Bulging or Leaning Masonry Walls

16th Century Cottage with distortion to masonry walls

Typical Causes of bulging or leaning masonry in old solid walled buildings

It’s a fairly common occurrence to survey old solid walled buildings and find some distortion in the masonry; the walls often found to be bulging or leaning to some degree; and indeed we’ve encountered two such properties within the last four weeks, the latter case was particularly interesting and forms the subject of this blog. 

16th Century Cottage with distortion to masonry walls
16th Century Cottage with distortion to masonry walls

The building in question is a 16thcentury building, originally built with an Oak timber frame and brick infill panels. In fact the building was originally separated into three small cottages but over time was converted to a single large cottage. The building is grade 2 listed but has been derelict for some years but about to undergo substantial renovation and improvement works. With that in mind we were commissioned to carry out a full condition survey, focussing on structural condition and causes of dampness within the property. Little remains of the timber frame and the external envelope of the building is now of solid walled construction; only two timber end frames exist internally, along with the timber roof structure. which is now almost wholly supported on the external masonry walls.

Checking for Distortion in the Masonry

A 2 meter spirit levels shows a bulge of 80mm to the front wall
A 2 meter spirit levels shows a bulge of 80mm to the front wall

Commonly we’ll use a simple plumb bob and line or a large spirit level to. check for distortion in masonry. A large spirit level can be used quickly and effectively, and in this case we noted bulging to both the front and rear walls of the property. The front walls were at a much reduced height due to the cat slide roof, and despite 80mm of distortion, they were less worrying than the 50mm distortion measured to the rear wall, due to the greatly increased height of the wall.

50mm bulge measured to rear wall
50mm bulge measured to rear wall
Checking masonry distortion using plumb bob and line
Checking masonry distortion using plumb bob and line

Acceptable Limits for Leaning or Bulging Walls

Generally speaking you should be concerned with anything more than 25mm of distortion as it lowers the stability of the wall. There is a general rule known as the V3 rule, which asks that you consider the walls centre of gravity. When viewing a 225mm solid wall in profile, a plumb line dropped from the head to the foot of the wall, which passes through the walls centre of gravity, will not fall outside that centre of gravity at the wall base, (if the wall is perfectly vertical.) Where walls are leaning or bulging then the plumb line will fall outside that centre of gravity, and should be considered unsafe, where the plumb line falls beyond the outer edge of the wall base. In these cases you should seek advice from a qualified structural engineer.

V3 Rule
V3 Rule

Common Causes of Bulging or Leaning Walls

Bulging of the walls is caused by a number of factors: 

  1. Vibration from road traffic. 
  2. Increasing the floor loads or building on additional floors 
  3. The original walls being insufficiently thick in relation to the height. 
  4. Lack of lateral restraint between the external walls and floor joists, beams and partitions. 
  5. Thermal or moisture expansion of the walls outer surface

In our experience, the type of bulging seen in older buildings built with lime mortar, such as this, is often a lack of lateral restraint. It is a well-known principle that lateral restraint should be provided to arrest any potential movement in the masonry walls, and this is usually achieved by building floor joist ends into the masonry, or by bonding in internal partition walls at right angles to the outer wall. In this particular case, lateral restraint was meant to be provided by the first floor joists, running from front to rear in the building. However, when viewed internally, the joist ends were set in sockets cut in the central spine bresummer

Joists. supported by sockets cut into bresummer
Joists. supported by sockets cut into bresummer

Visual Inspection of Joist Ends

In this case visual inspection was fairly straightforward, since all the floor joists were fully exposed. However, it is more common for the floor joists to be hidden as they are sandwiched between the ground floor ceiling, and upper floor. Often, we may need to take up floorboards at first floor level to inspect joist ends, and in particular, to assess how they are tied to the masonry walls, and whether lateral restraint is being provided.

The joist end lap joints, seen above, are seen to have pulled clear of the sockets cut into the bresummer, which accommodate them. Ideally, these lap joints would have been secured with oak pegs, but they are unfixed, and have simply pulled clear of their sockets as the external walls have bowed. This technical detail was simply unable to provide the lateral restraint required by the front and rear walls of the property.

When checking some joist ends, we found as little as 20mm of bearing surface at the joist ends, which essentially tells us, that if another 20mm of deflection occurs to the outer walls, then the floor joists could collapse.

Only 20mm of bearing surface to joist end
Insufficient bearing surface to joist end

The floor joists are now too short and cannot be re-used, unless the outer walls are taken down and rebuilt so as to be perfectly vertical again. In this case, the joists should then be able to be fully inserted back into their sockets formed in the spine bresummer. However, with the amount of deflection measured, the walls should be perfectly stable, so long as adequate lateral restraint in re-instated to prevent further ongoing distortion in the masonry. Joists will be replaced with longer joists which will be adequately tied into both the outer masonry walls, and the central spine bresummer.

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Timber Frame Defects

Common Timber Frame Problems

Neo-Georgian with Timber Frame

Neo-Georgian with Timber Frame

We were recently called to investigate some damp and structural issues to a 12 year old timber frame Neo-Georgian 3-storey apartment block across the water from mainland UK. The building had been affected by both water ingress and a number of structural issues for quite some time and two previous technical reports had reached broad agreement of the fact that the timber frame was showing signs of distortion due to shrinkage, shrinkage commonly occurs in timber frames after construction and one engineer estimated the height of the timber frame may reduce by up to 30mm, a degree of shrinkage that wouldn’t be replicated in the outer non-structural leaf of masonry. In fact the outer masonry leaf tends to expand as it takes up moisture during the first couple of years after construction, so it is in fact moving in the opposite direction to the timber frame.

Wall Ties

It is for these reasons that timber frame movement ties are specified for developments over four storeys, as these are required to accommodate the additional vertical movement in the timber fame and differential movement between the inner and outer leaf. However, this is a 3 storey development and so long as vertical movement stays within expected limits then a standard fixed wall tie should suffice.

Standard Timber Frame Wall Ties

Standard Timber Frame Wall Ties

Timber frame movement tie

Timber frame movement tie

 Structural Cracking & Movement

Significant structural cracking

Significant structural cracking

Structural cracking to the outer masonry leaf of timber framed buildings can often occur where this differential movement between the inner and outer leaf falls outside of acceptable limits due to inherent design flaws or poor build quality.

When inspecting the building externally we noted that door and window frames were often slightly deformed and out of square, which resulted in extreme difficulty in opening the softwood timber french doors leading out onto the apartment balconies. We also noted significant stepped cracking in a number of areas to the outer leaf of masonry.

Starting from the Top

It was initially thought that defective balcony detailing and waterproofing arrangements were the cause of water ingress into the building and in fact the initial instruction was very much about investigating potential balcony defects, but of course you must approach these investigations with a blank canvas and an open mind. Whilst there were a number of relatively minor issues with balcony upstand detailing and parapet wall box gutter outlets, it was clear that these were not responsible for the water ingress or the structural defects seen.

Logically, I like to start from the top and work my way down once I start the internal inspection  and starting from the top meant inspecting the balcony that fully surrounded the building at top floor level.

Keeping the Timber Frame Dry

Open bed joints to parapet copings and no throating detail to underside

Open bed joints to parapet copings and no throating detail to underside

I found a number of serious and critical defects relating to the high level parapet walls that in my opinion have been allowing rainwater ingress into the wall cavity for a number of years, possibly since the building was constructed. Of course, if this was the case and water ingress was as bad as I believed it to be then the the greater probability is that the timber frame has swelled and expanded, rather than shrunk. The net result of course is the same, which is the potential for excessive differential movement between the inner and outer leaf. Moreover, there is a further potential for timber decay in the structural timber frame and perhaps even structural failure as timbers are affected by fungal decay.

Open perp joints between coping stones

Open perp joints between coping stones

Defects Causing Consequential Damage

Adhesive and cohesive failure of sealant to coping bed joint

Adhesive and cohesive failure of sealant to coping bed joint

We noted that the parapet wall copings were not fit for purpose and had been poorly installed off centre so the outer wall face had a 70mm overhang, whilst the inner parapet wall face only had a 30mm overhang. To meet the requirements of BS5642 then a minimum 45mm overhang was required to either side. However, more critically there was no throating detail to the underside of the parapets meaning that rainwater would flow along the underside of the coping overhang and straight into  cracks or open joints that may exist to the coping mortar bed.

Lead apron proved not to extend across the width of the cavity

Coping stone removed. Lead apron proved not to extend across the width of the cavity

 

Of course, this shouldn’t be a problem, because there’s bound to be a physical damp proof course installed under the copings as a secondary line of defence… or at least there should be!  We removed a parapet coping and as we suspected there was no physical DPC installed. So water was entering the wall cavity from the underside of the failed bed joint to the copings and the open joints and cracks in the coping mortar perp joints.

On finding these defects we of course had serious concerns as to what effect this long term water ingress was having on the timber frame. On checking the timber moisture content to the head of the timber frame we recorded a moisture content of 21.2%, proving the real current and ongoing risk of timber decay to the structural timber frame.

High moisture content to timber frame

High moisture content to timber frame

 

 

 

 

 

 

 

 

We recommended and specified urgent works to correctly waterproof the balcony parapets and further recommended opening up sections of the wall cavity where cracking had occurred to inspect the integrity and condition of the structural timber frame.

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