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Green Building Advisor

Fiberglass Lath Works Better than Metal (and Won’t Rust)

Covering foam insulation on the outside of a foundation or exterior wall just got easier
— and this product works for interior stucco features, too

By Michael Chandler | February 4, 2010

Fiberglass Lath

SpiderLath stands proud of the felt. SpiderLath has foam-rubber furring strips that hold the lath off the wall. Mortar easily squeezes through the lath and locks into it with a solid grip.View Gallery 6 images

In order to provide a positive mechanical bond for exterior stucco and manufactured-stone veneer, builders install lath over felt paper or housewrap. But if you have ever worked with conventional galvanized expanded-metal lath, you know that the chore is anything but a pleasure. Metal lath is sold in large sheets that are sharp, prone to rust, and must be fastened through housewrap or felt paper in such a way that there’s a chance of water penetration. To me, the future of lath lies in products like SpiderLath.

I was first drawn to SpiderLath because it comes in a 4 ft. x 75 ft. roll that weighs only 25 lbs., and because the material has no sharp edges. That means I can lath a 25-ft.- tall chimney by climbing to the top, unrolling a 4-ft. x 25-ft. piece — which weighs only 7 lbs. — fasten it at the top, and then work my way down, without cutting up my arms in the process.

Product Info: Manufactured by SpiderLath, Inc.
www.spiderlath.comCost of a 300 sq. ft. roll: about $135

I was also pleased to learn some additional advantages of this alternative material. SpiderLath has foam-rubber furring strips spaced 6 in. on-center; these strips not only hold the material off the wall — allowing mortar to squeeze through and get a good grip — but they seal every staple penetration through the felt paper or housewrap.

Made from alkaline-resistant fiberglass — coated to make it stiff and to protect the fibers from the abrasive elements in the stucco or stone adhesive — SpiderLath is actually stronger than expanded metal lath. It also can be cut with a utility knife or scissors, and though it is stiff enough to lie down smoothly it is still flexible enough to be easily folded around corners. Because it’s so easy to shape, we’ve recently used it for decorative stucco range hoods and interior fireplace surrounds. (Of course, there are a few tricks that you can do with galvanized lath — for example, crush it into a self-supporting three-dimensional shape — that you can’t do with fiberglass lath.)

I also like SpiderLath for its durability on the job site. Expanded-metal lath that sits around too long before being used always seems to get mangled into a crinkled mass of unusable rusty scrap metal. But other than the fact that the rubber furring strips get a little gummy when left in the sun, SpiderLath is pretty indestructible. I can keep a roll or two in the back of my trailer and cut off pieces as needed.

Fasten it with staples or roofing nails
Like galvanized lath, SpiderLath needs to be mechanically fastened 6 in. on center. We use galvanized roofing staples with a 1-in. crown and 1 ¼-in. leg or roofing nails. Since stucco expands and contracts at a different rate than the substrate, it’s a good idea to seal all penetrations. We use a Tyvek and a double layer of asphalt felt, and we install the SpiderLath with the rubber strips oriented vertically to allow moisture to drain to the weep screed at the bottom. On large projects, I use galvanized or PVC corner bead, drip screed, and expansion joints, but I omit these trim pieces on smaller jobs like chimneys and decorative interior stucco work.

On my jobs, the same carpenters who are responsible for the Tyvek, tape, flashings, and asphalt felt also install the lath, so the stucco crew just has to deal with the stucco. I think I get a better price when the stucco contractor’s scope of work is simplified, with no responsibility for the water-tightness of the system.

SpiderLath costs about the same as galvanized expanded-metal lath (about $0.50 per sq. ft.), but has to be ordered ahead of time from the manufacturer. (Some manufacturers also offer a more expensive type of non-metallic lath that isn’t mechanically fastened — it’s rolled into thinset over foam board. I priced this system for a recent job and, not counting the cost of the foam, it was 30% more expensive than the SpiderLath and two-coat type-S stucco system.)

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