In this blog post, I will be looking into bay window movement and exploring the causes of this common issue.
I have inspected many properties in the past few months that have all suffered from movement, with the most prominent being cracks around the bay window. These cracks can appear externally in the masonry and internally in the plaster.
Establishing the cause of cracks can be challenging at times and the main question about movement is whether it’s historic or progressive.
The majority of buildings will show signs of movement that occurred in the past. Walls out of verticality and cracking in brickwork are often signs of old foundation settlement. A crack that has been filled with mortar and hasn’t subsequently reopened suggests that the movement has stopped.
There are two types of cracking found in masonry: Vertical cracks that extend through the bricks and stepped cracks that run through the mortar joints (either tapered or uniform in thickness).
Bay Window Movement to a property in East London
Difference between old and new structures
One key difference between period and modern properties is the way they handle movement.
Modern properties are rigid structures built on deep foundations extending below the upper unstable soil that is prone to frost penetration and seasonal moisture changes.
Where as old properties were built to accommodate a certain amount of movement rather than prevent it. The use of lime was important in allowing subtle movement to occur through the mortar joints. On the other hand, hard modern cement is rigid and inflexible, thereby resulting in cracking.
Before carrying out any remedial works, the cause of the movement must be dealt with.
Different foundation depths resulting in differential movement
Where you have a modern property extension built with deeper foundations, cracking can occur near the interface, due to different rates of settlement between the two structures. This does not necessarily mean that differential movement is new.
Victorian bay windows were often built with shallower foundations than those to the main house and may be prone to travelling at different rates over the course of the year.
One inherent defect Victorian properties suffer from is shallow foundations. These foundations are known as ‘footings’, because they were built in a stepped fashion to spread the load at the bottom.
Shallow foundations can cause cracking through the brickwork, often via window openings, which is the weakest point. Builders often skimped on foundations to structures such as bay windows, so they tend to move around more in tune with seasonal ground changes. Therefore, bays can be prone to settling at a different angle to the house with its deeper footings due to ‘differential movement’.
Vertical Crack to a defective Bay Window in North London
You can insert an expansion joint to accommodate future movement with a flexible mastic sealant to keep the structure weather-tight.
Subsidence affecting the ground under shallow foundations
Subsidence effectively means that the ground beneath the foundation is giving way, as a result of external conditions.If the soil beneath the foundation shrinks (eg due to prolonged periods of dry weather exasperated by trees extracting moisture or a leaking drain), it can deprive the wall of support and thereby cause cracking to the masonry.
Bay Window Movement in North London
Where there is extreme cracking to a wall due to subsidence, the standard approach is to excavate beneath the foundations level to stable ground and underpin with concrete.
Movement can occur as a result of poorly executed structural alterations. This will require an engineer to design a new method of additional strengthening.
It is not unknown for works such as illegal basement conversion excavations to undermine party walls/ shared walls. All structural alterations should be carried out with building regulations consent.
Once you have solved the cause of the structural movement, then you can repair the crack. One thing that is agreed is that cracks should be repaired to prevent water entering and causing further damage.
Stepped cracks can simply be remedied, by pointing with lime mortar, whereas vertical cracks often require the defective brickwork to be cut out and replaced.
Another technique for substantial cracking involves drilling holes along the crack and injecting them with special thixotropic resin grout. Engineers sometimes recommend cracks are stitched using stainless steel ‘Helifix’ rods inserted into the wall to stabilize the masonry.
If you’re planning on purchasing a property I would always recommend having a pre-purchase survey undertaken in advance of completion. The survey ensures an experienced and qualified Surveyor full checks the property ensuring there aren’t any hidden defects.
All of the photographs used in this blog post were taken on pre-purchase surveys!