Building Defects

University Degree Architecture, Building and Planning

Paula Quigley B00343872

OUTLINE The attached sketches, prepared on site by the building surveyor, show the details of cracks discovered in a 15-year-old extension to a 40 year old bungalow. The most severe defects are the external crack running the full height between the extension and the extension structure and the dislodgement of the wall plate to the existing roof. In addition to the defects noted on the sketches the surveyor noted numerous other cracks in the extension.

Before any works are carried out there would be a number of questions that I would ask, first being has the owner of the property carried out any works in the house and on the grounds lately? Have the cracks appeared over time or suddenly? Are there any cracks internally on the walls or around the windows? Has there been another movement in the property such as doors? I would also ask to see a copy of the plans to see what type of foundation have been used. It would also have to be ensured any works are carried out met with building regulations and that all work is carried out by qualified professionals.

Having looked over the building surveyor's sketches there are a number of issues that may need addressed; the first issue being further investigation of the numerous cracks on the external walls. According to the surveyor’s report they are the most “severe defect”. It also states that the cracks are “running the full height between the extension and the extension structure”; there may be a number of reasons for these cracks appearing, for instance the building may have settlement, thermal movement, lack of lateral restraint or wall tie failure. Due to the fact that the surveyor had not noted whether there were any trees within close proximity to the property it will be assumed that the damage has not been caused by trees extracting moisture from the subsoil and therefore causing the foundation to move or crack.

The first issue that would require investigation is the defect on the external wall and the dislodgement of the wall plate on the existing roof. The size and placement of the crack can tell us what may be the problem. For instance if the crack narrows at the top and bottom and is wider in the centre it may be lateral restraint (bulging) or if the crack narrows with height it maybe heaving. However in this case the surveyor states that the crack runs the full height of the building and is 19mm at the top and 0mm at the bottom, so this may indicate that the source of the defect is settlement. It may also be that the extension was not properly connected into the existing building and therefore both structures are moving away from each other.

Most buildings crack at some time during their lifespan; the appearance of cracks is a symptom of distress within the fabric of the building and may be the first sign of a defect. All cracks would also need to be filled in; this is when a flexible caulk is in placed in the joint to seal the crack to make it weather-tight. It must be ensured that the caulk expands and contacts with seasonal temperature changes or it will crack.

Cracks can be classified into three separate categories, aesthetic only, affecting serviceability and affecting stability. Any cracking ...

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Cracks can be classified into three separate categories, aesthetic only, affecting serviceability and affecting stability. Any cracking may be the first sign of a serious defect in a building and therefore must be investigated; one way to do this would be to establish if the crack is getting any bigger by monitoring it over a set period of time, and measured in the direction of movement this can be done using several different methods:

2 Disks – This is when two metal disks are fixed to either side of the crack at the same level by using epoxy resin; the distance between the two metal disks is then measured at intervals to establish any movement.

Direct measurement – This is where the defect is measured at a fixed point with such equipment as a ruler, a standard vernier caliper measuring gauge or a digital vernier calliper measuring gauge.

3 Gauge – This is when two part of a gauge is fixed to the wall using screws or glue and the measurement is taken from the scale on the gauge.

Most building structures will also have a slight degree of settlement. Settlement is caused by the compressing force exerted by a building onto the substrata and foundations below; these forces are made up of dead load of the walls, live such as furniture and roofs, on the ground underneath. Substratas vary in strength from rock having a very high resistance to compression and settlement whereas sand has less resistance and compresses easier and therefore is more susceptible to settlement when under compression. The foundations of a building have direct contact with the ground and transfer the loads from the building to it.

According to Riley (2006, P97) foundations must be strong and rigid enough to ensure that it is capable of withstanding the loads imposed on it without bending or suffering shear failure at the point of loading. Foundation failure will usually result in building movement and subsequent cracking; there are a number of issues that may cause failure such as, seasonal movement, variable ground conditions, frost heave (mostly in sandy soils) and also poor workmanship.

Due to the fact we have not received any information regarding the foundation of this property it will be assumed that Strip foundations have been used as they are the most common type of foundation used in the UK due to their simplicity and subsequent cost effectiveness (Stroud). A fault may have occurred with the foundation, such as concrete failure; this is where the concrete was not properly reinforced and therefore makes the tensile strength of the concrete low, or the concrete may not have set correctly in the first place.

Due to the fact the settlement is not uniform (which is known as relative or differential settlement) it could suggested that while the extension was being constructed the foundations may not have been wide or deep enough for the relevant substrata upon which the extension was built, therefore the load from the building were not uniformly distributed. This may be the source of the defect and if so the property may need underpinned. This is a process carried out to strengthen and stabilize the foundation of an existing building by expanding the existing foundation in depth or breadth. This will also help distribute the load across a greater area.

The two main types of underpinning are continuous and piled: continuous underpinning is carried out in sections, where no more than one third of the structure should be unsupported at any given time. Section lengths will depend on the condition of the structure being underpinned. There are three ways to carry out continuous underpinning:

Shallow – This is carried out by digging down the side and under the foundation. Concrete is poured into this hole beneath the foundation to the level of the old foundations; the hole is then filled back in up to ground level.

Deep – This process is the same as the above in terms of digging down the side and beneath the foundation, however the hole is made much deeper, due to this fact the sides of the hole need supported. The hole is then filled in.

Grouting – This is when bore holes are drilled down through the existing foundation at several points; the holes are then injected with a structural resin, which then hardens, acting as the underpinning to the foundation.

There are a number of ways to carry out piled underpinning:

Pile and Beam – This is carried out by piling down either sides of the wall and fixing a beam across the top of each pile through the wall, thereby creating an underpin to support the load.

Cantilever Piling – This is when two piles are bored or driven on the outside of the structure and a beam is placed over both piles and extended under the wall to create the underpin.
Mini Piling – This is when a number of smaller piles are placed through the wall at an angle from both sides. The piles cross each other directly under the foundation which creates an underpin.

Another area may need addressed after the building has been stabilized will be the wall plate. A wall plate is a piece of timber which is placed at the eaves of a roof, and it is designed to take the weight of the roof timbers and coverings. According to the surveyor’s report and sketches the wall plate has dislodged and moved forward by 25mm. This could have happened due to the settlement or due to the roof not been adequately strengthened when the extension was added to the original bungalow, which would have been carried out by adding additional support to the rafters of the roof. Due to the fact this may not have been carried out the roof may have suffered roof spread. This is where the eaves of the structure move from a lack of lateral restraints, which would therefore cause the wall plate to move. A remedy to this after all other works have been carried out to stabilize the building would be to provide more lateral restraints to the roof which could be done by running rafters horizontally from the rafter on each side of the roof construction, or to fit vertical and diagonal bracing to the rafters.

Overall my hypothesis is that the sources of the defects are settlement and roof spread. The remedial treatment that I would recommend would be to underpin the property and to strengthen and stabilize the roof and wall plate. The cost of all works would also have to consider and it would have to be weighed up as to whether it would be cost-effective to repair the defects in the property or rebuild the extension.

According to the Royal Institute of Chartered Surveyors the underpinning alone could cost between £5,000 and £50,000. If all the works are carried out I would recommend that the property was fully inspect, to ensure that the out leaf of the property was weather-tight and elements such as the lead flashing were not damaged.