Weeps, Slots and Vents That Are Meant to Be Open

These openings share a form. They are small, they are irregular against the surface around them, they appear at intervals, and they are usually the only visible evidence of an arrangement that is otherwise entirely concealed. Nothing about a gap in a mortar joint announces that there is interception behind it.

That is the whole of the problem. The cavity tray record makes the point directly: weep openings are the only visible part of the arrangement, and they are routinely filled during repointing, rendering or painting by someone who reads them as defects. The invisible part is what gives them their purpose, and it is invisible to whoever is holding the pointing trowel.

The consequence in a cavity wall is stated exactly. Without outlets, interception holds what it collects until the water finds the top of the upstand or a joint in the arrangement, which puts it on the inner leaf at the one point specifically arranged to protect it. The record describes filling the openings as converting a working arrangement into a reservoir.

The base of a wall has the same exposure from a different direction. The veneer record notes that drainage openings are not optional if the masonry looks sound, because they are the only exit for water the design assumes will arrive, and that ground, paving or planting raised against the lowest line closes the drainage the wall depends on. A landscaping decision can close an opening as effectively as a trowel.

In curtain walling the openings are part of a designed path rather than a discharge point, which makes closing them worse rather than merely unhelpful. The record explains the route: water passing the outer gasket is meant to reach a channel, run down, cross to the outside and leave, and every stage of that depends on the next.

It then names the failure precisely. Sealing over a vent slot, filling a rebate with sealant or breaking the downward path at a transom converts a zone designed to be wet into a closed one that holds water against a glazing unit's edge seal, which is where it does the most harm. The record's own caution is that applying sealant to a curtain walling face without knowing which openings are drainage can close the path the design depends on.

The window perimeter record raises the same issue at domestic scale and adds a second category. Perimeter seals and frame drainage need occasional attention, yet both are routinely painted, filled or resealed by someone treating them as gaps. Its conclusion is that what is a seal, what is a drainage opening and what is a movement joint should be recorded where an owner can find it.

The record also gives an owner the question to ask before any decoration or maintenance begins: what should we never paint over, fill or seal around these frames? It is one of the few published questions written for an occupier rather than for a designer, and it exists because the answer is not deducible from the frame.

The ventilated pitched roof record lists blocking eaves openings to stop draughts in the loft among its common misunderstandings, and states the consequence plainly: doing so removes the mechanism the roof space depends on to stay dry. The opening is a draught by design, and the draught is the point.

The same record and its flat-roof counterpart describe a quieter version of the same closure. Fascia, soffit and trim details that close the edge neatly can close the void with them, which turns a ventilated concept into an unventilated one without any visible change. Nobody blocked anything; the edge was simply finished tidily.

This is the case with the largest consequence and the least obvious appearance. A roof arranged to hold rainwater and release it slowly looks, while it is working, exactly like a roof that is draining badly. The record says so: water standing on this roof is the design working, not a drainage fault, and clearing the restriction to make it drain faster defeats the purpose.

The record also describes what happens next. Removing or enlarging the flow restriction disables the system the site's drainage was designed around, because the restriction is what spreads the discharge over time. And because the restriction is the element the concept depends on and the element most easily blocked by leaves and silt, somebody does need to reach it and clear it, which is a different operation from enlarging it.

The published question is therefore aimed squarely at this problem: how will a future owner or contractor know that standing water on this roof is intended?

A weep, a drainage slot in a glazed facade and a flow restriction on a roof look like entirely different things, and the error made about each of them is identical. In every case an opening that is part of a designed path is read as a fault in a surface, and closing or enlarging it removes the function without producing any immediate sign.

The delay is what makes the class coherent. None of these changes announces itself. The wall holds water for a season before anything shows; the glazing unit's edge seal deteriorates over years; the loft accumulates moisture through a winter; the roof performs as designed until the storm the attenuation was for. By the time there is evidence, the work that caused it has been forgotten.

Every record that raises this reaches the same conclusion, and it is not that owners should be more careful. The cavity tray record states that recording the purpose of the openings is the only realistic protection an owner has. The window record asks for what is a seal, what is drainage and what is a movement joint to be written down where an owner can find it. The attenuation record asks for the fact that the behaviour is intended, and how the restriction works, to be recorded so anyone arriving later has a reason not to clear it.

The practical form of that is a short document handed over with the building, saying where each kind of opening is, what it does, what may be done to it and what may not. It is the only item in this guide that anyone can act on before the damage, and it costs nothing while the building is still being finished.