Cavity wall construction

Cavity wall construction has almost entirely replaced solid wall construction in the UK. It evolved in the latter years of the 19th century and was used widespread from the early 1900’s. Cavity walls work on the principle that masonry (brick and block) is a an absorbent material and will slowly draw rainwater into the wall from the outside or inside of the house. The cavity serves as a way to drain water back out through weep holes. The weep holes allow wind to create an air stream through the cavity and the stream removes water from the cavity to the outside.

A traditional masonry wall should be built using an inner and outer leaf and a cavity should be provided between them. The cavity should meet the following provisions;

  1. The cavity should have a minimum width of 50mm.
  2. The cavity should be kept clear from mortar snots to stop cavity bridging.
  3. The two leaves should be tied. Wall ties should be a maximum of 900mm horizontally and 450mm vertically.
  4. Cavity wall should always be insulated, whether partially or fully. If partially insulated a minimum clear cavity of 50mm should be provided.

How can I tell if my wall is a cavity wall?

1) As  previously stated, older houses were usually built with solid construction. So the rule of thumb is below;

If your house was built;

Before 1932 – Cavity walls unlikely

1932-1982 – Highly likely

After 1982 – Almost certain

2) You can also check the brick patterns on the outside of the wall. The common types of brick wall are – Stretcher bond, Flemish bond, and English bond.

Stretcher bond, where all the bricks are laid on the long side often indicates a cavity wall.

Flemish bond, where the bricks alternate between a full length brick and a half length brick are unlikely to have cavities.

English bond, where there are alternating rows of full and half length bricks are also unlikely to have cavities.

3) You can also measure the thickness of your external wall to determine if your wall has a cavity. Measure from the outside face of the wall through the door window opening to the inside face of the wall. Anything less than 30cm isn’t a cavity wall. Approximately.


What is an AOV system

AOV is an abbreviation for ‘Automatic opening vent’. These systems are used to control the ventilation of smoke in a fire. When a detector or call point is triggered an AOV control system will open actuators, windows or vents to create ventilation.

AOV’s are required in multi storey residential buildings principally to protect the stairs to assist escape in the event of a fire. In a multi storey residential, the main escape route is always via common corridors and lobbies into the stairs. The aim is to keep the stairs reasonably free of smoke and to improve conditions in corridors and lobbies.

When the door to the flat which is on fire is open, a significant amount of smoke can quickly fill the corridor, making escape difficult for occupants. Legislation limits the distance between fire doors in corridors  to 30m and the length of dead end corridors to 7.5m to limit the distance people have to travel through smoke. BS 9991 permits 15m if the flats have sprinklers.

Smoke control systems should be provided for each stair and for any lobbies opening onto a stair. Ventilation may be mechanical or natural. Pressurisation systems can also be used.

Which render should I use?


There are a number of factors to consider when choosing a renders system for your property such as; the existing substrate, age of the property, desired colour and finish.

Modern system – Choose for an exterior render that will increase the aesthetics of your property whilst providing a flexible shell that protects from the elements.

Traditional system – To go for a classic look or to reinstate an exterior finish, including mouldings or features returning them to their original condition.

Polymer Render – Polymer render is a cement based system with specially selected polymers added to the mix. These polymers make the finished coat strong & flexible. Silicone water repellents are also an integral part of the premixed polymer/cement based render system. The silicone gives the render a high degree of water repellence to the render surface whilst allowing vapour to pass through the render, letting the substrate breathe.

Polymer render is available as monocouche (one coat) render, which eliminates the need for a base coat. Nylon reinforced base coats do provide a high strength base but monocouche is b only applied becoming increasingly popular. Monocouche renders are usually applied to lightweight blocks or breeze blocks. Expansion joints are often needed to ensure the render doesn’t crack. These joints do need to be mastic’d over for water protection. PVC corner beads and stop beads can be used to enhance render appearance.

Polymer renders come in a wide range of colours and can be finished in different textures, from a sponged float finish or a rougher scraped finish as well as Pebble dash.

Polymer renders are usually premixed and only need additional clean water to produce the final product. This ensures the quality of the render remains consistent and ensures a superior finish.


Acrylic Render – Acrylic render is a finishing coat for render, containing acrylic aggregates to display an attractive finish. It is applied to new and existing render to seal the substrate layer. Acrylic render is available in a variety of colours and textures, ranging from 1mm to 3mm aggregate size.

Scratch Render – Scratch render is a relatively new process that originated from the continent around 10yrs ago. It is a coloured cement based render with a surface that looks similar to weathered stone when finished. Scratch render is applied so the elevation of a house is made to look plaster flat. The surface of the render is then scratched with a nail and float. The colour of scratch render is never damaged and can be power washed.

Brick Effect Render – Brick effect render is an alternative to traditional brickwork and is ideal for use on projects where bricks can’t be used. Brick effect render is applied in 2 x coats. First a wet coloured base coat then a 2nd top coat. The top coat is then cut through to expose the base coat ‘mortar layer’  crating the brick effect finish.





EPDM is a extremely durable synthetic rubber membrane manufactured from ethylene, propylene and a small amount of diene monomer. These ingredients are then synthesized to produce a product that exhibits a high degree of ozone, ultraviolet, weathering and abrasion resistance & outstanding low temperature flexibility. These ingredients also contribute to resistance to acids, alkalis and oxygenated solvents. The elastomeric compound is black, has a smooth surface similar to natural gray slate but does not contain surface granules.

EPDM membrane is used throughout the glass and glazing industry to provide a weather-tight seal around the perimeter of windows, doors, cladding, glass screens and curtain walling. As previously stated the EPDM system can accommodate structural movement, is flexible and east to install by using an EPDM adhesive to adhere the membrane to a substrate. The EPDM system not only prevents water and air leakage but uses sealants and adhesives which are safe and solvent free. This green system helps the building achieve Document L.

Following the introduction of document L (Conservation of fuel and power), it has become a requirement to provide a robust seal to the internal interfaces between different facade components of a construction. The purpose of this seal is to reduce heat and air leakage from a building and to help prevent interstitial condensation. To aid a rapid construction techniques, contractors demand that a building is made substantially waterproof as early as possible during construction. The use of EPDM helps achieve this. The membrane is usually applied between the window or curtain wall and the outside of the inside wall.

EPDM window gasket

Things to consider when waterproofing a basement

Waterproofing your basement

Waterproofing your basement can be a useful way to add value to your home. During these times of stringent planning legislation the people of London have decided to build downwards instead of up. The problem is, the lower you dig into the ground the higher the water table is. There is a distinction to be made here between the ‘perched’ water table and the ‘true’ water table. The perched water table is defined as;

‘A reservoir of water in the ground, maintained temporarily or permanently above the standing water level in the ground below it usually caused by the presence of stratum which is impervious.’

That’s the Building regs approved documents explanation which is a bit of a mouthful. In normal person’s terms, water may stand in the ground at high level, before draining down to the true water table at lower level. Being below ground, the earth surrounding the basement acts as a route for water to enter through the walls, as we all know water will find its way anywhere. The walls can either be sealed using a surface coated tanking (wet basement tanking) or through the use of membrane systems.

Tanking systems

These systems work by applying a waterproof coating directly to the inside of the porous basement walls to prevent moisture from seeping in. They don’t remove the water they just stop the water from entering in by creating a barrier. For a tanking system to work well (they can come in the form of cementitious coatings, bituminous coatings, membranes, paints and sealants) a good key or scratch coat should be provided. Tanking systems also require the brick/block work of the house to be stable. In older homes, preparing the walls to take whatever coating is being used can include hacking off old plaster, raking out old mortar, re-pointing and applying salt neutralising products.

Waterproof coatings can be applied using a trowel or can be sprayed on. Plaster can then be applied on top of these coatings. Attention must also be applied to weak points in the structure – typically the wall/floor junction where water is most likely to enter. Here overlapping of the membrane system can be used, or vapour tape can be used, or a membrane skirting can also be used. Service pipe penetrations are also weak points and there are specific waterproofing products out there to fill these gaps. Similarly, a of water bar can be used to seal up that penetration gap. Newton and Delta have suitable products out there.

Cavity Drain Membrane Systems

Cavity membrane systems are used on most cellar conversions, especially in areas of high water tables and older properties as they don’t rely on sound substrates for their effectiveness. Most of these systems use a studded membrane, often made from a high density polyethylene being fixed to the walls with plastic plugs. Water flows down within the cavity into the floor and into the drainage system. Aco drains have been regularly used and a drainage channel is provided in the gully’s created in the concrete slab. This water then flows to a chamber then to sump and is pumped out.

The importance of of concrete cover


Reinforced concrete is one of the mostly widely used structural materials in the world. The introduction of CARES (Certification Authority for Reinforcing Steels)

…has ensured that quality problems for reinforced steel in the UK are rare. Most of the concrete now in the UK comes from quality assured ready mixed concrete suppliers and the biggest problem is failure to position the concrete within accurate cover which affects the concrete durability as a result. This type of failure costs the UK £550 million/ year in the UK alone.

The type and location of the spacers and chairs used to position the steel reinforcement are important because if they’re not in the correct position it can cause for the strength of the structure to be compromised.  This is especially important for cantilevers where the reinforcement is designed to be near the top of the concrete but can end up in the middle or bottom if the support is inadequate. This can lead to collapse. When spacers aren’t correctly placed, steel reinforcement can begin to corrode and the structure is significantly weakened.

As an example; for an external concrete structure sheltered from the rain 30mm of cover will give approx 135yrs of protection to the reinforcement, but 10mm of cover gives only 10yrs. In marine locations, deficiency in the specified cover can also reduce the life of the structure. During a fire the time before the heat reaches the reinforcement is dependent on the cover. When the reinforcement bars heat up the steel softens and can no longer take the stresses it was designed for which may lead to collapse.

Concrete spalling

Concrete spalling is when the chips or fragments of a material is broken off a bigger object. The most common reason for spalling in bricks is due to excess moisture. In concrete, especially older concrete structures spalling is usually caused by corrosion of the steel reinforcement bars. The rebar is used to offer strength to a material that is extremely high in compressive strength but has very limited flexible or tensile strength. One of the properties of reinforced concrete is that the high alkaline content of the concrete protects the embedded steel from corrosion. The correct cover should reduce the chances of the rebar corroding in reinforced concrete.

concrete spalling

Surveying existing buildings

There isn’t any published national guidance on how to achieve the specified cover before 1989. Back in the day reinforcement was positioned using site made mortar blocks, pieces of brick, or anything else found. Sometimes the rebar was placed into the wet concrete so its highly likely that buildings built before 1989 won’t have the specified cover for reinforcement. Therefore, they should be checked for cracking, spalling or excessive deflection. Using a cover meter makes it possible to check the actual cover however this can be difficult when there is congested concrete. You can also use radiography and even investigation by drilling and measuring. The latter however is semi-destructive.

Spacers and chairs

Spacers are used to create the cover for the rebar in reinforced concrete. They can come in plastic, cement or metal.

Single cover plastic ‘A’ spacers can be used for most purposes, including foundations, columns, beams, slabs and walls. They can be used for up to 20mm sized rebar’s. The spacer clips on to the rebar and is the cheapest option because it doesn’t need tying on to the wire (cost and time effective). They can be used for covers from 20-90mm.

Soft substrate ‘A’ spacers are used when there is a soft substrate involved, like insulation. The spacer has a spreader base which clips to the bottom of the A spacer which spreads the load carried by the spacer into the insulation.

End spacers are used at the ends of the wires or the rebar to ensure the correct end cover.

Circular spacers have been used on vertical concrete members such as walls and columns.  They contain more plastic than necessary so are not the best resource.

Cementitious  spacers are used where the surface of the concrete may be subject to abrasion. They should be wired on to the reinforcement. The wire is usually 16 or 18 gauge soft iron wire but in marine environments stainless steel should be used.

Cementitious line spacers are used to support rebar which is 25mm or above. They are manufactured in 1m lengths and in sizes from 25mm to 60mm. They should be used in small lengths not exceeding 350mm.

Chairs are used to support the top reinforcement from the bottom reinforcement. They are manufactured as continuous, individual or circular.

Reinforcing bars

Reinforcing mesh bars should be clean, free from rust and placed in accordance to the structural drawings.

The appropriate cover is shown below as according to LABC standards;

Concrete in direct contact with the ground – 75mm

External applications, shuttering – 50mm

Floor slabs, and other applications where concrete is cast onto membrane – 40mm

Concrete over blinding concrete – 40mm

Internal conditions 25mm