Internal Hydraulic Lime
Plastering |
Preparation of the Wall Surface The successful application, bonding and correct hardening of hydraulic lime mortars, requires that the background should be clean, free from vegetation, free of containments and reasonably dry throughout the wall mass. The wall should be structurally sound and the masonry and bedding mortars in good condition. The application of various dubbing out coats should not be seen as remedial repair work to the masonry or brickwork background, the replacement of missing or damaged masonry or open joints should be repaired before any plastering of dubbing out coats are applied. Where excessive hollow or unevenness is present the areas should be corrected by use of mortar and stone or brick slips bedded into the mortar, with the aim of presenting a reasonable flat background before plastering commences. Suction Control and Bonding Before the application of any new lime coatings, hydraulic or non-hydraulic, it is vitally important to check to the degree of suction within the background, poor or excessive suction can result in a weak bonding with the substrate caused by rapid de-maturing of the newly applied render, which will result and a weak and powdery interface which will lead to later failure and separation. Where there is little or no suction further action will be required to help bond the coating to the substrate. In situations where suction needs to be controlled, wetting down will be required, on dense blocks or near impervious masonry, simply dampening the surface with a mist spray may be all that is required, but on very porous surfaces such as old brickwork considerable wetting will be required. Wetting the wall by use of a hose, working from the top of the structure, downwards, may need to be carried out the previous day or several times throughout the day before rendering commences. The objective of the suction control is to achieve a thoroughly damp surface, but not wet, i.e., the surface must not have running or standing water remaining on the masonry or brick, this will form a barrier between the coating and substrate, also lime mortars adhere and stiffen through a certain amount of suction. On dense or near impervious background, it may be necessary to apply a sand/splatterdash coat to the background to act as a mechanical key. Salt Contamination Where new lime coatings are to be applied to masonry which is salt contaminated, the masonry should be allowed to dry fully before applying new renders. This will allow salt to be detected on the masonry and mortar joint surfaces, if excessive salt is identified clay or lime mortar poulticing may be required. Specialist advice should be sought, if the technique is considered where salt is detected on the mortar joints, rake out the joints to a depth of 50mm, as this is likely to be heavily contaminated and in a weakened condition and repoint (See repointing). Salt contaminates should never be washed from the surface, as this will result in the crystallized salt returning to a soluble state and retreating back into the pores of the masonry or brick. Where detected on the masonry surface, the salts should be brushed from the surface and cleaned away from the structure. Techniques of Hydraulic Lime Plastering The techniques employed in the application of hydraulic lime plasters should be to ensure a correct bonding with the background while striving to minimise shrinkage and rapid drying, these techniques should be followed throughout the plastering process. Lime plastering is generally applied in 3 coats, but it is common to find 2 coats or even single coat work in vernacular or early structures. In 3 coat work the first coat on masonry or brickwork is generally known as the scratch coat or render coat, this coat is applied in a coat of approximately 10mm thickness, this coat can be applied by use of a steel trowel or thrown onto the wall by use of a harling trowel and then flattened in by the steel trowel. When this coat has firmed up but has not gone hard, the plaster is keyed or scratched up to produce a key for the following coats. The keying up is carried out by use of a lath scratcher or similar toothed instrument and care should be taken not to cut through the plaster coat back to the background. The keying up is generally in the
shape of diamonds of approximately 30mm spacing. This coat should be
allowed to harden for 72 hours minimum before further coat of
plaster are applied, before applying the second coat the first coat
should be checked for shrinkage cracks, and these should be filled
with plaster before proceeding with further coats, before the second
coat is applied the first coat should be brushed down to remove any
dust which may have blown onto the surface. The first coat should
then be damped down to ensure that the second coat is applied to a
damp but not wet surface. This second coat is called the floating
coat and is the coat which is straightened to ensure a flat and even
surface, after this coat has been straightened, the surface of this
coat is scoured up with a timber or polyurethane float (rubbing up
with a float similar to finning up render).
Finish Coat Pages To Print | Sand Selection |
Mortar Mixing | Pointing
| External Rendering |
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