The Finish Line: Cast Stone and EIFS
April 25, 2007
In the early days of EIFS in North America, one of the gripes about EIFS was its vulnerability to impact damage. The advent of heavyweight reinforcing meshes partially solved the problem, but there’s a limit to how much glass you can add into an EIFS base coat, and hence there’s a limit to how damage resistant you can make an EIFS wall.
Lately I’ve been working with some cast stone products and have found that product to be a good complement to EIFS in terms of dealing with the damage-susceptible edges of EIFS. This month’s column gives some examples of how cast stone can be used with EIFS.
WHAT IS CAST STONE?Cast stone is a general term for a man-made building product. It’s actually real minerals – limestone, cement, some resin and other ingredients – but it is cast – like pouring Jello into a salad mold. Since it is a real mineral material, it looks and behaves pretty much like real stone, but is much cheaper and lighter than carving a profile out of a solid block of real stone. Cast stone is not precast concrete. It’s used indoors and outdoors.
Cast stone can be cast horizontally in open molds as solid pieces, or cast vertically into molds like making candles. The vertically cast pieces can be “hollow” (one open side), and the shape can be almost anything that can be extruded. The hollow type of cast stone usually has a wall thickness in the range of 1/4 inch to 3/4 inch. Those fancy stone-like fireplace surrounds you see in upscale homes are usually cast stone. Fancy kitchen countertops and massive boardroom table tops can also be made with cast stone. The surface of this cast mineral material can be burnished with a grinder and sanded just like real stone to give a beautiful appearance. The stone mix can also be tinted in a range of colors and can have numerous attractive, natural looking variations, just like real stone. Cast stone can also be stained and painted.
To make cleaning it easier and to inhibit dirt pickup, cast stone usually has the surface sealed with a clear sealer. Sealers are applied with a brush, sprayer or a sponge.
Tops of WallsAnother handy way to use cast stone is as a parapet cap or a railing. On balconies, for instance, you can make a shallow slope cap that fits over the EIFS. This topping is more damage resistant than an EIFS foam top, and can have a shallow slope, which looks nice and gives an almost level place to put your beer.
WindowsYou can also use cast stone as window sills. This is common in Europe and is a big help in keeping EIFS foam sills from getting crunched by window washing ladders.
Foam CoresSome “cast” stone products are more like precast concrete - they are heavy and solid (not hollow) and can be made in an extruding process or in large permanent molds. But real cast stone is often made in molds that are fabricated from a block of EPS that is cut with a hot wire. When demolded, sometimes the EPS core is left in place. There are cases where you might want to rip out the foam core, such as the balcony cap shown in illustration B. The EPS foam molds are used one time, and then are ground up and the EPS recycled.
Leaving the foam core in place gives a large surface to attach the EIFS to the supporting wall. Most of the time adhesives are used, but on big cast stone pieces, a cleat is sometimes molded into the backside of the case stone piece, and this provides a sort-of hanger to support the weight.
The surface to which the cast stone is bonded provides the primary water resistive barrier for the wall; the cast stone is thus a finish material. This “substrate” for the cast stone could be building paper, lath and stucco over sheathing, or an EIFS base-coat-like material applied over sheathing. While the cast stone adhesive is setting up, temporary mechanical means are used to keep the cast stone piece in place. This temporary support can take the form, for instance, of screws around the perimeter or wires that act as straps.
Cast stone weighs much more than EIFS foam shapes, but is still easy to work with. One thing you do not want to do, however, is to glue a big, thick, heavy piece of the cast stone onto the EIFS base coat. Big pieces of cast stone need support. Think of cast stone as being like using ceramic tile on the surface of EIFS, as an accent material, and you’ll get the drift of how this material can be used with EIFS.
Some cast stone comes with factory-mitered ends. This allows the cast stone piece to “turn a corner” without the hollow core being visible on a “cut end”.
Buy It By The PieceUnlike EIFS, which is shaped to fit a given wall area, cast stone comes in discreet pieces. Thus, drawings are needed for each piece. Cast stone can be trimmed at the jobsite to fit openings, etc. The drawings of the cast stone pieces show the profile and the lengths of the various pieces. Numerous “stock” shapes are available, as well as custom profiles. The ordering process for cast stone is not unlike aluminum extrusions for making a storefront system.
The joints between cast stone pieces are often handled in a manner that looks like real stone, namely, a synthetic grout is used, and the ends of the cast stone pieces do not abut, but rather a 1/4-inch or larger gap is provided for the grout.
Cast stone is also available as curved pieces (for doing arches at window heads) and various 3-D molded shapes like plaques and balustrades.
The thickness of the “wall” of hollow cast stone is such that there is enough contact area that caulking can be adhered to the cut ends of the hollow pieces. Most cast stone comes in pieces that are up to 6 feet in length. Thus, the completely monolithic (jointless) look that is possible with EIFS is not possible with cast stone. Cast stone looks like real stone – it has mortar joints.
One of EIFS’s big draws is its attractive appearance for a reasonable price. Cast stone costs more than EIFS, but the amount of it that would be used as described above, on most buildings, would be small. When the use of the EIFS is a high-end commercial storefront, a fancy house or a snazzy hotel entrance, cast stone “sets off” the EIFS and adds some interest and style to an otherwise plain façade. To my mind, I would use cast stone sparingly with EIFS – as a special accent and for impact resistance, and use EIFS foam shapes for the bulk of the trim – such as quoins and other common EIFS “foam shape” techniques.