Repairs: walls

When it comes to walls, many surveyors (and a fair few builders too) grasp instantly for one of two options:

• condemn the defective building element out of hand; or
• endeavour to repair the element without due regard to the fact that it is beyond its economic lifespan.

A lot of defects also endure insensitive repairs, which cause further problems – creating a domino effect of defect/repair. So, step back for a moment and examine the 'apparently obvious' to test your initial response to any defect.

Before you begin to plan a repair:

• be sure what you are repairing and why;
• consider carefully the age of the building and the materials used - in most cases it is appropriate to match the new material to the old;
• don’t skimp on investigations - it is usually better to make a large mess to start with than an even bigger one later.

If cracks are present, establish whether the crack is historic or ongoing.

Although the smaller buildings that fall within the scope of this section tend to be of quite conventional design, you may come across a surprisingly wide range of unusual circumstances arising from the day-to-day process of condition survey inspections and subsequent rectification.

Basics

9 inch English solid wall/Flemish wall bond

Both these types of solid wall construction are prone to poorly maintained pointing, or the wrong mortar being used. This means they can become wet, especially if in an exposed position. Joinery built into the wall can deteriorate over time. Watch out for cavity walls built to mimic traditional bonds – if in doubt, measure the wall thickness at door/window reveals to verify.

Half-brick walls

Nominally only 100mm (4 inch) thick, these walls (often encountered at first floor only in Sussex and Surrey) have limited strength. Most mortgage lenders will not accept a 2-storey half-brick dwelling. If repairing, consider a rebuild/upgrade to a cavity wall (or 215mm/9 inch, if space is tight). With very limited weather resistance, they are often slate or tile-hung to improve this. They can be easy to upgrade with insulation and render.

13 inch wall

Effectively a 100mm (4 inch) skin directly laid into a 215mm (9 inch) wall, this construction has similar performance characteristics to a 215mm (9 inch) wall, but being thicker has better resistance to moisture and temperature changes. The 100mm (4 inch) 'skin' can separate, but this is usually easy to re-attach using helical ties, providing the problem is caught in the early stages.

Flint wall

Unlikely to have a damp-proof course (DPC) unless on a brick base, this construction can drain quite well because of the interstices in the wall behind the pointing. This type is very expensive to repair, however, and it is difficult to match repairs to existing work. Pre-formed flint wall blocks/modules are available for new work or extensions. These need face pointing, and provide a fair match and good compromise for new work. Local defects in flint walls often lead to wholesale reconstruction of whole wall sections or walls, so proceed with caution.

Coursed rubble

Effectively a 'big stone' solid wall. Performance characteristics are broadly similar to 9 inch brick, depending in part on the quality of the stone. Weak bond laps can develop if the rubble is not skilfully laid, and this can lead to cracks and bulges (e.g. at door or window openings). Helical tie repairs are often effective, a solution that would not work in a flint wall.

Stone

A proper stone wall would probably have been expensive to build. It is also likely to be expensive to repair, and difficult to match the new work to the old. Some reconstituted stone 'equivalents' are available, but these have variable weathering qualities. After 20 years or so, some revert to the appearance of concrete.

Cob

A traditional wall built of mud/dung/straw, often found in listed buildings. This material has to have an inherent minimum moisture content – if it dries out it will fail. However, too wet and it will collapse. Do not insert a DPC or a French drain – both usually have dire consequences. Consider appropriate training before getting involved in specifying repairs. There are numerous books on this construction technique.

Half timber

Traditional in England and Wales, the timber frames are filled with wattle and plaster or brick. Give great care and thought to any repair; the basics are simple but are frequently ignored. Use sympathetic materials: conservation groups may call for cosmetic solutions, which a robust and equally valid traditional repair would not fulfil.

Modern timber-framed construction

'Timber frame' in various guises has been with us for many years, and was popular in the 1920s and 1950s when various systems were developed.

The 'modern' timber frame really dates from about 1980. Usually clad in brick, the timber frame behind the cladding bears the loads of the structure. The frames essentially take 2 forms:

• Closed panels: the timber frame is enclosed by a sheet board material on both sides to create a sandwich. This offers good panel rigidity but it is difficult to incorporate built-in service pipe and cable drops, and it is not forgiving of any errors if they are wrongly positioned.
• Open panels: only one side of the timber frame is delivered enclosed with a racking board to stiffen it. It is much easier to install services and insulation before enclosing the internal face of the board with plasterboard lining or similar.

Done properly, timber frame offers advantages of speedy and accurate off-site manufacture. Done wrongly (not as hard as it might seem), delay and cost to the build programme can ensue.

Timber frame has been promoted by the modern methods of construction (MMC) lobby because of the perceived advantages and use of a material with a low(ish) carbon footprint. The may change in the future, as concrete or brick mass products offer other energy advantages, which may perform better in the warmer, wetter climate predicted for the UK.

Modern cavity wall construction

Usually comprises a facing brick with a concrete block-work inner skin, which often includes a lightweight block to improve the insulation. Cavity widths have tended to increase with each revision of the Building Regulations to enable more cavity insulation material to be added.

Advantages of this method are:

• easily modified or extended post-construction; and
• less prone to damage from water penetration (flooding, or poor detailing causing water ingress might have a greater effect on a timber-framed equivalent).

Types of brick wall bond

(a) Brick wall bonds shown in imperial brick coordinating measurements Double Flemish in plan

(b) Three and one variant of English bond (in elevation)

(c) English bond

Materials

The majority of bricks found in older properties (from about 1800 to 1939) are likely to be of a traditional fired clay. If 'London red brick' has been used, the brick is probably quite porous and relatively soft, with (by implication) a limited resistance to crushing.

'Special' buildings, such as old warehouses that might be subject to high floor loads (in addition to the loads and self-weight generated by being comparatively tall structures themselves), might use a more dense engineering brick. This is probably of similar colour to the London red but very different in its behaviour; its resistance to crushing and improved moisture resistance being the more obvious features.

More modern buildings (Second World War to the present) use bricks made to a variety of formulas based on concrete or calcium/alumino-silicates. These can have very different characteristics from traditional clay bricks, which means that they may absorb more/less moisture, depending on the precise composition of the material.

Think carefully before mixing combinations of materials in repairs or extensions.

Mortar

Portland cement was not common until after the Second World War so most mortar joints up to about 1939 would probably be lime-based.

Lime mortar and Portland cement-based mortar do broadly the same job, but have very different characteristics. Unfortunately this is frequently ignored by specifiers, those supervising the job and even more by builders or their subcontracted plasterers or bricklayers, who seem determined only to use Portland cement mortar either for renders or for brick jointing and bedding or subsequent repointing.

The effect of using the wrong type of mortar can be disastrous, though the reasons behind it are seldom acknowledged.

Older walls of soft clay brick tend to absorb rainwater and atmospheric moisture. The old lime mortar joints did too. The ability of these materials to absorb moisture meant they also would lose it fairly readily.

A 215mm (9 inch) solid brick wall might not be as water resistant as a modern cavity wall under exposed conditions, but it would generally dry out again if left alone (unless touched by a repair in Portland cement mortar).

Portland cement mortar is a much denser material and retains water content, for example, trapping it in the lime mortar joints behind new repointing work or where render had been applied over an entire wall surface.

Being dense, the Portland mortars and renders are prone to 'map pattern' shrinkage cracking – hairline cracks then absorb more moisture through capillary action and make the problem worse. Cracking is initially caused by the dense Portland render and its tendency to shrink back. It does not have the plasticity of the traditional limes, so any water absorbed will freeze in winter, expand, and then shatter the cement.

You may have seen a wall of soft red brick that has been beautifully repointed in a modern Portland mortar – and a good proportion of the bricks will show a bright red/orange where the face has blown away due to winter frost action. The old joints might well have been weathered, but the brick probably survived intact for well over 100 years, until it was repaired! Some builders point out the inadequacies of the underfired old soft red bricks that have 'become porous' – ignoring the fact that they had survived perfectly well until repointing was undertaken.

Stone walls can be similarly damaged by incorrect repairs.

Portland cement has its place. It cures quickly compared with some limes (although modern lime formulations are now surprisingly quick curing) and the adhesive qualities are normally well in excess of those achieved in lime mortar. Portland mortar does set in a very rigid joint and does not accommodate building movements well.

Portland cement pointing on an old wall

The golden rule is: where there is original lime render or pointing, it is usually better to repair like-for-like.

Render

As with mortar, render coats are likely to trigger defects for moisture ingress. Portland renders are commonly applied to 'weatherproof' a wall but have the reverse effect because moisture becomes trapped behind the render layer within the solid wall. Even where render is applied to a modern cavity wall, care needs to be taken with use of appropriately gauged and mixed mortar layers.

Often, mortar has never been properly graded or the appropriate number of coats set. Dense single coat render is likely to lead to shrinkage cracking, moisture ingress (and frost damage them blowing off the render) and so on. Use of Portland-cement-rich renders (often applied in only one coat to a wall) can rapidly lead to differential thermal expansion, when compared with the rate of movement of the wall behind. Any hairline cracking can then encourage capillary action to draw moisture into the wall (rainwater running down the face of the wall, etc.), accelerating deterioration and eventually blowing the mortar away from the wall (frost action damage).

On the other hand, renders can also be a sensible way to dress and clean up a wall where the cosmetic finish to the brickwork is now beyond help.

Key items to remember are:

• Sufficient key needs to be left on the surface of the wall for the mortar to adhere to.
• Renders need to be built up in appropriately gauged coats, becoming progressively coarser towards the outer face of the wall in order to help control the passage of moisture through the wall and to accommodate some movement in the render itself.
• Guidance is set out in the Approved Document section of the Building Regulations (Approved Document C – Section 5.9). This is explained in further detail in British Standard EN 998-1:2003 and in the British Standard 5262:1991 Code of Practice for External Renderings). This is no longer a current British Standard but nevertheless is the version cited in the Building Regulations and continues to set out good advice.
• Even on new work, use of a traditional lime mortar rather than Portland cement base is worth considering, because it is much less prone to shrinkage and cracking. Not all limes are 'traditional' and there are modern formulations that set much quicker than the historic mixes.
• Further advice is available in the British Standards and from websites sponsored by organisations such as The Lime Centre and Hampshire Building Preservation Trust.

A typical scenario for lime rendering on an older building – for example to replace a section of defective or mismatched render (perhaps where Portland is being used in place of lime where original rendering is already in lime) is given in Weathered or desiccated mortar joints.

Mortars and renders - mix guides

The Building Regulations 2000 (Materials and Workmanship Approved Document to support Regulation 7) and British Standards both set out appropriate Codes of Practice for mortars and renders: BS 8000-3:1989 Workmanship on building sites. Code of Practice for masonry, AMD 6195 1990; and BS 8000-10:1995 Workmanship on building sites. Code of Practice for plastering and rendering, AMD 9271 1996.