Building a Brick Veneer Wall That Drains

The Anchor Grid Before Anything Is Bedded

Tie and anchor get used interchangeably on site and they are two different components with two different jobs. A cavity wall of two structural leaves uses ties, whose business is to make the pair act together. A veneer over a framed or concrete backup uses anchors, whose business is one-way: hold the leaf back against suction, hold it off the backup under positive pressure, and let the two move independently in their own planes while doing it. Corrugated sheet metal anchors screwed through sheathing into studs are the cheapest version and the least forgiving, because they only work when the bend lands within a narrow distance of the backup face; the two-piece adjustable plate-and-pintle anchors cost more and tolerate a wall that is not perfectly parallel to the frame, which every wall eventually is not.

An anchor fixed to sheathing is fixed to nothing. The fastener has to reach the framing member, the block web or face shell, or the concrete behind, and the length, thread form and corrosion class come from the anchor manufacturer's own literature and its evaluation report rather than from whatever box is open on the scaffold board. Corrosion protection for veneer anchors normally means hot-dip galvanizing to ASTM A153/A153M Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, or stainless steel throughout. Pairing a stainless anchor with a plain plated screw inside a cavity that is wet several days a year builds a small galvanic cell at every fixing point, and the fixing is what corrodes.

At the other end the anchor has to be in mortar rather than near it. The American specification asks for real embedment into the bed joint with mortar cover maintained to the outside face — the figures it carries are one and a half inches of embedment and five eighths of an inch of cover — and it also limits how far out of alignment the two halves of an adjustable anchor may sit before the assembly stops behaving as one component. That limit is the reason an anchor row set to the wrong gauge produces a wall that passes a headcount and fails a pull test. Where joint reinforcement carries the anchor, the wire itself is covered by ASTM A951/A951M Standard Specification for Steel Wire for Masonry Joint Reinforcement, and the reinforcement has to be continuous through the anchor rather than stopped short at it.

Spacing is where the codes stop being advisory, and they do not all say the same thing. The North American prescriptive route caps the field grid at 32 in horizontally and 24 in vertically on centre and also caps the wall area any single anchor may serve, with wind speed, seismic design category and veneer height all capable of tightening both. British practice works from a density instead, roughly two and a half ties per square metre for an ordinary cavity, with additional ties run at closer vertical centres alongside every unbonded edge. Openings carry their own rule everywhere: extra anchors close to the reveal and spaced tightly around the whole perimeter, because the corner of a window is simultaneously where suction peaks and where the field grid has a hole cut in it. Count that perimeter work as its own line — it never falls out of an area calculation.

The Air Space Has to Stay an Air Space

The gap behind the leaf is not slack in the setting out; it is the drainage path, and it has a minimum and a maximum. North American practice works to a nominal one inch clear behind the veneer, with a maximum beyond which the prescriptive anchor tables no longer apply and the anchors have to be engineered. British practice with partial-fill insulation holds a residual clear cavity of 50 mm in front of the boards. Either way the number is a clear dimension after everything else in there is accounted for, which means the insulation thickness, the fixing discs, the drainage layer and the tolerance on a backup that is never dead flat all come out of the cavity width before the mason gets what is left.

What fills that gap in practice is mortar. Every bed squeezed against the cavity face sheds a fin, every perpend drops a sausage, and by the third lift the bottom of an undefended cavity is a continuous dam sitting on the flashing with the weeps buried underneath it. The defences are all cheap and all have to be running from the first course: bevel the bed away from the cavity so the squeeze goes inward, draw a cavity batten up the wall on the anchors and lift it every course, and keep the base of the cavity swept out through the clearing openings until they are closed. None of this is retrievable later. A mirror on a stick will show you the dam at the fourth lift and nothing on site will remove it.

A cavity drainage mat is the version of that defence that does not depend on anyone lifting a batten. A dimpled or entangled-mesh sheet stood against the backup holds a drainage channel open through whatever falls into it and carries the water down to the flashing regardless of what the cavity has collected. Some products run full height and are bought by wall area with an allowance for overlap and for cutting around every anchor; others are a deep mesh strip laid only in the courses above the flashing and bought by the metre. The failure common to both is stopping the material short. The mat has to land on the flashing and reach the weep line, because a drainage layer that ends one course above the tray simply delivers the water onto the top of the dam it was installed to defeat.

In at the Bottom, Out at the Top

Flashing is the floor of the cavity and everything above it is only as good as that floor. It has to be laid before the base course, turned up the backup by the height the specification states and lapped behind the water-resistive barrier so water arriving down the barrier lands on top of the flashing rather than behind it. TMS 602 lists the materials that are acceptable in that position and the project specification names the one you are using; membrane, sheet metal and composite products all behave differently at a lap and at a corner. The two details that get skipped are the ones that decide whether it works: end dams turned up at the termination of every run, and a front edge that projects clear of the veneer face and is hemmed so the drip breaks away rather than running back under the brick.

Weeps are the discharge. Open head joints are the simplest and the largest, mesh or cellular vent inserts trade some free area for pest resistance, and rope wicks move the least water of the three and rely on capillary action to do it. The North American codes set a minimum opening size and a maximum spacing for weeps at the base of a veneer; open head joints are commonly run at around 24 in centres and wick or tube weeps closer than that, at roughly 16 in, because their capacity is lower. Whichever type is specified, it belongs in the first course above the flashing and nowhere else, and it stays open — a weep pointed up at the end of the job is a wall with a bathtub in it.

Vents are the part crews leave out, and they are not the same component as weeps even where they are the same plastic insert. A cavity that only has openings at the base drains, slowly, and dries barely at all, because there is no path for air to move through it. A cavity with openings at both the base and the head of each drainage compartment develops a small convective flow that carries moisture off the back of the leaf and out of the assembly, which is the difference between a leaf that dries between rain events and one that stays saturated through a wet fortnight. Head vents belong under the soffit, under every shelf angle and immediately below every cavity tray, and their spacing follows the same order of magnitude as the weeps below them.

Compartments are what make that arithmetic bigger than the base of the wall. Every interruption to the cavity — a shelf angle, a lintel tray, a roof abutment, a change of plane where the flashing steps — closes the drainage path and starts a new one, and each new compartment needs its own weeps immediately above its flashing and its own vents immediately below the flashing over it. Count the openings compartment by compartment rather than as one run along the base course, and remember that insect screening and the geometry of a vent insert both cut free area below the raw size of the hole, so the effective ventilation area is always smaller than a headcount of openings suggests.

1. Lap the water-resistive barrier down over the upturn of the base flashing, not behind it.
2. Dress the flashing across the cavity, turn up end dams at both ends of the run, and hem the front edge clear of where the veneer face will sit.
3. Set the drainage mat or mesh so it lands on the flashing rather than stopping a course above it.
4. Lay the first course with the weep openings formed as you go, never cut in afterwards.
5. Sweep the flashing clear through the clearing openings at the end of each day, until the lift closes over it.
6. Mark the head vent line on the storey rod at the same time, so the top of the compartment is set out before anyone needs it.