Planning an extension

williamglossop · 5 Mar 2015

Hi all

I'm planning on building a small extension to the rear of my house. Its an 1900's stone built end terrace.

The issue is that the ground at the back of the house is level with the inside finished floor level of the kitchen. This floor level then runs through the entire ground floor of the house.

Basically, the finished floor level of the extension will need to match the existing floor level of the kitchen, which will mean it will be at the same height (ever so slightly above) as the outside ground level.

The issue then being the DPC and DPM. Whats the simplest way around this? I have looked at Approved Document C but it always shows the DPC being 150 mm above the outside ground level, and the FFL being at the same height.

Can any of you more knowledgeable members point me in the right direction.

Many thanks

Will

williamglossop · 5 Mar 2015

I was wondering if this would be OK, and if it would get passed the building inspector

The idea is to run the DPM through the inner leaf, and into the cavity. I would then lap the DPM up the inside of the outer cavity wall and then use concrete infill above and below the DPM inside the cavity up to the height of the finished floor level.

StevieV1 · 5 Mar 2015

Hi there

Ive just built an extension and have the same issue.

You will need a minimum of 150mm from dpc to ground level and there is no getting around that - and nor should you want to as your wall will end up getting damp otherwise.

I will be diging the ground out to create a new patio area 150mm below dpc going out from the extension wall and then creating a small retaining wall with a step up to wherever the ground level then is.

Suggest this is the way to go.

StevieV1 · 5 Mar 2015

That picture is the 'oversite' i.e. build up of the floor inside the new extension and doesnt really address what I thought you were talking about.

If the ground where the floor of the new extension is going to be is at the same level as your existing floor then it will need to be dug out to the depth of the oversite. So measure your type 1 (hardcore), sand blinding, slab, insulation, screed and whatever flooring your are having and dig down that much from where finished floor level will be.

williamglossop · 5 Mar 2015

hi steve,

thanks for the reply

I would do that - but sadly it wouldn't be possible. There is a drive way running up one side of the proposed extension. I cant dig that out as there would be a step in the top third of the drive.

In my idea, the DPC on the external leaf would be 150 mm above the outside ground level. The issue would be that the DPM would run up the middle of the cavity rather than on the inside of the wall. I think it would work, just dont know if the building inspector would agree.

williamglossop · 5 Mar 2015

Yeah, I don't mind digging out the oversite to the required depth - its only a small build.

Basically, I need the finished floor level of the extension to match that of the existing kitchen. The issue is that the floor level in the kitchen is at (or ever so slightly above) the ground level outside the house.

Ordinarily (as i'm sure you're aware) the finished floor is above the outside ground level, and there'd be no issue. But I cant raise the level of the floor.

StevieV1 · 5 Mar 2015

Probably best to speak to your BCO and see what they suggest.

tony1851 · 5 Mar 2015

williamglossop said:
I was wondering if this would be OK, and if it would get passed the building inspector

That would not be acceptable.

Assuming you have a cavity wall, then rain getting in through the outer skin - which it will do - will run down and be directed to the inner skin by your upturned dpm.

If you have a normal cavity wall, it will not matter that your outer dpc is higher than the inner. Those diagrams in Approved Doc. C are really just conceptual rather than practical construction details.

williamglossop · 5 Mar 2015

Thanks Tony,

I get what you're saying. Is there a way around the problem i'm describing?, i.e. where I cant have a step up into the extension.

williamglossop · 5 Mar 2015

Could I use a cavity tray above to get around the problem? As shown here

[img=http://s24.postimg.org/4njjal6ld/foundation_for_extension_alternative.jpg]

Or is there a simpler way?

Thanks chaps, I appreciate your time

Will

williamglossop · 5 Mar 2015

Anyone else got any ideas? Its doing my simple mind in trying to work out a solution.

I cant be the first person to have to overcome this kind of situation, lol.

freddiemercurystwin · 5 Mar 2015

I dunno what the problem is, it's a very typical situation, you just lap the dpm from under the slab up the outer face of the inner skin and tuck it in under the stepped dpc, I did a 2 minute doodle to show you, this drawing would put tony to shame.

Untitled

freddiemercurystwin
5 Mar 2015
1

tony1851 · 5 Mar 2015

tony1851 · 5 Mar 2015

....why even use a cavity tray?.....

Untitled

tony1851
5 Mar 2015
1

or have I missed something?

williamglossop · 5 Mar 2015

you're a bloody legend Freddy - I owe u a few beers.

I knew there would be a simple solution, but just couldn't see it.

Thanks man

And Tony, you too buddy. Cant thank you enough - seriously, this was doing my head in. Tony in your picture, when you refer to full fill what do u refer to filling the cavity with and to what height? Whats that horizontal line that bridges the cavity above the dpc?

Thanks for the sketches

Will

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Planning an extension

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