4.9 Coatings and finishes
Aircrete blockwork is suitable for drylining or plastering internally and for rendering, tile hanging, weatherboarding and other cladding systems including external insulation and brick slips externally.
Walls required to be fairface both sides should be designed as cavity or collar jointed walls. Because of block thickness tolerances, although small, it is not possible to build any single leaf solid masonry fairface both sides.
When the building is to be rendered or weather boarded externally, it presents the opportunity to use Aircrete blockwork as the outer leaf as well as the inner leaf of a cavity wall, or as a solid external wall.
4.9.1 Drylining
Drylining with gypsum plasterboard avoids the need for wet plastering trades.
Drylining should be applied to the Aircrete walls using purpose-made light steel or traditional timber battens or by fixing direct to the wall with dabs of bonding compound. Battens should be used to produce enhanced thermal insulation.
Joints between drylining sheets should be carefully sealed to avoid weaknesses in acoustic insulation.
4.9.2 Plastering and rendering
When plastering or rendering on Aircrete walls built using general purpose mortar, conventional thicknesses should be used.
Suitable plasters include two-coat pre-mixed gypsum-bound lightweight aggregate undercoat plaster with pre-mixed, gypsum-bound lightweight aggregate finish coat.
Suitable renders include 1:2:9 or 1:1:6 Portland cement:lime:sand by volume.
Normally the render should be applied in two-coats with the second coat being slightly weaker than the first. Strong render mixes should be avoided.
Rendering as a substrate for ceramic tiling should be 1:4 Portland cement:sand by volume.
The wall surface should be specified to be sound and uncontaminated and neither saturated nor very dry. The suction of dry surfaces should be adjusted with water or a bonding agent may be used. When plastering or rendering on Aircrete walls built using general purpose mortar, the mortar joints should be raked back to provide an additional key for the plaster or render.
External render on Aircrete outer leaves or solid external walls may be carried down to the ground level dpc, but must not bridge it.
Between just below finished ground level and dpc level a bituminous finish is recommended rather than render.
4.9.3 Thin sprayed plasters and renders
When the blockwork has been laid in thin-layer mortar, sprayed proprietary renders and plasters 3mm to 4mm thick which reduce completion times may be used.
4.9.4 Internal tiling
Aircrete block walls built using general purpose mortars should generally be rendered before tiling and the render allowed to dry thoroughly before the tiling is applied.
In areas likely to be wet or steamy, tiles should be solid bedded using a water resistant adhesive. Thin-joint Aircrete block walls may be tiled direct when they are sufficiently plumb and plane.
Large tiled areas should be divided into panels with movement joints at 3m to 4.5m centres, both horizontally and vertically in the tiling, adhesive and rendering. The vertical movement joints should extend through the tiling, adhesive, the rendering and the blockwork. |
4.9.5 Painting
Aircrete blocks in internal walls built fairface can be painted in accordance with the paint manufacturer's instructions with an alkali-resisting paint such as plastic emulsion which will allow the walls to breath.
When smooth-faced Aircrete blocks are to be painted, small blemishes in the face of the blocks should be filled first.
| 4.10 Cladding, tile hanging and |
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weather boarding |
With all of these types of finish it is recommended that a moisture barrier such as breather paper should be provided between the blockwork and the cladding. Vapour barriers such as polyethylene sheet are not recommended. Brick tiling, tile hanging and weatherboarding are particularly suitable for single-leaf external walls.
4.10.1 Cladding
A suitable vapour-permeable membrane should be used between the cladding and the blockwork.
4.10.2 Boarding
Weather boarding of most types can be applied to Aircrete blockwork. The boards are fixed to vertical timber battens secured to the blockwork.
Horizontal or vertical pre-treated fixing battens can be nailed directly to aircrete blockwork as for tile hanging. Battens are spaced typically at 1.2m maximum centres for vertical battens with 25mm horizontal boarding and at maximum 600mm centres for horizontal battens with 25mm vertical boarding.
4.10.3 Brick tiling and tile hanging
Brick tiling and tile hanging on horizontal battens are suitable forms of cladding. Counter battens are not normally required, but to improve thermal insulation vertical battening can be in-filled with insulation and lined with a breather paper before installation of the tiling battens.
Pre-treated timber battens can be nailed directly to Aircrete blocks using cut nails or spiral nails, which are less prone to splitting of the battens. The tiling is then fixed to the battens using tile nails as recommended by the tile supplier.
| 4.11 Fixings and ancillary |
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components |
4.11.1 Cut nails
Light fixings such as skirting boards, light door frames, linings and cladding battens may be made using cut nails skew driven at not more than 500mm centres and penetrating at least 50mm into the Aircrete.
4.11.2 Proprietary plugs
Commonly available proprietary fixing plugs should be used to support heavier weights, such as heavier door frames, battens and small fixtures, specifically designed for Aircrete when driven into a slightly undersized pre-drilled hole. A wood screw can then be screwed into the hollow centre of the fixing plug.
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