The property
A two-storey 1930s semi-detached house in Greenford, with a single-storey enclosed front porch that had been rebuilt a few years earlier on a deeper foundation, a two-storey side extension and a rear extension. British Geological Survey records show London Clay in this part of Greenford, in places with river terrace deposits on top.
The client's concern
The owner had noticed cracks in the front porch and a gap opening up where the porch meets the front wall of the house. It had appeared during the summer, and there was some slight distortion around the first-floor windows above.
What we saw
Two horizontal cracks up to 1.5 mm wide ran across the front wall of the porch (BRE Digest 251 Category 2, slight). At both junctions with the house there was a vertical separation, about 10 mm on one side and 3.5 mm on the other, wider at the top than at the bottom (Category 3, moderate). The narrow brick piers beside the porch side windows had bowed outwards by about 20 mm, with cracking at their base, and one side window was sticking.
Elsewhere there was only minor, long-standing cracking at the two front bays and hairline cracks inside, typical of a house of this age.
How we investigated
A General Structural Inspection: a visual, non-intrusive inspection of the accessible rooms and of the front and rear elevations, with particular attention to the porch. We measured and classified every crack, checked the street tree in front of the house against NHBC Standards Chapter 4.2, and reviewed the British Geological Survey records. Nothing was opened up, and the foundations and drains were not exposed or tested.
What we found
The tapering gap, the bowed piers and the distorted windows all pointed the same way: the porch was rotating outwards and pulling away from the house. In our opinion the most likely reasons were that the porch was not properly tied into the main wall; that no lintel could be seen over the side windows, so the slender piers and the window frames were carrying the wall above; and that the porch sat on its own foundation, of unknown depth, which behaves differently from the house. Seasonal shrinkage of the clay during a dry summer may have added to the movement.
The street tree, a low water demand species about 7 m from the house, was outside its zone of influence and was not considered the main cause. The main house showed no significant signs of structural movement and no evidence of subsidence.
What we recommended
A staged repair. First, obtain the records for the porch rebuild and carry out a CCTV survey of the drains at the front. Then open up the render to check the brickwork, lintels and ties; tie the porch back to the house with stainless-steel restraint fixings; reduce the side window openings and provide proper lintels, so the frames no longer carry any load; and stitch any cracked brickwork with helical bars. Finally, monitor with tell-tales for at least 12 months. Underpinning was only to be considered if monitoring showed that movement was continuing.
We also recommended checking that the roof can take the added weight of solar panels on a concrete-tiled roof, and how a retained chimney stack is now supported.
Related services
This was a General Structural Inspection (GSI). Read more about our structural inspections, cracks in walls and our work in Greenford, or see more case studies.
