RMMI Technical Q&A
Full Bed-Depth Stone veneer
MASONRY TECH QUESTION
Architect: Is the cavity behind a sandstone ashlar veneer and the backing wall to be filled with mortar or an open airspace? It seems better to have an air space like brick veneer.
RMMI Technical Director: Full Bed-Depth Stone veneer (in this case defined as sandstone ashlar, web wall, river rock, field stones, and dry-stacked stone patterns) is required to have a minimum 1-inch solid-filled mortar or fine grout cavity (re: IBC 1405.7). Even though the cavity is filled solid, it is advised to use a drainage plane to allow moisture to migrate to the typical horizontal through-wall flashing locations and allow the moisture to wick to the exterior using wicking cotton ropes every 2’ on center. The wicking ropes are installed with a long tail (12 – 16”) turned in the cavity. This allows more wicking surface area than if the rope is cut just to the length of the veneer thickness.
Here’s the reason for treating full bed-depth stone veneer differently than brick or concrete masonry veneers: Stone veneer performs much differently than unit masonry veneers, either brick or concrete block. Because of the irregular stone shapes and thicknesses, joint treatments, in-and-out-of-plane projections, clipped ends, dry-stacked patterns, etc., it is advised to fill the cavity solid with mortar as the stonework is laid. This provides more uniform loading of the stone, both in compression and wind-loading because stone veneer is so much heavier than unit masonry.
Here's the exception:
A caveat for stone veneer: If the stone selected is cut from quarry blocks (such as limestone, granite, or even some types of sandstone) and is fabricated in uniform bed-depth thickness (3-inch minimum), and has square corners, then it should be designed as a cavity wall with an air-space.
Brick and concrete block veneer both benefit from a cavity because: both of these materials expand and contract, are typically more porous, have many more mortar joints, the joints might not be completely solid, the cavity assists the masons with finger room to grip and lay the units, and allows for air circulation to dry-out these materials more quickly.
Rebar positioners are no longer a code requirement in TMS 402/602 as of the 2011 version. TMS 402/602-13 (and -16) in 602 Specification 3.4 B. Reinforcement, Item 1 states “Support reinforcement to prevent displacement caused by construction loads or by placement of grout, beyond allowable tolerances”. Both Phil Samblanet with The Masonry Society and Mike Schuller with Atkinson-Noland & Associates have interpreted this new language to mean it is allowed for the mason to “support” the bar against displacement and is not entirely dependent on rebar positioners. I think this change was a result of more mason contractors opting to use low-lift grouting techniques (lifts up to 5’-4”), than high-lift grouting techniques (lifts up to 28’). Mason contractors have opted to use low-lift because it reduces the risk for walls blowing over during construction, although low-lifts require the masons to work over the bar lap more frequently. The use of rebar positioners really depends on which type of grouting technique the Project Specifications and/or the structural engineer’s Structural Notes for Masonry allow. If the Specifications allow a contractor to use high-lift grouting techniques, then I would recommend specifying rebar positioners similar to the one shown below (high-lift only). The older TMS code required rebar positioners were to be placed not to exceed 198 bar diameters (around 10’ for a no. 5 bar).