Subsidence problems

[Nigel Watts]

My 1840 semi in North London, sitting on London clay, was originally built with a small two storey extension at the back (sticking out only about six feet). We extended this by a further six feet or so about four years ago. The new extension, unlike the original one had some chunky foundations, as required by building regs.

The whole rear extension (new plus old) is now doing a leaning tower of Pisa act, and has detatched itself from the main house. The crack at the top is now more than an inch wide. To be more precise, it seems to be tipping like a see-saw, with the new bit sinking and the old bit next to the main house rising slightly.

The engineer who we used originally came back to have a look and concluded that the ground must either be too dry (she pointed accusingly at a large Eucalyptus tree some 20 feet away) or too wet - there is a also a damp problem in the new extension, but that is another story - at which juncture she pointed accusingly to the drains.

Digging around the extension shows little penetration of tree roots at the level of the foundations and a detailed survey has just concluded that the drains are basically sound. The house is not on a hill.

My suspicion is that there is no culprit here, just that old house and original part of the extension are quite content where they are, having had 160+ years to settle down, but the new bit is still settling in. The brickwork of the original extension was probably not very well connected to that of the main house, so that is where the crack has formed.

Any ideas/similar experiences? Is it likely to be covered by insurance?

 

[Penners]

Does your insurance policy have a subsidence clause? Insurers have got very nervous about subsidence in recent years, and many of them load this problem with a much heavier excess than other risks.

Did the extension have Building Regs consent? It occurs to me that, if they were to kick up a fuss about covering this problem because of "sub-standard workmanship", you could always rely on your BR consent and inspections to show that everything was done properly.

 

[hobbaabboh]

Sounds to me, to be poor bonding to the inner leaf brickwork of the original back projection joining the main house brickwork - i have come across this before on victorian properties , maybe worth removing a few bricks to the outer leaf to investigate what's happening behind, and the groundworks to the new extension may have unsettled the stability to the ground of the original projection - might be a case of having to re-bond the inner leaf to the main house using ties/ or patching in new bond work and lime mortar repairs from inside the house. I don't suspect the tree to be the culprit - i can't advise on the damp problem without more information. Would change your engineer and check the small print in your insurance policy and penners advise reg. BR on the extension.

 

[joatex]

Plenty of old buildings on the blue clay of Vale of Aylesbury have had iron rods threaded through the building and S or X plates at the wall surfaces. Seems to hold the vertical cracks.

 

[Nigel Watts]

Thanks for the advice. The extension was done using a professional architect with a professional engineer designing the foundations, all with full consents, inspections by building control etc. I will dig out the small print of the insurance policy to see if there are exclusions.

The new part of the extesion seems perfectly well attached to the old part of the extension - the problem is where the old extension joins the original house. All the brickwork is rendered, so inspecting it isnt that easy.

The mistake, perhaps, was preserving the relatively small amount of old extension wall (all is in 9 inch solid brickwork, rendered). If the whole thing had been knocked down and rebuilt, properly connected to the house, perhaps this would not have happened.

 

[Mark]

Did the engineer who designed the foundations do a site visit before the design? What did they do?

Yeah, the old brickwork is coming apart because its where the stresses are going to overcome the strength of the brickwork, the real issue is that the extension is subsiding - and that has to be a foundation issue doesnt it? If you'd rebuilt the whole back then I think you'd still have the same issue. If the ground is unable to support the weight of the building then the building will sink.

Tie bars are usually on old slow moving stuff. This isnt. I guess you could patch it and see if it stabilises?

 

[MdB]

That was my feeling too. If it had been better bonded to the back of the house, it would have just have pulled the back of the house off.

I'm interested in this as I will be butting new build to old walls and I expect movement. The plan was to have a flexible join between the two.

 

[Nigel Watts]

The foundations were pretty deep and solid looking (I have quite a few pics of them). The extension is at basement and ground floor level, but the basement is only half sunk into the ground:

The builder was a bit dodgy, but I am pretty sure the whole thing was propertly insepcted.

 

[Mark]

deep is one thing - but isnt always the important thing. Were they wide? piles or ribbon or a float? what do they sit on?

 

[Gareth Hughes]

The bathroom and kitchen outshuts on the rear of my 1880 terrace are well-founded on the remains of the city wall. The rest of each house is built on infill in the ditch outside the wall (presumably consisting largely of 4-500 years of unmentionables being thrown over the wall). The result is that the meant-to-be-horizontal yellow and black brick string course along the front elevation bucks and rolls like a distant line of hills, while I can lie in the bath safe in the knowledge that I’m not about to be buried in the rubble when the rest of the place collapses.

Or at least I could until a few months ago when I noticed a fine horizontal crack running through three tiles at the end of the bath. I’m sure it wasn’t there before. So now I have the minor discomfort of sitting with my back to the taps instead, so I don’t see it.

 

[Stephen]

Nigel

I suggest you speak to your insurers and let them appoint a loss adjuster and get them to investigate. If they turn down the claim you have lost nothing, but if they accept the claim the costs (subject to excess) should be covered.

Loss Adjusters tend to work one of two ways - all-in service where you get very little say once they hav accepted the claim, etc. but they pay everything (except the excess) OR reimbursement where you can use engineers, builders, etc of your choosing (within reason) and the insurers reimburse costs as it progresses.

As for the extension I would generally advise forming a movement joint where a modern extension adjoins an older part so that if the extension settles, or the main house moves independently then one does not affect the other. I am slightly concerned that your comments imply the new extension was positively 'fixed' to the older parts as I would generally steer clear of trying to tie the new to the old.

As for the matter of roots, it is the fine roots you do not see easily that often cause the damage! Just because you cannot see roots does not mean the tree has no bearing on the matter. This is where proper investigation comes in - trial holes and root analysis, etc - and this is what the loss adjuster will sort out.

 

[Penners]

a few months ago when I noticed a fine horizontal crack running through three tiles at the end of the bath. I’m sure it wasn’t there before. So now I have the minor discomfort of sitting with my back to the taps instead, so I don’t see it.

In the event of a sudden problem, how quickly could you invert the bath over your head?

 

[Moo]

The bathroom and kitchen outshuts on the rear of my 1880 terrace are well-founded on the remains of the city wall. The rest of each house is built on infill in the ditch outside the wall (presumably consisting largely of 4-500 years of unmentionables being thrown over the wall).

I don't want to alarm you, Gareth, but you are, of course, aware that this being Norwich, there's every chance that beneath the city walls and the ex-ditch are some of those even more ancient chalk-workings. Which are beginning to collapse.

 

[chuckey]

what are the odds of building a bathroom extension over gateway in a city wall such as a salle port? - no gate towers.
Frank

 

[Gareth Hughes]

The bathroom and kitchen outshuts on the rear of my 1880 terrace are well-founded on the remains of the city wall. The rest of each house is built on infill in the ditch outside the wall (presumably consisting largely of 4-500 years of unmentionables being thrown over the wall).

I don't want to alarm you, Gareth, but you are, of course, aware that this being Norwich, there's every chance that beneath the city walls and the ex-ditch are some of those even more ancient chalk-workings. Which are beginning to collapse.

Too low-lying for that - though three houses on the slightly higher ground two streets away did fall into a hole in 2005 :?