Drained, Bonded or Coated: Three Facade Water Strategies

A ventilated rainscreen splits work that a solid wall does in one plane across several. The visible skin intercepts wind-driven rain and most of the wind pressure, but nobody expects it to exclude water. A cavity behind it is held open so that anything crossing runs down a defined face and leaves at a defined point, and so air can move through and carry away moisture that would otherwise sit against the construction.

The record is explicit about what holds the concept together: each layer accepts a smaller job than it appears capable of. The cladding is allowed to leak, which is what makes an open joint acceptable; the cavity accepts that leakage, which is what makes drainage compulsory; the control layer on the backing wall accepts whatever reaches it, which is what makes its continuity more consequential than the fixing of any single panel.

A masonry veneer reaches the same arrangement with a self-supporting leaf instead of a hung screen, and a sheeted metal wall does not: its outer sheet is the weather line rather than a screen in front of one, so sealing the outer laps and closing the sheet at every edge matter in a way they would not on a screen.

An adhered tile or stone facade is held on by bond, in a continuous adhesive layer distributed over the whole back of every unit. There is no cavity, and the record states that this single absence explains most of the assembly's behaviour: no drained space to receive water that gets past the units, no ventilated gap to dry the back of them, and no separation between what the background does and what the facade does.

That leaves water only two routes: through the surface into the bed, or down the face to the bottom edge. Which makes the grouting and the base termination the whole of the water strategy, and makes freezing conditions a question about what is behind the units rather than about the units themselves.

A brick slip facade sits between the two answers, and the carrier decides which. Over a mechanically supported carrier standing clear of the wall, pointing that lets a little water through hands it to a drained space. Over a carrier bonded flat to a substrate, the same water has nowhere to go and sits at the back of the facing.

A direct-applied render is a graded build-up worked straight onto the wall, with no board, no cavity and no fixing chain. The bond between the first coat and the background is the only thing holding the facade on. Water is dealt with at the face by a continuous skin whose coats are conventionally graded so that nothing stronger or tighter sits over something weaker, relieving movement and salts outward.

Reverse that grading and the tightest layer sits at the front, restricting the route by which moisture already in the wall leaves through the face; the consequences then concentrate at the bond. And a cracked render is worse than no render where water can enter and cannot leave, because the coats now shelter a wetted wall from the drying it once had.

A rendered external insulation facade is the same family of finishes applied over a compressible board rather than a hard background. It has no cavity either, so the base of the system, the reveals and every penetration carry the risk, and water entering behind the boards is beyond the reach of any surface repair.

Every one of these records can be interrogated with the same four questions: what happens to water that crosses the visible face, where does it go next, where does it leave the wall, and which surface is relied on to keep it off the construction behind. A facade that cannot answer the third question has a collection with no exit, which the junction record describes as worse than not collecting at all.

The answers are physical and often visible. Open joints with shadow behind them say drained. Continuous pointing over a bonded bed says the surface is the defence. A render stop with no drip at a sill says run-off returns to the wall below it.

A drained facade puts its working parts out of reach. The control layer, the trays and the bracket penetrations are photographed or unrecorded, because cladding covers them permanently, and a later leak investigated without that record turns into an exploratory strip of finished facade.

A bonded facade cannot be taken off and put back. The record treats inspection, the ability to detect units that have lost bond before they come off, and access to a wall above ground as ongoing questions belonging in the design discussion. A coated facade is repaired by matching a texture and a colour that have weathered, so the practical unit of repair is often a whole panel between joints, and planning where those boundaries fall is planning what a future repair will look like.

It is common for a building to use more than one. The records note veneer at ground level with slips above where weight matters, and facings on a carrier over insulation with render elsewhere on the same elevation. That is not a problem in itself, but it puts a line on the elevation where two water strategies abut and one has to discharge in front of the other.

Where a bonded facade is applied over the outer leaf of a cavity wall, the record raises a specific version of this: the trays that used to discharge at the face are now covered, and whether and how that discharge is brought through the facade has to be resolved before the units go on, since it cannot be added afterwards without breaking the bonded field.