Construction Tolerances: Masonry Veneer and Construction Tolerances

Tolerances for wood-framed wall and concrete foundation construction exceed masonry tolerances, leading to conflicts if carpentry and concrete work is sloppy.

On a recent project the architect designed brick veneer over wood stud framing to have a 2-inch drainage cavity. The mason contractor was having a difficult time maintaining the cavity width as designed because the stud wall leaned outward at every story, by 1 inch or more, and the owner’s rep was critical that the cavity was only 1” wide due to the stud wall construction.

From a functional standpoint, the 1” cavity provides a barrier to moisture infiltration in the same way as would a 2” cavity. Water won’t “jump” across the cavity, and if the drainage space is constructed with little or no mortar bridging across the cavity, then it will still drain water down to the flashing. Wood framed construction will have a weather-resistive barrier facing wall sheathing that also helps to keep moisture from penetrating the wall and direct water down to flashing.

Ultimately the dispute was resolved by looking into the drainage cavity at 2 locations each wall panel using a videoscope inserted into small-diameter holes drilled into mortar joints. These observations verified that the drainage cavity was reasonably clear of mortar obstructions and weeps were clear to drain water.

But what should the contractor do if they run into this situation on a future project? It is important to understand how building codes and industry guidelines address tolerances related to concrete foundations, wood framing, and masonry veneer construction so you can be ready if you run across sloppy concrete work and wood framing on your project.

Tolerances for Concrete Foundations and Wood Framing

According to the American Concrete Institute (ACI 117, Specifications for Tolerances for Concrete Construction and Materials), the width and alignment of concrete foundations supporting masonry can vary from design requirements by ±1/2”and still meet tolerance guidelines.

Tolerances for wood frame construction are not published and are instead left up to the designer to develop for each individual project. One industry guideline that mentions construction quality for home construction is published by the National Association of Home Builders (NAHB). In their 2011 Residential Construction Performance Guidelines, the NAHB provides construction tolerances as a listing of observations. For residential homes, concrete foundation walls are permitted to be ±1” out of square in any 20-foot length, and can be ±1” out of plumb or bowed in any 8-foot height. Framed walls are permitted to be 3/8” out of plumb in any 32” length – that means that rough-framed wood stud walls can be ±1-1/8” out of plumb for a typical 8-foot story height.

Masonry Drainage Cavity Requirements

Masonry veneer design requirements are listed in Chapter 12 of the 2016 TMS 402 Building Code Requirements for Masonry Structures. Section 12.2.2.8.2 states: “A 1 in. (25.4 mm) minimum air space shall be specified.” By code the cavity width should be designed as 1 inch or greater.

Construction tolerances included in Chapter 3 of the 2016 TMS 602 Specification for Masonry Structures are intended to address structural and performance issues, and masonry veneer is excluded from meeting tolerances as stated in both the IBC and TMS codes. Section 1404.6.1 of the 2016 IBC, Tolerances, states that: “Anchored masonry veneers in accordance with Chapter 14 are not required to meet the tolerances in Article 3.3 F1 of TMS 602.” 2016 TMS 402 Section 12.1.1.3 states: “Articles 3.3 B and F shall not apply to anchored veneer.”

Although codes do not require masonry veneer to meet listed construction tolerances, it is reasonable to apply one of the listed requirements to veneer drainage cavities. TMS 602 Section 3.3 F (c) requires: “Grout space or cavity width, except for masonry walls passing framed construction........-1/4”, + 3/8”. That means that if the cavity was specified to be 2” wide, it can be between 1 3/4” and 2 3/8” and still be within code limits. This tolerance does not apply for “walls passing framed construction” including where veneer extends past floor slabs, spandrel beams, or other structural elements, for the exact reason discussed above: construction tolerances for framed walls exceed that for masonry, and TMS 602 statement relieves the contractor in cases of conflicting conditions. (See Figure for comparison of concrete, wood framing, and masonry tolerances.)

Jobsite Practices

Tolerances for masonry construction are notoriously tight, much more so than tolerances applied to other trades including carpentry and concrete. This can lead to conflicts if work of other trades is not well-built and situations where it is not possible to build masonry meeting all project tolerance requirements.

Make sure to check construction of other trades before laying the first course. Run plumb lines, string lines, or laser lines up the face of the backup and horizontally at each floor line to identify any problem areas. Check the foundation placement to make sure the veneer will be properly supported by brick ledges. Submit an RFI or otherwise notify the contractor and the architect (in writing) that the actual cavity width will vary, having an effect on your ability to place insulation, mortar diverters, and meet project and code requirements.

If a mesh-type mortar diverter is specified at the base of the wall, have several different thicknesses available on site. Mortar diverter mesh should fit reasonably tight in the cavity, to prevent mortar droppings from slipping down into gaps and clogging weeps. Use the correct thickness to match the as-built cavity width.