Extension/lean-to foundations

[Rubyblueshoes]

We're planning to add a small extension at the back of our house. The size of the extension will be 1.4m long by 2.8m wide and will be a light-weight, wood-framed structure with one window and a single external door. It is effectivly going to be a lean-to (or conservatory) as it won't be tied into the existing structure (it's for a separated utility room).

There is a patio already behind the house which consists of a concrete slab about 150mm thick on a base of hardcore which is about 400-500mm deep.

So my question is; do you think this existing slab is sufficient to act as a foundation for the new extension, or do you think we will need to modify it?

Thanks in advance.

 

[noseall]

Depends how long you want the building to last.

What i can't work out is are you building a glorified shed or a heated structure?

Any space that you intend spending time in, complete with electrics and electrical equipment and clothes drying and painted walls etc, will need to be able to withstand cold wet winters.

It's one thing using these spaces in the summer months but as soon as winter is here you won't want to be in there.

 

[Rubyblueshoes]

Ok, I'm playing it down I guess hoping to get the answer I want

It was going to be a full extension but to avoid building control it'll basically be like a conservatory separated from the main house by a door.

The plan is for it to be a heated, fully insulated, wooden structure, rendered externally, tiled and painted internally. It'll have a large double glazed window and a door, electrics and plumbing primarily for the washer and dryer.

Hopefully that's a more accurate description.

 

[noseall]

The plan is for it to be a heated, fully insulated, wooden structure, rendered externally, tiled and painted internally. It'll have a large double glazed window and a door, electrics and plumbing primarily for the washer and dryer.

Basically an extension then.

If there was one thing you would want to get right, regardless of BC or any other excuse to save money, the foundations should always be built to last. It would be deemed false economy to build the uppermost part of the structure to a high spec only to forsake the found's.

 

[Rubyblueshoes]

Agreed, fair enough. So what would you recommend?

 

[noseall]

Agreed, fair enough. So what would you recommend?

Standard strip foundations.

 

[Rubyblueshoes]

Ok thanks, just two final questions if you don't mind.

1. You don't think there would be any way to reuse the existing slab then? Someone else suggested concrete lintels accords the slab tied between the house and post foundation.

2. Any thoughts on size/depth for strips?

Thanks for the help.

 

[^woody^]

That slab should be more than enough to support the building as proposed

You just need to take the usual precautions for damp prevention etc. It may be that you need a couple of courses of engineering brick to lift the frame up off the slab

 

[Rubyblueshoes]

Thanks ^woody^ that's certainly what I was hoping.

Is that a controversial opinion? noseall seemed to be saying that it wouldn't be enough.

 

[^woody^]

I don't know what nose was thinking, but my thinking is that you've got effectively 600mm or so of foundation, which is now [presumably] stable after being there for a while, and this as good as many houses have.

 

[Rubyblueshoes]

Just noticed I missed one detail in this thread (I had included it in an earlier thread elsewhere) but I assume it could make a difference... The existing slab I mentioned is mostly above ground (it also forms the base of a patio), you may have guessed this anyway.

As you say the slab is solid and stable, no cracks or damage and has been down for many years. No idea how many but we removed two layers of tiles, so long enough for the patio to have been refinished at some point.

Cheers for the advice.

 

[theoldun]

If there was one thing you would want to get right, regardless of BC or any other excuse to save money, the foundations should always be built to last. It would be deemed false economy to build the uppermost part of the structure to a high spec only to forsake the found's.

The opinion that nose has given you is factual, honest and based on fact.

I don't know what nose was thinking, but my thinking is that you've got effectively 600mm or so of foundation, which is now [presumably] stable after being there for a while, and this as good as many houses have.

Sorry, whilst the existing base could be with the emphasis placed on (could be) and (presumably) strong enough to support the proposed light weight structure, with out knowing the ground conditions, whether the existing base is the make up you have specified, then in this instance we consider this opinion to be contentious.
Your choice.
Regards oldun

 

[freddiemercurystwin]

What on earth is the point in going to all the expense and hassle of building a good quality 'conservatory' and avoiding Building Control? Madness I tell thee!

 

[Rubyblueshoes]

freddymercurystwin, I want to avoid BC for three reasons: 1. It costs over £500 to get signed off here which is a large proportion of a small budget. 2. BC has an appalling reputation around here. 3. The chap from BC that came over told us we need 1000mm deep footings and concrete lintels to bridge the existing slab which is by some accounts over spec'd and will cost more.

theoldun, that's a fair assessment. We have a very experienced builder working on this so no rash decisions will be made but I did want to get a range of opinions. The guy from BC put a spanner in the works, so we're re-assessing our options.

Certainly what noseall said is right, without question, however I guess I really should have phrased the question differently at the start to say removing the existing slab is not really an option, so if we can't use it as is, how can we modify it?

 

[noseall]

My main disagreement is not only the quality of the base but also the footprint. If the intended structure is smaller then the base then the o.p. will suffer the same problems that many concrete garage structures encounter, i.e. Water seepage under the baseplate.
A slab foundation is avery poor way to support a structure unless it is a sub soil raft.