Cavity walls

Brickwork and Blockwork


Brickwork & Blockwork

When laying bricks, care should be taken to ensure that all joints are fully filled to avoid the risk of rain penetration through the brickwork.

Diagram C63 - Typical brickwork outer leaf

When laying blocks, ensure building is plumb, level, and square. Ensure all joints are filled fully and that vertical joints are in line; this is especially important in brickwork.

Ensure brickwork remains free from mortar splashes. All blockwork and brickwork should be built at the same or near to the same level. No part of the can be more than 1m higher than any other part at any time. This also applies to the leaves of a cavity.

As the entire weight of the structure is supported by the lower blockwork, it is important that all walls are supported properly and are bonded securely to support the entire load.

Note: “Toothing in” will never achieve an appropriate strength.

It is important to protect work from the weather at breaks in work sequence and when the walls above are being rendered.

You should ensure that all blockwork/brickwork is built together, i.e. there is no difference in level greater than 1m.


Should a corbel be provided in the external wall construction it is important to ensure that the overhang is within the permitted limitations. The overhang of any corbel should not exceed 1/3 of the thickness of the original wall or, in the case of cavity wall construction the overhang should not exceed 1/3 of the thickness of the leaf which will contain the corbel. Deviations from the above limitations may be permitted if the corbel detail is designed, by a suitable qualified person, taking into account the circumstances specific to the site in question.

Diagram C64 - Typical corbel detail at eaves level

Diagram C65 - Typical corbel detail above and below window ope


Exposure Conditions

Using full-fill cavity wall insulation is and should be limited regarding external wall construction and conditions of exposure. An unrendered masonry outer leaf and its requirements demand particular attention.

Facing normal exposure, fairfaced unrendered brickwork is adequate for full-fill cavity wall insulation ≤2 storeys in height with cavity width of 90+mm; ≤3 storeys’ height with cavity width of 140mm.

If cavity width is greater than 110mm, an engineer must design the walls and foundations according to IS 325 Part 1: 1986. They must be qualified, in private practice, and possess professional indemnity insurance.

Severe Exposure

Facing severe exposure, unrendered brickwork should not be used for full-fill cavity wall insulation. In addition, full-fill cavity wall insulation should not be used for cavity walls with an outer leaf comprised of unrendered fair-faced blockwork.

Any full-fill cavity wall insulation must be appropriately certified for its intended use and use conditions, and installed according to its Agrement certificate requirements. The Agrement certificate map indicates normal and severe exposure areas.


External Rendering

Any external rendering should be durable, resist moisture penetration, weather uniformly and be attractive in appearance.


This depends predominantly on accurate detailing, the building’s exposure level, mix proportions, bonding between wall and rendering, and workmanship quality.

Moisture Resistance

This depends predominantly on building design detail, rendering mix proportions and number/thickness of applied coats. Water is less likely to seep through absorbent character rendering than rendering cracks induced by a strong, dense mix.

Uniform Weathering

Scud blockwork with the following mix: cement:sharp sand 1:1.5-2. Finish with 2 render coats. Mix the render as following: cement:lime:sand 1:.5:4-4.5.

If a mix contains plasticiser, this should be according to manufacturer’s recommendations only. Avoid overdosing/overmixing; this may cause excessive air content, leading to strength/durability loss.

Volume Batching

Do this using properly constructed gauge boxes.


A finish is the decorative coating/texture on top of the undercoats, i.e. it is not an undercoat.

Textured finishes are less crack and craze-prone than plain finishes; any cracks are less likely to be obtrusive. They are also easier to achieve uniformity with – which is important for coloured rendering.

A rough texture evens out discoloration, making any dirt less apparent than in smooth finishes, even if it does allow for greater dirt lodgement on the whole. Flow/rainwater distribution over the surface also diminishes risk of penetration risk through rendering.

A finish is the decorative coating/texture on top of the undercoats, i.e. it is not an undercoat.

Trowel Finishes

1. Plain

These should be worked using a wooden float. They require high-quality workmanship to minimise crazing and/or irregular coloration. For an external render finish, a steel trowel shouldn’t be used.

2. Scraped/Textured

The freshly applied final coat of these should be worked with a trowel or hand tool; alternatively, they can be applied directly from a rendering machine nozzle. For a scraped finish, allow the final coat to harden for several hours then scrape with a suitable tool.

Scraped textured finish

Image - Scraped textured finish

Thrown Finishes

1. General

For facing severe exposure, thrown finishes are typically more durable regarding weather resistance, durability, and crack/craze resistance. Use roughcast (wetdash) and drydash finishes on strong backgrounds. Modify mix properties for any use on weaker backgrounds.

2. Roughcast (Wetdash)

Achieve this by applying the final rendering coat as a wet mix and leaving it untrowelled. The shape/size of the mix’s coarse aggregate determines ‘roughness.’

Image result for plaster finish types

Image - Wetdash plaster

3. Drydash

Achieve this by throwing pebbles or crushed rock chippings at a freshly applied buttercoat (float coat) using a scoop or small shovel; leave it exposed. Apply the buttercoat onto the second undercoat.


Any newly rendered surface must be prevented from drying out too quickly; however, sun/wind protection or water spraying may be necessary in hot, dry weather. To ensure adequate spatterdash hydration, wet it down an hour after application.

Allow each coat to shrink and dry as long as possible – for a period of several days typically – before applying the next coat. This will take longer in cold, wet weather. In adverse weather conditions, including hot weather, wind and rain, you should consider advanced protection of any coloured rendering and/or decorative finish.

Craze Resistance

Crazing is the result of rendering surface shrinkage relative to its interior. Cracks are narrow, typically not far below the surface in depth; however, these can develop into shrinkage cracks. Cement-rich steel trowelled finishes are especially craze-prone; scraped, textured or other rough finishes of leaner mix are craze-resistant.

Minimise crazing by:

  1. Using graded sand; avoid an excessive proportion of very fine material – e.g. silty, dirty sand.

  2. Use a mix lean in cement.

  3. Avoid over-working the mix.

  4. Avoid rapid drying out of the final coat.