External Plastering (Rendering on Metal Lath Using St. Astier Products)
The main difficulty in working on lath is ensuring the lath is suitable for the job. Metal laths come in a wide variety of types and designs, some of which have specific functions and are not suitable for a broad range of rendering applications. It is imperative to check with the manufacturer of the lath what is it designed to cope with.
The lath Manufacturers recommendations for fixing should be sought, however as most fixings are specified to accommodate cement renders or plasters, it may be necessary to decrease the centers of the fixings to 4″ (100mm). to reduce to the minimum any movement during application. Laps should be firmly fixed.
Lath can be plastic, metal, galvanized or stainless steel. All can be used with NHL mortars.
Simple expanded mesh lath usually requires greater numbers of fixings to reduce play.
The ribbed laths span more readily between supports and are generally more robust.
Brick, block, concrete or even straw bales. It is also commonly used on framing, either timber or metal studs or to bridge between two completely different backgrounds and supports. If the work spans over two different types of backgrounds, the choice of application and mix will be determined by the weaker support.
Ensure that the first coat covers the laths by a minimum of 5/16″ – 3/8″ from the outside face of the metal (including any ribs or overlaps), particularly where the lath is spanning between framing.
First coat: use NHL 5 and a well graded sharp sand at a ratio of 1:2. The mortar must be allowed to cure and harden sufficiently, at least 1 week in good weather conditions, longer if possible, before a second coat is applied. Internal work should not be force
dried under any circumstances.
Further coats can be applied as per the general guidelines for NHL Renders and plasters (2 or 3 coat work).
Where lath is well fixed to a solid background, use of a spray gun with a NHL 5 1:1.5 can be used for the first and all subsequent coats.
Protection and curing:
Always follow best working practice, pay particular attention to protection against knocks and bumps on fresh mortars.
If lath has been fixed by another trade, the plasterer should only apply NHL mortars when he is fully satisfied that the lath work is rigid and sound.
|All above information is given as general indication only. Dosages may vary in accordance
to materials and aggregates used and site conditions. Please liaise with your St. Astier distributor to finalize mortars mixes.
4 thoughts on “Render on Metal Lath – Guide”
If I put a finer weave mesh, like polypropylene, or a thinner glass mat, like fiba-fuse, fastened and tensioned behind the steel lath, would a sprayed NHL5/sand mix blot though it?
I’m already using PP mesh behind hand-hurled structo lite to stop product loss with success, but I don’t own a pressure sprayer yet!
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Hi, I am lime plastering an internal brick wall and part of the wall has a void (about 25 cm wide 1.8 m tall) where a door has been budged along. I want to use steel lath but should I also back this with something like more wood. I’m thinking of either laddering the void with timber or backing the whole void with a sheet of hard wood. Any advice is aporiciated
I would frame the area with wood and then apply wood lath ( if possible). I would stay away from metal lath due to the platelet structure of lime plaster as it has a square platelet structure as opposed to the needlelike structure of gypsum plaster, or cement plaster. The square platelet structure allows lime to move freely and self-heal, with this slight movement the keys could eventually get cut, or work loose from the sharp galvanized lath. This will take many years, but I believe a proper repair could last 100 + years. I would also stay away from metal lath because it will eventually rust with the vapor permeability of lime plaster and could cause failure and possible staining leaching through the lime plaster. You could use a fiberglass lath,( LimeWorks does stock this, see attachment). This would prevent the aforementioned issues.