What foundations for out building?

[perky416]

Hi guys,

Me again with another question on my latest project. I know there are other similar questions, but i havent found an answer yet.

In case you havent seen my other posts, i want to build a large shed/out building in the garden within permitted dev.

Please see the picture attached.

Basically, the out building will cover the entire green, an area of 30m2. The walls will be pretty much where the current brown edging is, but obviously it will be a square room. Im thinking single skin block with supporting piers. Each wall will be roughly 5.4m long. 2.5m overall height. Likely a cement fibre roof.

The room will be a golf simulator room, so very little live load.

As you can see, there are trees and bushes literally touching the edges, especially the big one to the right. These will all be ripped out, to create access around the perimeter of the building, however im reading that the type of foundation required will be determined by the trees and soil type.

Ive read that typically outbuildings use a concrete slab rather than trench footings. The red garage you can see in the picture is also built on a 100mm slab. However, ive also read that the presence of the trees, even when ripped out, could mean deeper foundations.

The big brown thing at the back of the garden is supported by a 150mm timber post at each side, i dug these 800mm down and postcreted them in. From memory, the soil was sandy.

How do i determine what type of foundations i need?

Will a 100mm concrete slab suffice considering it is single story, single skin, with very little live load?
Or will i need to go deeper trench foundations?

Ive done foundations in the past but ive just followed the orders of the builder, ive never had to do the working out for myself.

Thanks

 

[johnny2007]

Firstly, how dare you call a slab..."slab"!
Apparently us from the south are not allowed to call it that way, and accordingly to someone who knows all, it's called a "rafter" or some other shyt like that.
Secondly, you're very much right in saying that fundations are built accordingly to many factors.
Even though we're talking about a single skin outbuilding within permitted development, you'll need to get some professional advice, possibly from a structural engineer.
Failing that, building control have certain standards which apply widely to the area you're in, so they could advise.
But personally I would ask a professional, unless you're prepared to dig deep and err on the safe side.

 

[perky416]

Haha ive read it as a slab all over google. Rafter makes me think of a roof lol.

How deep do you recon id have to go to be safe?

 

[noseall]

Haha ive read it as a slab all over google. Rafter makes me think of a roof lol.

A plain piece of concrete is a slab. An engineered foundation for buildings constructed on poor ground, is called a raft. jonny is extremely ignorant regards foundations and building. I would treat his advice with trepidation. He's made himself look a right tw@ and got into many a pickle, being out of his depth.

 

[johnny2007]

Haha ive read it as a slab all over google. Rafter makes me think of a roof lol.

How deep do you recon id have to go to be safe?

It depends what you find when you dig.
Around here at about 2 feet you find solid ground which looks like compacted subbase aggregates.
I'm not gonna say how we call it down here, otherwise surely someone will moan about it and come up with some other stupid words only spoken in their 200 soul village.

 

[noseall]

Hi guys,

Me again with another question on my latest project. I know there are other similar questions, but i havent found an answer yet.

In case you havent seen my other posts, i want to build a large shed/out building in the garden within permitted dev.

Please see the picture attached.

View attachment 300973

Basically, the out building will cover the entire green, an area of 30m2. The walls will be pretty much where the current brown edging is, but obviously it will be a square room. Im thinking single skin block with supporting piers. Each wall will be roughly 5.4m long. 2.5m overall height. Likely a cement fibre roof.

The room will be a golf simulator room, so very little live load.

As you can see, there are trees and bushes literally touching the edges, especially the big one to the right. These will all be ripped out, to create access around the perimeter of the building, however im reading that the type of foundation required will be determined by the trees and soil type.

Ive read that typically outbuildings use a concrete slab rather than trench footings. The red garage you can see in the picture is also built on a 100mm slab. However, ive also read that the presence of the trees, even when ripped out, could mean deeper foundations.

The big brown thing at the back of the garden is supported by a 150mm timber post at each side, i dug these 800mm down and postcreted them in. From memory, the soil was sandy.

How do i determine what type of foundations i need?

Will a 100mm concrete slab suffice considering it is single story, single skin, with very little live load?
Or will i need to go deeper trench foundations?

Ive done foundations in the past but ive just followed the orders of the builder, ive never had to do the working out for myself.

Thanks

Any foundation deserves respect. Look at ground conditions and how the walls impart their loads. Masonry walls that bear the roof laod will generally require strip foundations. These will need to go down to decent ground. The thickness of the concrete (around min 200mm) is superseded only by the condition of the ground upon which it sits.

 

[cdbe]

2.5m height including a roof, you must have a short golf stick.

 

[perky416]

Is there an actual formula to calculate the required foundation?

For example:
X dead load + Y soil type + Z distance from a tree = foundation

How do structural engineers calculate it? Ive seen BC poke the floor and say "thatll do", but there must be more to it than just guess work.

 

[mikeey84]

Take a read of approved document a, that's likely to have more detail on how to work out what you need

 

[Jake2021]

I usually go over kill on my home stuff lol I dig 1m down x 60 then trench fill, that’s based on my ground but I think that would be a good place to start

 

[noseall]

Is there an actual formula to calculate the required foundation?

For example:
X dead load + Y soil type + Z distance from a tree = foundation

How do structural engineers calculate it? Ive seen BC poke the floor and say "thatll do", but there must be more to it than just guess work.

Visual on soil conditions - i.e. if you are still pulling out coke cans and rusty scaffold clips - go deeper. Soil colour change is a good indicator when breaking through virgin earth. Once past all the dark fertile stuff, the ground usually changes colour and consistency.

Width of trench is usually 200mm - 300mm wider than the wall on top, i.e. giving a centrally positioned wall a minimum 100mm 'toe' each side of the wall.

Thickness of concrete - min 225mm, though mass filled foundations tend to be more economical.

 

[jeds]

How do i determine what type of foundations i need?

Work out the total load per metre>work out the load bearing capacity of the ground> design a suitable foundation accordingly.

That's the simple answer. Your problem will be establishing the bearing capacity at the correct depth, which may (will) be deep due to the trees? You could do a trial excavation but if that ends up being 2m+ deep you might end up digging up the whole garden.

You've said the ground was sandy down to about 800mm. You could build on sandy soil but a foundation might be 800, 900, 1200 wide? A lot of excavation for 30M² build at 800 or 900 depth.

I would consider a mat foundation. (basically a reinforced slab) It will do what you want and will probably be less work in the long run.

 

[perky416]

Work out the total load per metre>work out the load bearing capacity of the ground> design a suitable foundation accordingly.

That's the simple answer. Your problem will be establishing the bearing capacity at the correct depth, which may (will) be deep due to the trees? You could do a trial excavation but if that ends up being 2m+ deep you might end up digging up the whole garden.

You've said the ground was sandy down to about 800mm. You could build on sandy soil but a foundation might be 800, 900, 1200 wide? A lot of excavation for 30M² build at 800 or 900 depth.

I would consider a mat foundation. (basically a reinforced slab) It will do what you want and will probably be less work in the long run.

The mat/raft/slab foundation is likely what i will go with. How do i determine the thickness? All i can seem to find is advice like "extensions require 200mm, sheds require 100mm", nothing specific on load requirements.

My build isnt an extension, but it isnt a shed either, what thickness should i go for?

 

[noseall]

The mat/raft/slab foundation is likely what i will go with.

How do you propose keeping it watertight?

 

[perky416]

Dpm