We are often asked about underpinning walls.
This is a typical question we are asked:
"We are refurbishing and extending a 140year old stone cottage in Oxfordshire. It has shallow foundations on a very sandy soil and a structural engineer has said it should be underpinned. We keep reading that this is not advisable to allow the building to move as it was designed to. We need to lay a new floor slab (currently a dirt floor) and are looking at a Limecrete base, digging at a 45 angle to stay away from the foundations. We are not sure what to do for the best?"
So what are the issues here?
The house is solid walled, built with stone, on shallow foundations. The walls usually rest on wider stones at the base which spread weight. Internally, floors were either earth or stone slab / brick / tile. The building has survived very well for well over 100 years with no ill effects. Because it is built with soft, flexible mortar - often just mud with a little lime added if funds allowed, it will expand and contract with the seasons - and as the ground underneath moves and heaves, the building will do the same - settling with the years. Its done this for 100 plus years and not fallen down - so why would anyone want to interfere with this natural equilibrium?
Unfortunately we are surrounded by structural engineers who are programmed to think concrete and steel. There are few if any calculations that can be done on a naturally flexible, movable structure. Most structural engineers are out of their depth completely, and their knee jerk reaction to being confronted with a house like this is to shove concrete everywhere. It causes more problems than it solves. If you STOP the movement that has been able to take place over hudreds of years, the building becomes stressed, cracks start to appear, and the structure can deteriorate.
Underpinning is the worst thing you can do - firstly it blights the house - once done, its on the record and you'd never sell the place. Anyone knowing a house they are about to buy has been underpinned will run a mile. Secondly - (our structural engineer deals with this all the time) - if you do a dig out, and put the floor back in straight away, he's happy for a steep slope - so long as the ground is dry and compact, it wont move in the short period of time it is exposed. We've done 350mm dig outs with vertical sides under the foundations - which are then immediately backfilled with compacted insulation and the limecrete floor laid.
There ARE situations where underpinning is needed - if, for example ground has been leached out under the corner of a building by escaping stormwater from a drain, you can easily end up with a large cavity under the wall, associated cracking, and structural problems. In this situation, you have to put something back under the corner of the building to prevent further movement. Personally I'd not use concrete - it would be a limecrete block, which at least will move at a similar rate, even though there is a lot more mass to that corner of the building.
The same sort of issues occur if you build an extension onto an old building. Architects always specify new build materials, concrete floors etc - so now you have two buildings, each moving at a completely different rate. You can't successfully joint them, without associated stress and movement - it is inevitable - but architects building extensions on old buildings just never think about it. They aren't trained to think outside the box! In these cases we always advise clients to use similar materials - and use lime mortar for any brick or stonework. Limecrete floors instead of concrete.
Buttresses to hold walls up are another thing that we often see - and modern thinking is somewhat against them. A buttress can put huge weight on the ground - even if it is supporting a wall, it is also pushing down at a different rate to the rest of the building, and tends to push down, and drag the offending wall outwards in the process. If a wall is moving, you are far better off strapping and tying it - look for the cause of the movement - if it is roof spread for example, install collar ties, or tie the wallplates to prevent further movement. One of the walls in my own farmhouse was like this - we ended up holding the wall in place with a JCB bucket parked against it for a week or two - whilst straps were fixed to the inside of the wall and tied back to the carcassing of the floors. JCB was moved back, and the wall hasn't moved a millimetre in the last 20 years since it was done.
Don’t be bullied by building regs into insulating either - it’s a breathable structure exempt under Section 11 - breathability.
