A Little bit about me...

I'm a Structural Engineer, specialising in conservation, at Hurst Peirce + Malcolm in London. I don't wear tweeds, am not particularly "cultured" and I'm not that old, but I do care passionately about the conservation of old buildings. I am a Chartered Structural Engineer, a member of The SPAB (formerly on the founding committee of the reborn Berks, Bucks & Oxon Regional Group), a Friend of SAVE Britain's Heritage and have recently completed a PD Diploma in conservation at West Dean College, with a view to achieving CARE accreditation. I hope to give you regular updates on my trials and tribulations as well as some insight into projects I am involved with and things I believe in.

Be patient with me - I'm no writer and I'm normally up to my eyeballs at work........

All views expressed here are my own.

Hurst Peirce + Malcolm, Celtic House, 33 John's Mews, Holborn, London WC1N 2QL

07854 624692 - richardsalmon9@gmail.com


Sunday, 17 April 2011

A Cautionary Tale

The Old Manor House in Cholesbury, Bucks, is an unimposing little brick cottage that hides a big secret (although the name is a bit of a clue). This Grade II listed cottage sits at the foot of Cholesbury Camp (Iron Age hill fort) and is considered to contain two bays of a much larger timber framed manor house/manorial court building dating from the 16th century. (I am hoping that @wallstroker will be given the chance to investigate this further). None of the ground floor timber framing remains, so the 9" brickwork here is loadbearing, but the first floor and roof framing are original and largely intact, so the brickwork at this level is primarily just cladding.

The Old Manor House - as existing
I was initially called in with building works already underway (extension), to give a second opinion on another Engineer's report advising that the existing bowing gable wall would need to be taken down and rebuilt (On inspection it was clear that this wall had already been rebuilt at least once before). The experienced builder had raised concerns over the necessity of the proposed demolition and the client obviously wanted to avoid the cost of rebuilding if at all possible. Quite unbelievably, the local Conservation Officer had seemingly accepted the Engineer's report on the most flimsy basis; that the wall was "unstable". But, other than some additional tying-in for the new wall, no other works were recommended in this area.

The "offending" gable wall.

On my first site inspection, I really didn't think the wall looked that badly out of alignment, but could see where the centre of the bow was located and then went about trying to find out why this had occurred. The first noticeable defect was a large separation crack between the gable wall and the chimney breast. This separation was at its worst at about first floor ceiling level and narrowed above and below this point. The next-most obvious defect was a strange oak corbel detail springing out of the corner of the chimney breast and apparently providing support to the original oak spine beam. A third defect was discovered up in the roof space. After marvelling at the original arch-braced central roof frame, I discovered that the original oak collar to the end frame at the gable wall had effectively rotted away where embedded into the chimney breast, leaving the purlin ends without support and thus applying load to pockets in the gable wall brickwork. There were also very shallow foundations (concrete) under the rebuilt gable wall.

So, by now we had a number of possible causes. I then undertook a plumb survey of both wall and chimney breast, which confirmed my earlier visual inspection, that the wall was only borderline unstable and could easily be tied back into the structure. Further analysis of the plumb survey results showed that the prime suspect to be the loading of the outer corner of the chimney breast, had caused a slight inward bowing of the breastwork, leading to a much larger complimentary outward bowing of the gable wall (the two not being effectively tied together).

Having found the problem, the solutions were relatively straightforward. Result: One happy client with a slightly less light purse!

The annotated photos below show some before, during and after repairs, but in essence, the moral of this tale is that one really has to get to the bottom of the problem, before suggesting a solution. Had the other Engineer just had the wall rebuilt, the same thing would have happened again, over time, with the new wall.

There are still too many old buildings being worked on by architects and engineers without the right attitude and/or experience, so just a quick plea to homeowners and developers to think twice before appointing your next design team.

As an engineer, mainstream philosophy says it's much easier to condemn something than to save it. That's where conservengineers come in.....

Part of the original framing visible at first floor.

Original arch-braced roof frame, purlins and rafters - still sound, even if they've moved about a bit down the years.

The gap between chimney breast and gable wall. Lovingly papered over by the previous owners.

The existing oak corbel detail. When we uncovered the extent of the structural damage to the original beam, the reason for the length of the corbel became clear. I'm sure a well intentioned, but an ill advised structural solution.

The spine beam from above. The steel flitch plate allowed us to invisibly "extend" the beam span so the load is carried directly onto the back of the chimney breast and gable wall. (Note: the timber section at the top of the pic is a new make-up piece to replace previously lost material)

A little steel assembly up the loft to provide support to the end of the purlin and ensure roof loads are applied onto the eaves level beam as originally intended.
As an aside, I was asked to asses the load capacity of the existing ceiling/loft after the loss of a large section of existing lime plaster. It transpired that excessive deflection (under imposed load) of relatively poor quality scantling joists had sheared off the grips between laths. The client was keen to utilise the loft space, so an independent deck was installed above the original ceiling. 

The beam repairs completed. The lowered section of ceiling is as original and signifies an historic infilled staircase.
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The cottage, with extensions and repairs (very nearly) completed. The bowing gable wall was retro-fitted with ties into the existing chimney breast, first floor and roof construction, along with some bed joint reinforcement (in a lime grout) above and below the existing windows where cracking had occurred.