TM TECHNOLOGY BRIEF April 2004 RESOURCE INFORMATION FROM THE INTERNATIONAL MASONRY INSTITUTE
Section 7.3 Stone Veneer Walls Laid in Mortar
Scope This guide provides general information on the construction of anchored veneer walls built using stone set in mortar. Current masonry standards allow the use of stone but provide no specific guidance to designers and contractors on their design, specifications, or installation of cavity walls. This guide is intended to provide general guidance and options for designers and contractors who want to use stone veneer with a specific emphasis on rubble stone. The recommendations have added application for ashlar stone.
Background their marble, the rubble cores construction has been relegated to remain. the homeowner wishing to build a Stone walls, and particularly rubble Through the ages, European practice landscape wall and the preservationist walls, have been a building system for has been to use rubble walls as restoring a historic structure. a millennia. They are composed of fortifications, retaining walls, However, in recent years there has irregularly-shaped and sized pieces of buildings, and as the backing for a been a renewed interest in rubble stone. Material costs were low if the more decorative and durable stone stone along with other stone systems. stone was gathered from a field or exterior. In recent centuries, these There are generally two options stream. Examples still exist world- walls were constructed with lime for constructing rubble veneer walls. wide. The walls were built dry-laid or mortars; many were also covered in The first is a barrier wall and the with mud mortar depending on local a stucco for improved durability and second is the cavity system. The practice and availability of materials. appearance. Even today, rubble walls International Building Code (IBC) These walls vary in thickness but are represent a significant portion of the allows stone veneer to be constructed rarely thinner than 16 inches (Figure 1). residential masonry walls in the as a barrier wall (see IBC 1405.6). world, particularly in southern Asia When this veneer is anchored to a and eastern Europe. masonry back-up, the code requires When the Europeans came to the a 1 inch grouted collar joint. Barrier United States, they brought their walls rely on the mass of the masonry masonry building technology. Thus, to absorb moisture which penetrates there are numerous examples of the veneer. Cracks create a pathway rubble wall construction including for water to enter the building. houses, forts, churches, universities, The second option uses cavity retaining walls, fences, and more. wall technology. This system uses While there are many structures with brick, thin stone veneers, and exposed rubble walls, an even greater concrete masonry, and has nearly use of rubble wall construction was replaced the barrier-type stone walls Figure 1 - Stone Wall for the backing for brick, stone, and of past centuries. Cavity walls were stucco. developed as a means to minimize With the introduction and water penetration. However, there availability of unit masonry such as are no specific standards for using Starting around 50 B.C., the Romans clay tile, brick, and concrete rubble veneer in this application. chose to add marble veneers to the masonry, rubble masonry generally This guide is intended to provide rubble walls to give them a more declined in use. Often, unit masonry general guidance and options for lavish appearance. While many of became the backing for a stone designers and contractors who want these early examples of veneered exterior. For the past 50 years in to use stone veneer in cavity wall rubble walls have been stripped of the United States, rubble wall construction. Definitions for Use as a Veneer For typical veneers, the drainage There are synthetic drainage path is provided by an opening cavity systems now available that can be Cavity wall - Wall construction with a between the veneer and the backing. used in conjunction with masonry to continuous air space. In recent years, cavity drainage provide a drainage path. They are Anchored veneer - A non-structural systems have been developed and generally a cellular material covered masonry facing anchored to a are becoming more popular as a in filter fabric developed for use in backing support. means to keep the bottoms of geotechnical applications. While cavities free draining. These systems these materials were developed to Rubble stone - Rough stone from a are commonly composed of synthetic filter out soil particles, they perform quarry or field or stream. The stones material and are inserted into the also as drainage systems for cavities. are irregular in size and shape. They base of the cavity to allow free Instead of constructing a cavity, a may be shaped or unshaped and can drainage even if mortar droppings collar joint is created with the be laid randomly or in coursing. spill into the cavity. (See Figure 2.) drainage material. To avoid clogging the filter, the collar joint should not Random rubble - Rubble stone be grouted. The mortar alone should constructed without regard to not clog the filter unless it is overly coursing. fluid. (See Figure 3.) Coursed rubble - Rubble stone constructed with approximately level coursing at regular vertical intervals. The courses may be discontinuous. Ashlar stone - Cut rectangular stone ashlar masonry which can be installed random or coursed. Figure 2 - Cavity Drainage System
Masonry Standards The use of stone veneer will The current standards for anchored generally not result in running bond. veneers in the U.S. are in Chapter 6 Therefore, joint reinforcement is Figure 3 - Stone Veneer of Building Code Requirements for required. However this requirement Masonry Structures (ACI 530/ASCE 5/ was developed for unit masonry and TMS 402) by the Masonry Standards is not considered applicable to stone Since the drainage system is in Joint Committee (MSJC). masonry except where required for contact with the back-up, a water- Specifications are in Specifications seismic concerns. proof membrane is recommended for Masonry Structures (ACI 530.1/ to avoid leaks through the back-up. ASCE 6/TMS 602). Construction The majority of the veneer Otherwise, there is a chance of information provided in the MSJC is Drainage Path directing cavity moisture into the directed toward clay brick and back-up. To meet the drainage intent of the concrete masonry veneers. masonry standards for anchored Anchorage In concept, cavity walls constructed veneers, the masonry contractor has with anchored veneers include four Not all veneer ties and anchors several options. The stones can be primary features: are suited to rubble walls. An individually cut and shaped to acceptable anchorage system must a drainage path for water that provide an open cavity, or a reliable accommodate the irregular sized penetrates the veneer, including drainage system can be installed. stones. Various suppliers are weep holes Cutting stone should be minimized marketing systems that are intended anchors to laterally support the for cost reasons. veneer The open cavity is primarily suited to be used with concrete masonry back-up. However, some do not minimum veneer thickness of to masonry units that have a meet the criteria for anchorage in 2 5/8 inches consistent dimension. Even with this control on the thickness, accordance with MSJC in that the veneer laid in other than a ties do not have two legs and the running bond is to have joint keeping the cavity clear can be a reinforcement of at least one challenge. Thus, there have been elements allow too much lateral wire spaced a maximum of 18 cavity drainage systems developed movement. The MSJC restricts inches vertically for unit masonry. movement of the anchor elements to 1/16 inch. Figure 4 shows one If portland cement is used in the mix, new research and testing are com- system which must be modified to it should be non-staining cement. pleted. The prescriptive requirements meet the MSJC. The mortar mix should be placed for anchorage and reinforcement of drier than is normal for clay brick or anchored veneer in accordance with concrete masonry to avoid settle- the MSJC vary based upon the ment of the stone. If needed, wood Seismic Design Category (SDC). wedges can be used to hold the SDC is a function of the site soil stone alignment while the mortar conditions, the seismic properties of sets. Remove the wedges and the site represented by ground repoint the void before the mortar accelerations, and the use of the fully cures. specific structure being designed. The thickness of the mortar joints is an important factor. Besides being For SDC A and B, there are no a costly item, the mortar will special requirements. experience more shrinkage and For SDC C, the veneer is to be possible cracking the thicker it is. isolated so that vertical and lateral seismic forces resisted by the Figure 4 - Stone Veneer Also, thicker joints take longer to strengthen during curing. Therefore, structures are not imparted to the mortar joints should be kept to less veneer. Figure 5 shows two systems that than 3/4 inch wherever possible. For SDC D, the requirements of meet the MSJC criteria and provide Initial shrinkage of the joints can be SDC C apply, plus: the necessary flexibility. While the minimized by pre-hydrating the a) Support each story layout of the ties and anchors is mortar. independently irregular to meet the joints of the Stone b) reduce the maximum wall stones, they must still be installed area of each anchor by 25 approximately 16 inches on center Anchored veneer must be at least percent, and horizontally and vertically. The 2 5/8 inches thick in accordance with c) provide joint reinforcement actual limits in accordance with MSJC. However, for rubble and (single wire) at 18 inches on MSJC are 32 inches horizontally and ashlar stone, this is too small. center vertically. 18 inches vertically, with an upper Preferably, stone veneer should be 4- to 6- inches thick. This allows for limit of 2.67 square feet per tie. For SDC E and F, the requirements variations in the stone and provides a of SDC D apply, plus: suitable bedding plane for the anchors. The bedding plane is most a) provide vertical expansion important for rubble walls. While joints at all returns and ashlar stone tends to be flat, rubble corners, and stone can be rounded. b) mechanically attach joint It is preferable to use full thickness reinforcement with clips or stones at anchor locations to ensure hooks. full engagement of the anchor. Small stones are common in thick walls as The lack of regular coursing in Figure 5 - Stone Veneer Ties fillers but they are not recommended stone veneer walls makes the and Anchors for use in veneer walls because the anchoring particularly challenging to thickness of the veneer is decreased achieve. locally. This reduces both the wall Meeting the seismic anchorage strength and the surface area for the Reinforcement provisions requires added modifi- tie to bond. cations. The seismic requirements In all cases, the stone selected For Seismic Design Categories A, sometimes require the joint reinforce- should be clean, hard, durable, and B, and C, no joint reinforcement is ment to be attached to the veneer ties. not highly porous. Some pre- needed. Therefore, only veneer wetting the stone is preferable to anchors are required. Mortar avoid absorbing the mortar moisture Joint reinforcement is required for too quickly. Type N mortar is the preferred SDC D, E, and F. Where it is necessary mortar for stone veneers on modern to install joint reinforcement, it is buildings. ASTM C 270 allows Type Seismic Provisions easier to use a tie which attaches the S also. For historic structures, Seismic provisions are continually joint reinforcement to the back-up. masons often use high lime mortars. being revised in the United States as This attachment of the anchor to the reinforcement is required for SDC E the joint reinforcement is more is kinked, it is necessary to add extra and F. suited to level courses, it can be bent anchors. At discontinuous ends, The selection of the individual to follow the irregular layout of the there should be an anchor within 4 veneer stones in SDC D, E, and F joints. Figures 6 and 7 show inches of the end of the rein- becomes a factor due to the require- examples to achieve the intent of the forcement. ment for joint reinforcement. While standards. Where the reinforcement
Figure 6 - Joint Reinforcement Figure 7 - Joint Reinforcement (Seismic Design Categories D, E, and F) (Seismic Design Categories D, E, and F)
Details area, it can not be supported vertically to most masons. Open cavities are by wood framing. not necessary because drainage Figures 8 and 9 show possible paths can be created without cutting cross-sections. The actual back-up Conclusion and shaping every stone through the and the use of insulation are design use of modern synthetic drainage choices. Refer back to Figures 6 and Cavity walls with stone veneers of materials. Seismic reinforcement 7 for the reinforcement placement, if rubble stone and ashlar stone can be can be installed in the veneers but needed. constructed using methods familiar extra ties and anchors are needed. Specifications A sample specification is available from IMI that can be modified and incorporated into a project.
Limitations All veneers have limitations as noted in the MSJC. One that is significant involves wood backing. Since stone veneer usually has a weight greater than 40 psf of wall Figure 8 - Uninsulated Section Figure 9 - Insulated Section
This document is intended for the use of industry professionals who are competent to evaluate the significance and limitations of the information provided herein. This publication should not be used as the sole guide for masonry design and construction, and IMI disclaims any and all legal responsibility for the consequences of applying the information.
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