PDD Leak Questions

Purpose of the Air Space in Brick Veneer Walls

In a typical brick veneer cavity wall assembly, there is an air space between the back side of the brick and the sheathing (or water-resistive barrier) of the structural wall. This space is typically 1 to 2 inches wide and serves several critical functions:

Moisture Drainage and Ventilation

Rainwater can penetrate brick veneer through joints and cracks.

The air cavity allows water to drain down the back of the veneer to flashing and out through weep holes.

It also provides ventilation to help dry out the wall assembly.

Minimizing Mortar Bridging

During construction, mortar can drop down into the cavity from bricklaying.

If mortar bridges across to the sheathing, it can create a path for moisture to move into the structure.

The 2-inch cavity helps reduce the chance that mortar droppings will fully bridge the gap, ensuring the drainage plane stays functional.

Why Other Options Are Incorrect:

A. To meet the minimum R-value — The air space in brick veneer is not designed as insulation; its thermal benefit is minimal compared to continuous insulation layers.

B. Allow for differential movement — Brick veneer differential movement is accommodated by wall ties and control joints, not by the air cavity.

C. Provide space for roof drain piping — Roof drainage piping is routed separately and is not part of the brick veneer cavity design.

NCARB ARE 5.0 PDD Study Guide References:

Content Area: Building Envelope Systems — Masonry Wall Assemblies

Source References:

Building Construction Illustrated (Ching) — Brick Veneer Wall Sections and Cavity Function

Architectural Graphic Standards — Masonry Veneer Construction Details

BIA (Brick Industry Association) Technical Notes 21 & 21A — Cavity Wall Design and Construction

Key Principle:

A 2-inch air cavity behind brick veneer is primarily to ensure proper drainage and to minimize mortar bridging, which would otherwise allow moisture intrusion into the building.

When a GMP (Guaranteed Maximum Price) is established before the design is 100% complete and specific materials are not yet chosen, the correct approach is to specify allowances in the project manual.

An allowance is a fixed monetary amount for a specific scope item (e.g., flooring finishes) that allows the contractor to carry a cost in the bid/GMP until the actual product is selected. This ensures the GMP covers the item without delaying pricing.

A. Contingency — Is for unforeseen conditions, not specific unselected finishes.

B. Schedule of unit prices — Typically used for variable-quantity work, not for unknown product selection.

D. Alternate — Provides an option for substitution, but does not guarantee a base cost in the GMP.

PDD Reference: ARE 5.0 PDD “Project Manual and Specifications—Allowances, Alternates, and Unit Prices,” AIA A201 §3.8, CSI Division 01—General Requirements.

In the provided pressure-plate curtain wall mullion detail, the thermal break is typically located between the interior and exterior aluminum components of the mullion to prevent conductive heat transfer.

✅ Thermal Break Location:

Look for the black or dark shaded strip in the middle of the mullion where the two separate metal parts meet — this strip is a non-metallic insulating material (usually polyamide or other plastic) that interrupts the metal-to-metal connection between interior and exterior.

In this image, the thermal break is located:

➡️ In the vertical center of the mullion, between the inner and outer aluminum extrusions, where a black isolator or separator is shown (just above and below the screw/fastener).

Clicking on this narrow dark band (thermal isolator) between the inner and outer components of the curtain wall mullion would be the correct hotspot answer.

The arrow in the wall-floor assembly indicates a horizontal structural member attached to a wall, used to support the end of a floor joist. This is called a ledger.

Ledger: Typically a dimensional lumber or engineered wood member fastened to a wall that supports floor framing members (joists).

Fire block: Installed to limit fire spread in concealed spaces—often mid-wall or mid-stud cavity, not in this floor-to-wall position.

Girt: A horizontal framing member in a wall, usually in steel-frame or post-frame construction, not in conventional wood framing for floors.

Lateral shear plate: A metal plate for shear transfer, not applicable here.

PDD References:

ARE 5.0 PDD “Structural Systems—Wood framing details”

IBC §2308.4—Floor framing requirements

AWC (American Wood Council) “Wood Frame Construction Manual” Ledger attachment details