Lean To Shed - Clay Soil Foundation advice

[Antony11]

Hello,

I am in the process of building a lean to shed at the side of my house which is a trapezoid shape... not the easiest space to work with but is a fair space that is never really going to be used for anything else... so a shed seems suiting. It measures roughly 6m long, 1.2m at the narrowest width and 2.3m at the widest width.

I want to attach the back and front shed walls (that I am going to make) to the house brick wall and use the house brick wall as an internal wall for the shed. I am planning to make a flat roof with a 5cm slope away from house wall into guttering on a 6 inch roof overhang and connect it to an existing house down pipe, then use lead flashing to seal it off against the wall. Possibly with a rubber roof?

As for the foundation the ground is absolutely full of clay.. green/grey/ orange in colour and can dig out thick slabs of it. I am guessing it has alot of movement depending on the season and moisture level of the ground.

I have already dug out the area 250mm deep.. and am thinking to put.. 150mm compacted hardcore, 25mm sand blinding, 1000/1200 gauge DPM, add formwork, add 12mm expansion strip between house and concrete slab (or possibly double it up to 24mm), then a 125mm or 150mm concrete slab reinforced with rebar mesh, on small concrete castles to elevate the mesh to the center of the slab.

So my main concern is if the shed is attached to house and also attached to the concrete foundation and there is movement in the ground that the concrete slab is sitting on, is it going to pull out the shed wall fixings and damage the wall?

Ultimately I am looking for some peace of mind for the best possible way to prevent movement in the slab so I don't have to worry for damage to the house in the future. Or does my current slab idea sound solid enough to resist or minimise movement enough to stop any wall damage with consideration given to the clay earth?

Also is it acceptable for airflow bricks to go into the shed from the house?

The heaviest singke item on it will be 210 litre water butt.

I have attached a picture of the area I am working in and a picture of the clay ground.

I did post another similar post recently about DPM but it was kind of a spur of the moment post but this is the main question giving me concern.

Thanks,

Antony

 

[frutbunn]

I would talk to building control first rather than on here, they may want calculations for the raft and debonding ties between the existing and extension.

 

[Antony11]

Thanks for your reply,

I understand why you have suggested that. But I imagine getting surveyors in is going to start racking up cost... was hoping someone on here has done something similar or is experienced in laying foundations in these kind of circumstances... but if not I will take that route as it will at least give peace of mind.

 

[frutbunn]

Thanks for your reply,

I understand why you have suggested that. But I imagine getting surveyors in is going to start racking up cost... was hoping someone on here has done something similar or is experienced in laying foundations in these kind of circumstances... but if not I will take that route as it will at least give peace of mind.

Building control have to approve both the excavation and foundation design, this is something you have to do. You don't really have a choice . there's not much point discussing it here as they may take a different view.

 

[cdbe]

Is this a lightweight timber lean-to shed on a lump of concrete or a full blown extension?

 

[JohnD]

Sir! Sir! I know Sir!

Lean To Shed - Clay Soil Foundation advice

 

[Antony11]

Is this a lightweight timber lean-to shed on a lump of concrete or a full blown extension?

Making it from treated timber on a concrete slab

 

[Antony11]

Building control have to approve both the excavation and foundation design, this is something you have to do. You don't really have a choice . there's not much point discussing it here as they may take a different view.

Why do I need building control for a shed? Its frame is basically a lean to pergola on a concrete slab

 

[Harry Bloomfield]

I'm no builder, but a few thoughts....

That sounds like a fairly substantial foundation, I would not expect much movement at all of the actual slab, but why not allow for some slight movement in your hut roof and the walls? The hut timber will twist and shrink as it dries out anyway.

 

[Antony11]

I'm no builder, but a few thoughts....

That sounds like a fairly substantial foundation, I would not expect much movement at all of the actual slab, but why not allow for some slight movement in your hut roof and the walls? The hut timber will twist and shrink as it dries out anyway.

Thanks for your reply, so when you say allow for some movement do you mean to keep it detatched from thd house and build a seperate wall? I thought of this but was concerned with the small gap between the house and shed getting damp

 

[cdbe]

This is what they do near me, not something I'd do but simple and effective:

 

[rougharsedbrickie]

For planning and building regs the above photo could be classed as a covered way, whereas a shed is an outbuilding and if attached to the property probably requires building regs ( so put doors either end and call it a covered way )

 

[Antony11]

This is what they do near me, not something I'd do but simple and effective:
View attachment 198682

Thanks for the suggestion. That is a fair idea and looks simple and effective like you say, but I want something that is going to be substantially secure and look decent to.

 

[frutbunn]

Why do I need building control for a shed? Its frame is basically a lean to pergola on a concrete slab

Its an extension regardless of its construction as far as B regs is concerned

For planning and building regs the above photo could be classed as a covered way, whereas a shed is an outbuilding and if attached to the property probably requires building regs ( so put doors either end and call it a covered way )

I would only class it as a covered way if it was permanently open ie no doors, if doors are fitted its an extension, unless its fitted with a clear or translucent roof. Your LA may take a different view but thats how I would deal with it

 

[Antony11]

For planning and building regs the above photo could be classed as a covered way, whereas a shed is an outbuilding and if attached to the property probably requires building regs ( so put doors either end and call it a covered way )

Thanks for your reply. I could do that but have a fence at the end so won't serve much purpose. So I am guessing if its not attached to the house I don't need building regs?