Battle of the bulge can be a tricky and expensive business.

Byline: By Peter Fall

One good thing about modern methods of construction is that they result in far fewer bulging walls than we had 30 years ago.

Indeed, I was chatting to a younger colleague recently about a bulging wall and he recounted that this was the first instance that he had come across in his 10-year professional life.

Back in the bad old days, our houses were built with solid brickwork walls that in some parts of the country, were only one brick thick. When we use the expression "one brick" we mean one brick lengthways, not widthways, so it is 225mm (9in) thick, not 110mm (4 1/2in). The one-brick wall was, in fact, quite slender for a two-storey house. If the first floor was built into the walls, then the slenderness was reduced by the buttressing effect of the floor.

The walls that didn't have the first floor resting on them, which quite often were the end gable walls stretching right up to the roof, could be quite thin in relation to their height. Added to this, the mortar of the old buildings, being lime-based were softer. It, therefore, had less resistance to movement in the wall. Indeed, the benefit of the lime-based mortar was that it allowed a lot of movement without cracks forming.

Modern cement-based mortar is a lot stronger and it holds the wall a lot steadier. This relationship between height and width is called the slenderness ratio. You have to be very careful when designing a wall to ensure that the ratio is not excessive, causing the wall to become weak.

With older buildings, we found the roof loads pressing down on the top of the walls would tend to push the wall not only down, but also outwards. The erosion of the mortar pointing by normal weathering, would add to this problem, softening up the outer mortar joints, and weakening the wall even more so.

The modern cavity wall, despite being made of two half-brick thick walls, is much more stable. The wall ties between each leaf of the wall give better lateral strength for the overall width. Also, the cement mortar increases the resistance to buckling.

That doesn't mean to say we don't have problems of bulging walls. Probably the worst instance is when the cavity ties fail. Unfortunately, wall ties are made of steel and in the past these have not been particularly well protected against rusting. The end result is that they corrode and snap, so depriving the cavity wall of the benefit of two walls tied together.

On top of that, the corroding steel tie expands where it is bedded into the mortar joints, forcing the joints apart and increasing the bulging. This bulging can get to the point where eventually the wall collapses.

It's not just cavity ties that cause problems however, any lateral pressure against the wall can give a bulge.

Another example is directly under roof slopes. The pressure of the sloping rafter on to the top of the brickwork wall can eventually push the upper brickwork out over. This is particularly the case where there are a series of windows across the upper wall, with just narrow stretches of brickwork between. These narrow stretches of brickwork are all there is to resist the pressure of the roof. Remember, a sloping roof not only gives vertical pressure, but some horizontal pressure, and it's this that pushes the top of the wall out.

The weather can also give rise to problems of bulging. The best example is a boundary wall. Here the wall is exposed to the weather on both sides causing saturation of the bricks, erosion of the mortar pointing and eventually, buckling of the wall. The hardest decision is what to do with the wall once it starts to bulge. The easiest answer is to completely rebuild it, but that can be very expensive, as you not only have to relay the bricks, but you will need to support any structure above. Because of this, we tend to restrain the wall in its buckled position to prevent it moving any further. However, you can only do this if the wall hasn't bulged too much.

So how much is too much? The rough rule of thumb is that the bulge should not exceed one third the thickness of the wall. That's fine with a solid wall, but with a cavity wall it has to be a far more refined calculation. For example, you cannot take one third of the overall width of the wall when there are cavity ties in the middle and, similarly, you don't just rely upon the outer leaf of the wall if new cavity ties have been installed.

The good news may be that we have less bulging walls, but the bad news is, they still cost a lot of money to put them right.

* Peter W Fall is the senior partner of Peter Fall Cowie, Chartered Building Surveyors. The company can be contacted on [phone]. To request back copies of the House Doctor articles, visit www.pfc.uk.com