Render on Metal Lath - Guide

External Plastering (Rendering on Metal Lath Using St. Astier Products)

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One of the primary challenges when working with lath is ensuring that the lath is appropriate for the specific job. Metal laths come in a range of types and designs, each suited to different purposes, and not all are suitable for wide-ranging rendering applications. Therefore, it is essential to consult the lath manufacturer to determine its recommended uses.

While the manufacturer's guidelines for fixing the lath should always be followed, most fixing specifications are designed for cement renders or plasters, which may necessitate reducing the spacing of the fixings to 4" (100mm) to minimize movement during application. Ensure that laps are firmly secured.

Lath can be made from materials such as plastic, metal, galvanized, or stainless steel—all of which can be used with NHL mortars. Simple expanded mesh lath typically requires more fixings to prevent movement. Ribbed laths, on the other hand, offer better support and are generally more robust.

Backgrounds:
Laths can be applied to a variety of backgrounds including brick, block, concrete, or even straw bales. They are commonly used on timber or metal studs and can bridge between different backgrounds. If the work spans across different backgrounds, the application method and mix should be tailored to the weaker support.

Application:
The first coat should cover the lath by at least 5/16" – 3/8" from the outside face of the metal, particularly when spanning between framing.

First coat: Mix NHL 5 with well-graded sharp sand at a 1:2 ratio. Allow the mortar to cure and harden for at least one week in good weather, longer if possible, before applying a second coat. Avoid force-drying for internal work.

Subsequent coats can follow the general guidelines for NHL renders and plasters, typically in 2 or 3 coats. If the lath is securely fixed to a solid background, a spray gun can be used with a NHL 5 1:1.5 mix for all coats.

Protection and Curing: Always follow best practices, especially in protecting fresh mortars from knocks and bumps.

Note: If the lath has been installed by another trade, ensure it is rigid and sound before applying NHL mortars.