Mortar, Render & Plaster

MORTARS

  • Airtec blocks should be laid adhering to the recommendations in the masonry Code of Practice, BS 5628 Part 3:2001 or BS 8000 Part 3:2001 workmanship on building sites.
  • A full bed of mortar should be used with the perp-ends fully covered.
  • If the walls are to be plastered or rendered, the mortar joints should be raked back to provide additional key.
  • Over-strong mortars should be avoided. Airtec is more suited to lower strength mortars.
 
 
Mix Proportions (by volume)
Notes
Above
dpc
Cement: Lime: Sand
 
Cement: Sand
(with plasticizer)
 
Masonry Cement: Sand
1: 2: 9, or
1: 1: 6
 
1: 6
 
1: 5
Class iv or iii as per
BS 5628-2
Below
dpc
Cement: Lime: Sand
 
Cement: Sand
1: 1˝: 4˝
 
1: 4
 
Below dpc, the mortar should be stronger. Sulphate resisting mortar may be required.
 
Thin
Joint
 
All Airtec blocks are suitable for thin-joint construction.
Thin joint mortar is normally supplied in 25kg bags.
Mixing and workmanship should be strictly in accordance with the manufacturer’s instructions.
 
 
 

RENDERS and PLASTERS

The quality and durability of plaster coats and render is greatly influenced by:
 
1.       The quality and suitability of materials
2.       The quality and thoroughness of preparation
3.       The quality of workmanship and adherence to codes and standards
 
Materials and workmanship should always comply to BS 5262: 1991 Code of Practice for external renders.
 

Preparation – all renders and plasters, Internal or External

  • The textured faces of Airtec blocks provide and excellent key for plasters and renders.
  • Airtec is suitable for internal and external walls.
  • With traditional mortar construction, the mortar joints should be raked back to provide an additional key for the render.
  • The blocks should be clean and free of dust and debris.
  • Fungi or algae growth must be removed using a suitable fungicide.
  • In certain circumstances, the initial natural suction of the aircrete substrate can draw the moisture from the render and adversely affect hydration and setting. If this occurs, apply water to the block faces with a fine spray or brush immediately prior to application. Do not apply too much water. Avoid saturation.
 

EXTERNAL RENDERS

  • At least 2 coats should be applied. Each successive coat should be weaker than the first. Avoid strong mixes.
  • The first coat should be applied to a thickness of approx 8-18mm and scratched. The final coat should be thinner and less strong than the first, usually 6-10mm.
  • Apply down to dpc, but do not bridge the dpc.
  • Apply a bituminous coating between finished ground level and dpc if external Airtec walls are used.
  • Avoid cement-rich renders because they use more water for hydration and can lead to pronounced drying shrinkage.
  • Sharp sand should be used, compliant with BS 1199 and BS 1200. Use only clean, well graded sand with very low fines content.
  • Recommended finishes include wood-float, scraped or textured.
 
 
Mix Proportions (by volume)
Notes
Preferred
Mix
Cement: Lime: Sand
1:2:9
Type iv: more suited to sheltered exposure conditions
Cement: Lime: Sand
1:1:6
Type iii: more suited to moderate and severe exposure
Alternative
Cement: Sand (with plasticizer) 
1:6
With the addition of an approved plasticizer to manufacturers instructions
Alternative
Masonry cement: sand
1:5
Avoid cement-rich renders because they use more water for hydration and can lead to drying shrinkage

 

INTERNAL RENDERS & PLASTERS

Guidance is given for the choice of plaster for a particular application in BS 5492:1990 and BS 5628 Part 3:2001. Consideration should be given to the differential expansion that can occur at the interface of two dissimilar materials. Gypsum plasters have greater thermal expansion. 
 
 
 
 
Base Coat
Cement: lime: plastering sand
 
Finish Coat
Notes
Dense Plaster
1:2:9
1:1:6
Approx 10mm thick
 
A skim coat of
gypsum plaster approx 3mm thick
 
Avoid rapid drying out of base coat
Light'wt
Plaster
 
Proprietary gypsum plasters requiring only the addition of water for both base and finish coats are available.
 
Manufacturer’s instructions must be strictly followed.
 
Never mix Portland cement and gypsum plaster.
 
 
 
 
 
 

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