. These functions may sit at different depths within the wall; they need deliberate connections when the construction changes.
A coloured line on a section is a useful starting point. The design must also explain how that line turns the corner, crosses a fixing, terminates at an opening and connects to the next material. Ask which component provides each function and how someone can install and inspect it.
Give water a route out
In a drained assembly, water passing the outer defence should be collected and discharged without reaching the protected interior. This secondary-defence principle is described in CWCT’s performance guidance [1].
At an opening, review the head, jambs and sill together. Water arriving from above needs a controlled destination. At the sill, discharge must remain possible after trims, finishes and adjacent systems are fitted. A detail that works in a two-dimensional section may fail at its end or corner.
During construction, check that sealant, debris or a misplaced component has not obstructed the intended route. Adding sealant to a visible gap without understanding the system can divert water towards a less accessible location.
Design the installation sequence
Imagine that one trade installs the window, another fits the perimeter membrane and a third closes the cladding. The membrane connection might appear complete on the drawing but become inaccessible once a bracket or return panel is in place.
An interface review should therefore address access, substrate preparation, material compatibility and the order of work. Identify the inspection point before concealment. Photographs should show the actual connection, its location and its state of completion; a distant photograph of the finished elevation provides little evidence of a hidden seal.
Use testing to answer a defined question
Agree what a proposed test is intended to demonstrate. A laboratory specimen, a site chamber test and a local diagnostic water test address different questions. A result only supports the configuration and conditions assessed. Test selection, exposure and acceptance criteria need a project-specific basis.
For a mock-up, include representative interfaces as well as the repeating façade area. For an existing leak, record when it occurs and how water may travel before selecting investigation locations. The place where staining appears may be some distance from the entry point.
A useful review before closing the wall
Can the team trace the air seal continuously? Can water drain at junctions and terminations? Are the substrates and connecting materials suitable? Can the connection accommodate the expected movement? Has responsibility for installation, inspection and corrective action been assigned?
Resolving these questions early makes the interface a designed part of the façade. It also gives the site team a clearer basis for deciding whether the completed work matches the intended construction.
References
[1] CWCT: Performance — sections on weathertightness and secondary defence. [2] CWCT: Technical Updates — consult applicable updates alongside the relevant publication.General educational guidance. Apply current requirements, the selected system’s evidence and project-specific professional assessment to design and construction decisions.