Clay and lime renders, plasters and paints: A how-to guide to using natural finishes by Katy Bryce Adam Weismann

Author:Katy Bryce, Adam Weismann
Format: epub
ISBN: 9780857842701
Publisher: Lightning Source Inc.

Applying lime render/plaster onto wood lath or reed mat

The main difference between applying lime onto wood lath and solid wall substrates, is in the application of the first coat – the ‘pricking up’ coat – onto the lath. The application of the remaining coats is very similar to the general techniques onto solid walls. Lime is normally applied in three coats onto the lath. It is vitally important that the wood laths are thoroughly wetted the day before application, and then about fifteen minutes before work commences. Failure to adequately do so will result in excessive cracking as the dry wood quickly draws the moisture from the plaster/render. The lath could also potentially swell once the mortar is applied, causing excessive cracking.

The application of the ‘pricking up’ coat onto the lath must be carried out effectively, as it provides the key onto the lath that holds itself and all the subsequent coats in place.

The mortar must be pressed on with enough pressure so it is adequately squeezed through the gaps in the lath, where it subsequently hangs over to form the key. These are known as ‘nubs’, and act as hooks to grip onto the back of the lath and anchor the plaster face in place. The pressure must not be too forceful, however, so as to squeeze too much material through the gaps, causing the nubs to fall off under their own weight. For similar reasons, the pricking-up coat must not be overworked.

Tips for application of the ‘pricking up’ coat:

1. Always start from the top of the wall and work down, so as not to knock off nubs formed below.

2. Work in a diagonal upward stroke.

3. Work with an even, direct pressure, slightly reverberating the trowel to work the material between the laths.

4. Always have enough mortar on the trowel to allow for a suitable amount to be pressed through the lath spaces.

5. Adding suitable amounts of hair to the mix is vitally important in creating a strong background, and for the formation of the nubs. Slightly more hair should be used in the ‘pricking-up’ coat than in subsequent coats.

6. The ‘pricking-up’ coat should never be scoured or devil-floated as this would apply too much pressure onto the lath, causing damage to the nubs and undermining the key.

7. The scratching of the ‘pricking-up’ coat should solely be carried out using a 3 or 4 pronged lath scratcher because it will cut less deeply than a comb scratcher, whilst providing the appropriate key for the subsequent coats.

8. Scratching should never be carried out in the same direction as the lath, as this will cause cracking along the line of the laths. Scratch diagonally at a 45 degree angle to the lath, creating a diamond hatch pattern, 25 mm (1″) apart from one another.

9. Application of a lime mortar directly onto lath should never be by means of harling, spraying or throwing, as the crucial nubs created on the back-side of the lath will not be formed.

Three steps to applying the

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