foundations, clay soil and trees

butterpie

Hi all,
I'm planning to build a garden room at the bottom of my garden. It will be within permitted development, and is exempt from building regs but I still want it to be built safely.

It will be double skin block approx 4.8m x 6m. A surveyor is drawing some plans and has specified 60cm deep trench foundations. I think this maybe too shallow.
It is clay soil, and next door have a holly tree and a laurel tree right on the boundary less than 1 metre from the proposed foundations. They also have 2 apple trees less than 4 metres away. Their garage which is next to the trees is falling down and is propped up, though it is quite old I think.

A building control surveyor directed me to the LABC foundation calculator online. I input just one of the trees, (the laurel) and the calculator said "The required foundation depth is in excess of 2.5 metres and is outside the scope of this calculator. A full structural foundation design is required."

Another surveyor I spoke to says the foundations need to either piled or a raft which needs to be designed by a structural engineer, to avoid the influence of the trees.
I just wondered what other people would do in this situation?
I know trees and clay soil can mean a lot of shrinkage but I thought 99% of the roots were in the top 1 metre anyway.

Its a long narrow garden so its not possible to site the building anywhere else.

thanks for any advice
bp

ds1980

http://www.planningportal.gov.uk/permission/commonprojects/outbuildings/

If it was me I'd want to follow building regs rules as much as possible especially in the foundations (even though they are not required) and that would probably require footings of 2.5m in the case of what you have described. As you don't require regs then it is entirely up to you and at your own risk.

Holly are renowned for having a very deep root system. You'd be surprised how far down they go.

Furts

A couple of points. I have not checked your trees or depths, but the 2.5metre rule comes about because of safety and because of cost. With deep foundations someone needs to be trimming the bottom (not always required) and fixing the Clayboard/Jablite/Polystyrene - which is an awkward process in narrow trenches. (I am assuming you have been advised on the requirements for this - it is a bit more involved than some think)

When the the above issues and the cost of the concrete are considered there is a recommendation to opt for piling, or a raft. In general, piling is expensive for small jobs such as yours. But equally, a raft needs to be designed, the reinforcement purchased to suit and the construction is outside the abilities of many people. An average groundworker will say they can build rafts, but many are only competent at laying drains, placing concrete and laying paving slabs.

27col

In the circumstances that you have described I think that I would opt for a wooden structure. Then you wouldn't need strip foundations, the structure could be built off of individual pad foundations. Which would be a lot cheaper to do.

brightontraveller

2.5 metres deep what you building a skyscraper Calculate dead load (basically building materials weight) and live load (whats going in it) use a slab foundation or sonotubes columns there stronger than traditional foundations require alot less concrete, less removal of soil etc and are easy to get level, same height etc dig hole, put in nail to required height and fill (half the surveyors architect etc don’t even know what they are) add steel, timber, concrete lintel frame lay bricks up from that etc

I_have_spoken

Yes, 2.5m for a fancy shed is madness.

jackyann

We have built a house near a tall leylandi hedge which we are obliged to keep. A structural engineer designed the foundations to take account of soil movement.
The neighbouring house (whose objections made keeping the leylandii a planning condition) built an extension a few years ago which took no account of the leylandii. This is now having problems because of soil "heave".

So I would say that you either need some good advice about foundations, or, as 27col says, opt for a cheaper structure that won't be affected, and accept that at some point it may need replacing.

Davesnave

jackyann wrote: »

....opt for a cheaper structure that won't be affected, and accept that at some point it may need replacing.

There's a lot of wood-based garden rooms that will last at least 20 years with proper maintenance.

I suppose it depends how old the OP is, and whether they might move later in life, but that would be long enough for many of us!

Furts

jackyann wrote: »

So I would say that you either need some good advice about foundations, or, as 27col says, opt for a cheaper structure that won't be affected, and accept that at some point it may need replacing.

The advantage with a raft is, if properly constructed (which is rare), it will move with the ground. Hence the building located on it is safe and sound. The risky areas, needing careful detailing and construction, are the drains and all incoming services