Finishes Applied to Celcon Blocks

Celcon manufactures a wide range of quality aircrete products, available in a variety of grades and dimensions, and all conforming to BS 6073. This is the excellence of Celcon aircrete.


External Rendering

Appearance and durability of rendering are influenced by:

  • Suitability of materials
  • Thoroughness of preparation
  • Quality of workmanship

Specifications for rendering and whether rendering should be on a support (lathing) system will depend on such factors as exposure conditions and type of finish required. Most specifications will require a two-coat system (undercoat and finishing coat); see "Thickness of coats". Single-coat work is not normally suitable for external exposure.

Materials and workmanship should always comply with BS 5262:1991.


Aggregates should consist of clean, well-graded sand, complying with BS 1199 and 1200:1976, without too high a proportion of fine material. For undercoats, a sand as coarse and sharp as can conveniently be handled should be used.


Mixes should generally consist of cement:lime:sand or masonry cement:sand in preference to those containing admixtures, because of the difficulty of accurate gauging under most site conditions. The use of admixtures as a precaution against frost damage is not recommended, since hydration of the cement may be adversely affected, and there may be other undesirable results (see BS 5628-3:2001, Clause 5.7.2).

For undercoats and finish coats, the generally recommended mix for Standard and Hi-Strength blocks is 1:1:5-6 cement:lime:sand. Equivalent mixes of 1:5-6 cement:sand with plasticiser, or 1:4-5 masonry cement:sand may also be used.

In moderate to sheltered conditions, a 1:2:9 cement:lime:sand mix may be used on Solar blockwork.

Recommended finishes include wood float, scraped or textured.


Blockwork to be rendered should have recessed joints, and should be cleaned of any dust, loose particles and contamination which may have occurred during construction. Fungi or algae growth resulting from bad storage in wet conditions should be treated with a fungicide applied in accordance with the manufacturer's recommendations.


Whenever possible, rendering should not be carried out in direct sunlight.

Dry blockwork should be wetted to balance the initial suction using a stock brush or fine spray immediately before the render is applied. This must be carried out in a controlled operation and on relatively small areas at a time; excessive wetting must be avoided.

Each coat should be allowed to dry out slowly and thoroughly, before the application of the next coat. In dry, warm weather rendering must be kept damp for a few days.

For the final finish, experience has shown that a porous rendering with a rough texture is more effective in resisting water penetration than a dense, impermeable plain finish.

For conventional finishes, the method of application should be appropriate to the type chosen (floated, scraped or textured). Proprietary finishes should be applied in accordance with manufacturer's recommendations.

Thickness of Coats

The following thicknesses are recommended:

Undercoats: the first coat should be 8-12mm thick; in three-coat systems such as are required for severe exposure conditions, the second coat should be 6-10mm thick.

Final coat: for plain (float) finishes, the finishing coat should be less than 8mm thick; for textured finishes, the thickness should be 8-11mm thick.

Note that the strength is related to mix and thickness, and that each coat should be weaker than that which precedes it.

External Tiling

Tiling should only be fixed to external surfaces which have been rendered using anchored reinforcement, and the recommendations contained in BS 5385:Part 2:1991 should always be followed. Table 2 of the Standard summarises the clauses which are particularly relevant to external tiling.


It is good practice to use a suitable vapour-permeable membrane between the cladding and the blockwork.


Weather boarding of most types can be fixed direct to Celcon blockwork, but it is usual, particularly in the case of single-leaf walls, to fix the boards to vertical timber battens attached to the blockwork.

Tile Hanging

Tile hanging on horizontal battens fixed direct to the Celcon blockwork is also a suitable form of cladding, and counter battens are not normally required.

Internal Plastering

The choice of plaster and its application should be made with regard to guidance given in BS 5492:1990 and BS 5628-3:2001; and the NFPC/AACPA Advisory Note 1 ‘The application of plaster to aircrete block walls’.

Consideration should be given to countering the effects of the differential movement which can occur between the plaster and its background, as well as between two dissimilar coatings, which can lead to cracking or loss of adhesion. It should be noted that gypsum plaster has a greater thermal expansion than most backgrounds.

Dense Plastering

Recommended base coat mixes for dense (sand-and-cement) plasters:

1:1:6    Cement:lime:plastering sand, or equivalent with plasticiser

1:2:9    Cement:lime/plastering sand

All mix ratios are quoted by volume.

Finish Coat

A skim coat of bagged gypsum plaster should be used as recommended by the manufacturer.

Lightweight Gypsum Plastering

Bagged prepared gypsum plasters only requiring additional water are currently available from British Gypsum, Knauf, Lafarge and Tilcon in both undercoats and finish coats. The recommendations of the manufacturers should be sought regarding the suitability of particular products in a given situation, and for an indication of their shelf life (e.g. British Gypsum White Book and the Site Book). Portland cement and gypsum plasters should never be used in the same mix nor allowed to contaminate each other at any stage of the mixing or gauging of materials.

Preparation and Workmanship

Under normal conditions, Celcon blocks have moderate suction and no special preparation is required prior to plastering. Joints should be recessed and the block surfaces should be cleaned of any dust, loose particles and contamination which may have occurred during construction. Fungi or algae growth resulting from bad storage in wet conditions should be treated with fungicide applied in accordance with the manufacturer's recommendations.

In dry conditions, it may be necessary to dampen the wall using a stock brush or fine spray immediately before plastering to balance the blocks' initial suction. This must be carried out in a controlled fashion and on relatively small areas at a time. Excessive wetting must be avoided. Reference should be made to BS 8000:Part 10:1985 for full recommendations of plaster application.

For gypsum plasters the recommendations of the manufacturer should be followed.

Thickness of Plaster

The total thickness of plaster is normally 13mm when applied to blockwork in two coats. The base or undercoat can be approximately 10mm thick and the finish skim coat 3mm.

This excludes any dubbing out which may be necessary when walls have been built out-of-plumb or alignment, or where architectural features require localised thickening of the plaster finish.

Dry Lining

Both standard plasterboard and laminated thermal boards can be fixed by bonding directly to the blockwork using proprietary dabs of adhesive, or attached to proprietary metal furring systems.

Direct Bonding

Plasterboard can be directly bonded to the background using plaster dabs in accordance with the instructions of the board manufacturer; those boards which have a layer of bonded insulation should be fixed by means of a suitable gap-filling adhesive. Selection of a suitable adhesive, and the most appropriate method of application should be made in accordance with the board manufacturer's instructions.

Secondary nailing using proprietary nails and plugs is also necessary to ensure that laminated linings remain attached to the blockwork in the event of fire.


Plasterboard can also be fixed by means of a timber or a proprietary metal furring system, which should be spaced in accordance with manufacturer's instructions.

Internal Tiling

Traditionally, internal ceramic tiling is applied to blockwork which has previously received a rendered or plastered finish. This is a very time consuming process; a period of six weeks must be allowed between the completion of the blockwork and the start of the rendering or plastering, and a further two weeks must be allowed before the tiling commences. For further information, see BS 5385:Part 1:1995, Clause 17.2.

More recently, proprietary adhesives have become available which permit tiling to be carried out on blockwork which is sufficiently plumb and accurate, within 24 hours of the completion of the blockwork. However, this method is not appropriate for every situation and advice should be sought from the manufacturers of the tiles and the adhesive.

Details of both the traditional and proprietary systems can be obtained as follows:

  • Technical specifications (materials and methods) for ceramic wall tiling are obtainable
  • Adhesives and fixing methods for traditional systems, in either wet or dry areas, and direct fixing in dry areas, BRE Defect Action Sheet 137 gives general advice to avoid problems in internal tiling.

Tiling Rendered and Plastered Walls

An undercoat of cement:sand (dense) plaster provides a stronger background for tiling than lightweight plaster and should be used wherever possible. Guidance is contained in 'Fixing ceramic wall tiles to plastered surfaces', prepared by the British Ceramic Tile Council and the National Federation of Plastering Contractors.


Cement and sand render is the preferred background for large areas of blockwork which are intended to receive a tiled finish, but the blockwork should be allowed to dry out for at least six weeks before rendering. On Solar blockwork the render should be reinforced with welded wire mesh secured to the blockwork. For further guidance see BS 5385:Part 1:1995, Clause 17.2 and Table 4(c).

The most suitable mix for Celcon Standard and Hi-Strength block walls is 1:4 cement:sand (by volume); for Celcon Solar block walls, appropriate mixes include 1:5 masonry cement:sand, or 1:6 cement:sand plus a plasticiser.


Under normal conditions, Celcon blocks have moderate suction and no special preparation is required prior to rendering. Joints should be recessed and the blockwork surface cleaned of any dust, loose particles and contamination which may have occurred during construction. Fungi or algae growth, resulting from bad storage in wet conditions should be treated with fungicide applied in accordance with the manufacturer’s recommendations.

Rendering should be left with a wood float finish (see BS 5385:Part 1:1995, Clauses, 17.1, 17.2.2 and Table 2) and should be completed at least 14 days before the tiling begins; the surface should be dry to receive the tiling. When conditions of service are damp or wet, a flexible, water-resistant adhesive should be used with a solid-bed technique (see BS 5385:Part 4:1992).

Large areas of tiling should be divided into bays of 3.0m to 4.5m square. Movement joints will be required at edges of tiled areas and at junctions of walls, floors and ceilings. Movement joints in the wall should extend completely through the tiling, bedding and rendering.

Unrendered Walls

Provided the walls have been well built and exhibit the necessary standards of plumbness and accuracy required by the recommended thickness of the bedding material, tiles can be fixed direct to Celcon block walls. The wall surface should be free from dust, oil and other forms of contamination, and should be dry or almost dry; the blockwork should be completed at least one month before the tiling begins.

The tiles should be fixed with a proprietary adhesive, using the solid-bed technique. A thin-bed or a thick-bed adhesive may be used depending on the flatness of the wall. For general guidance on internal tiling see BRE Defect Action Sheet 137.

The selection of a suitable adhesive will depend on the background to which the tiles are to be fixed; guidance will be found in Appendix B of ‘Fixing ceramic tiles to autoclaved aerated concrete walls in normal interior conditions’, published by the British Ceramic Tile Council.

Decorative Finishes

Celcon blocks can be left unfinished or finished with one of many surface treatments other than conventional render, plaster and tiling.

Natural or Painted Finishes

Where blockwork is protected from regular contact, or is out of reach, the most economical finish is for it to be left with the V-key exposed or painted. A painted wall of Celcon blocks will often be perfectly satisfactory in areas with limited access, for example in plant rooms. Plain face blocks are available, without the V-key. Paints of most types can be applied direct to the surface of the blocks. After normal preparatory work, the paint should be applied following the manufacturer's specific recommendations.

Textured Finishes

There are also many proprietary surface treatments which can be used on Celcon blockwork, either directly or on a rendered background. These should always be applied in accordance with the relevant manufacturer’s recommendations.

This information has been sourced, reviewed and adapted from materials provided by H + H Celcon.

For more information on this source, please visit H + H Celcon.


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