Stone Veneers: Mind Your Mortar
TYPE AND SELECTION OF MORTARS
Mortar is a very important component of stone-veneer installations; choosing the right mortar for the installation is as important as choosing the right color and type of stone – the mortar needs to fit the application.
Mortars work by creating a bond between the wall sheathing and the stone veneer unit. Bond strength is a key property of mortar. Workability is also an important factor, along with durability.
The materials used to make mortars are dependent on local availability, as well as the preference of the installer for workability. Mortar is comprised of a blend of either portland cement, hydrated lime and sand, or masonry cement and sand. Type N or Type S mortars are generally used for installing stone veneer. Basic characteristics include:
• Type N: General all-purpose mortar with good bonding capabilities and workability; 28-day compressive strength of 750 psi.
• Type S: General all-purpose mortar with high flexural bond strength; 28-day compressive strength of 1,800 psi.
Both mortars have the flexibility (or creep-and-extension) needed to move the stone around during placement without cracking. Type N has better creep-and-extension compared to Type S mortar and better workability. Type S mortar is preferable for mortar beds since it tends to be “stickier” and may set up faster than Type N.
PREBLENDED OR FIELD MIX?
Type N and Type S can both be field-mixed and many veneer manufacturers provide recommended field-mix ratios for sand and cement to achieve Type N or Type S properties. However, field mixed-mortars can be problematic.
For example, maintaining the proper ratio of sand and cement is difficult. Exposed sand piles can become contaminated with other materials and affect the mortar’s performance. And, field-mixed mortars are also more time-consuming, requiring extra labor to add multiple materials into the mixer before blending.
Preblended, bagged mortars provide better mix consistency than field mix and reduce waste by eliminating the guesswork out of field mixing and increase productivity. Mortars arrive ready to use on the job site, require just the addition of water, are made to ASTM specifications, and are quality-controlled throughout the manufacturing process.
Both field mix and preblended mortars can be custom-pigmented to match the stone or to enhance an area’s décor. Preblended pigmented mortars tend to have more-consistent dispersion of the pigment, resulting in less streaks or color pockets than field-mixed mortars.
Pigment should be added to the initial mix water with the cement when field mixing to ensure pigment is fully dispersed prior to blending. Other additives can be included in the mortar to affect bond strength, set time, water resistance when hardened and workability.
OTHER MORTAR OPTIONS
Recently, preblended mortars have been designed specifically for use with stone veneers. These stone-veneer mortars contain polymers and/or other additives that enhance bond between the mortar, the substrate and the stone.
Polymer-modified stone veneer mortar is less sensitive to substrate compatibility issues. Other additives enhance the mortar’s mechanical bond properties by retaining moisture and allowing more mortar to be drawn into any pores or void spaces on the stone veneer and substrate.
For interior and some exterior applications with heavier stone veneers – or to eliminate sag with lighter stone – thinset mortars can be utilized. Formulated for use with large-format tile or stone weighing up to 20-lbs. each, these thin set mortars eliminate sag and have excellent workability.
In addition, the creamy nature of the product allows for good water retention that helps to fill any open spaces on the product surface, improving bond strength. Thinset mortars are ideal for jointless or dry-stacked interior installations.
For a customized look, use colored tile grouts for applications where joints will be filled (overgrouting). Preblended pigmented grouts can add color enhancement for joint widths up to 1/2”, and have a wide range of preblended shades.
PROPER INSTALLATION
Specific surface preparation requirements must be followed for mortars to properly bond to the wall sheathing. Without proper surface preparation, cracking, bond failure or moisture problems can occur.
Stone-veneer-manufacturer installation instructions and requirements should always be reviewed prior to commencing work. Here are some general guidelines for various surfaces:
Drywall, plywood or other solid wall sheathing: Cover the surface with house wrap or similar material. Install metal lath and a ½” masonry scratch coat before installing stone veneer units.
Untreated brick, block, concrete or other masonry: No surface preparation is required but, for best practice, use metal lath and a scratch coat for optimal results. Newly poured concrete should be inspected to ensure no release agents or other substances are present that may interfere with proper bonding. Metal lath and a 1/2” masonry scratch coat should be used if these items are present and cannot be removed.
Painted concrete, masonry or stucco: Surface must be cleaned back to original surface material or use metal lath with non-corrosive fasteners and a 1/2” masonry scratch coat.
READY FOR INSTALLATION
With surface preparation complete, it is time to begin installation. Be sure to lay out the stone veneer units before beginning. Mortar should be mixed to a workable consistency but should stick to the trowel.
The stone is then set into place. Most jobs work from the top down. This eliminates the possibility of dropping mortar on lower stones.
Depending on preference, some stone veneer is applied directly to the scratch coat by buttering the back of the stone and setting it into place. Alternatively, a mortar may be placed directly on the scratch coat and the stone veneer unit.
For installations requiring grouting, most stone-veneer manufacturers recommend leaving about a finger-width space between each unit.
Installations should occur when the temperature is above 40o F; otherwise, the mortar may not set. For hot climates, steps should be taken to prevent the mortar from drying out too quickly.
Whether the project is for new construction or renovation work, stone veneer can be used to create a unique style for both residential and commercial structures. Choosing the right mortar is key to ensuring that the project offers enjoyment for years to come.
Kevin McFadden is the VP, Construction Products – Product Management for Bonsal American Inc. He holds a B.S. from the University of Akron, in Akron, Ohio; he’s also an active member of the TCNA Board of Directors Executive Committee, as well as several industry technical committees, including ANSI, TCNA Handbook and NTCA Technical. He is the past chairman of the ISO TC-189 working group III.