Why oh why do extension foundations have to be so deep?

[leew2]

Ok, thinking of building an extension on the back of my house. Dug a hole next to house to discover foundations about 330mm (13 inchs) deep. However building control want me to make the foundations for a single story extension to said house 900mm (3 feet(!!) ) deep, almost three times the size. The original house has stood for over 80 years with no evidence of movement or settlement but they still want me to put in ridiculously deep foundations. Clay soil with no major trees nearby.

 

[jeds]

Old houses are built from lime mortar which is flexible and able to accommodate small amounts of movement. The house most likely has been moving but it's not evident. New constructions built from cement mortar are very brittle and will crack very easily if there is any movement at all. Having said that I agree the 900mm deep rule sometimes seems onerous but research shows that clay will never dry out or freeze at that depth.

 

[RedHerring2]

I had to 'underpin' the old foundations up to 300mm with the new foundations. So be prepared for that.

 

[noseall]

900mm.

Deep?

You avin a larf?

900mm by todays' standards is not deep, especially when you think that mini diggers, grabloders, laser levels, on-site concrete mixing services, concrete pumping services are available to all, then 900mm is a cakewalk.

The question you should be asking is not why - but why not. Old houses along with their shallow foundations are pants. We are renovating one at the mo' and i wish we could just knock it down and start from scratch.

 

[leew2]

If building a totally new house then I can understand it but not for just adding a utility room to an existing house. Due to the very hard clay soild I'd have to use a mini digger to get to 900mm but could dig to 450-600mm with a spade and just mix some concrete myself.
If the whole house did start to sink for some reason, would it not be better for the utility room to sink with it rather than stay and break off?

EDIT: just read about lime mortar, that's a good point, but I don't think the house is old enough to be lime mortar and the walls are still all straight and square.

 

[noseall]

Due to the very hard clay soild I'd have to use a mini digger.

And???

We are building a 5m x 3m extension at the mo'. Small and sweet.

The foundations went down from 1.2m to 1.5m deep due to sediment type mud for the first four feet or so.

The concrete thickness ranged from 800mm up to 1200m and used up 12m³.

Ho hum.

 

[Possel]

Old houses along with their shallow foundations are pants. We are renovating one at the mo' and i wish we could just knock it down and start from scratch.

Not all old houses. I have sorted out the dampness due to water ingress from roof, gutters, downpipes and higher ground, and there are no structural issues on my house (170 years old). I would of course totally agree that if I hadn't fixed these problems, there would have been some major issues in the near future! Apart from severe neglect due to the above, structural issues really only happen in the event of subsidence, flood or drought, which affects many modern houses as well if built in the wrong place.

 

[bhm1712]

if you can dig to 600mm by spade, whats wrong with another 300mm?

 

[Richard C]

Rules is rules & 900m is now the minimum required depth; think yourself lucky, if you had any trees within 5m you could be looking at going down 3M or even more!

Don’t prart about with a spade, hire a mini digger & you’ll be all done in a couple of hours.

 

[clifford1]

Old houses are built from lime mortar which is flexible and able to accommodate small amounts of movement. The house most likely has been moving but it's not evident. New constructions built from cement mortar are very brittle and will crack very easily

Obvious silly question - so why don't we use lime mortar?

 

[RedHerring2]

Obvious silly question - so why don't we use lime mortar?

http://www.bricksandbrass.co.uk/diy_materials/lime/lime.php

 

[clifford1]

Obvious silly question - so why don't we use lime mortar?

http://www.bricksandbrass.co.uk/diy_materials/lime/lime.php[/QUOTE]

So that puts the case for using lime .

It doesn't answer the question, why use cement if it necessitates such deep foundations, and presumably much use of energy to manufacture it?

 

[Static]

Basically your options are dig below the trees influence on the ground conditions or put foundations in the zone and cross your fingers for the next 10-15 years that nothing moves..
A few millimeters movement is common but it can be upto 50mm which can make your new extension worthless almost over night.. take your pick.. and be sure the insurance company will chuckle over the wrong choice..

 

[RedHerring2]

So that puts the case for using lime .

It doesn't answer the question, why use cement if it necessitates such deep foundations, and presumably much use of energy to manufacture it?

Taken from that web site:

This rapid set was an advantage in cold or very wet conditions. Builders slowly abandoned lime mortar in favour of cement and sand mortars.

Lime is a much slower setting than cement, requiring work to be covered in the least possibilty of rain.
It may aslo limit the daily 'lift', i.e. number of courses that can be completed per day or subsequent days.

Other reasons, not specifically mentioned is the consistency of cement. Not the feel or texture, but the reliability of the consistent response of cement. Lime does vary in it's response, set, etc.

Also, hydraulic lime would be needed below ground or in wet conditions. Hydrated lime indoors, etc.

 

[noseall]

We might as well revert back to (hand) digging a lime slaking pit, have it all delivered by horse and cart, hand ball it off with shovels and mix it by hand.

Luddites, don't you just love 'em.