Monday, 17 January 2011

Replastering Specification


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Preparatory Work

1. Timber skirting, etc should be removed as outlined in specification.

2. Plaster in removed back to masonry to a height as outlined in the specification, but should not be less than a height of1 metre or 500mm above the maximum level of the visual rising damp/salt contaminated plaster.

3. Rake out all mortar joints to a depth of 12mm (0.5")

Mixing

the water must be clean, free from oil, dirt or other injurious chemicals (water suitable for drinking if available).

First Coat

1. Prepare 3 parts sand to 1 part cement using gauging water containing suitable waterproofing additive. The sand should be specified as washed sharp, concrete sand, loam-free, which satisfies the requirements for Zone 2/3 grading as laid down in British Standards 882: 1973. The cement should be fresh and free following.

2. Use minimum of water to ensure a dnse coat; an approximation is not more than 8 litres per 50 kilos of dry mix.

3. Compact mix into joints and render to give an overall thickness of 12mm. Do not over trowel. When the cement obtains its first set, scratch to form a key.

Second Coat

the mix is as for the first coat except that clean water only is used for gauging (no additive). This is applied as a further 12mm of render, giving, an overall thickness of the render coat of 25mm. This coat should be applied before the first coat has finally set in order to obtain a satisfactory adhesion between the rendering coats. Scratch surface to form a key for finishing plaster.

Third Coat

this should be 3mm of multi-finish or similar. Other finishes are acceptable provided they are porous.

Important points to note

1. Where masonry is unstable, this must be made good prior to the application of renderings. Where it is not possible to obtain a proper bond between the wall fabric and rendering, as in the case of cob walling for example, the rendering must be applied direct to the wall face but over expanded metal lath, previously fixed to the wall surface.

2. Renders and finish coat must be cut short of finished floor level in order to prevent any damp within the solid floor from bring transferred into the soft setting coat.

3. Gypsum plasters, lightweight premix plasters must not be used to bond metal angles to corners.

4. Where conventional timber skirtings are to be fixed they should be coated with wood preservative or ideally pre-treated. It should be remembered that walls will take a considerable time to dry out and it is possible that sufficient moisture would ingress int the new joinery to cause fungal decay.

5. Where practical, fixings onto the walls of all joinery should be by means of adhesives particularly where the areas concerned are below ground level.

6. Our water resistant render work needs to be applied to a minimum of 20mm thick. Occasionally existing wall finishes are under this thickness in which case it may be necessary for the clients to make good above our re-plastering to achieve wall surface continuity.

It is essential that the specification is adhered to and not varied unless by previous agreement with ourselves. No other additives must be added to the mix, unless approved by the manufacturer.

Lightweight gypsum premix backing or bonding plasters (ie. Carlite) must not be used.

1 comment:

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