Building for flood resistant measures in Flood Zone 2.

30 Dec 2010
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United Kingdom
I have been looking over the requirements set out in our planning consent for a new extension that has to be considered due to flood risk. We are in a flood zone 2.
I am looking particularly at;
- The floor level within the proposed development shall be set no lower than existing levels and flood proofing of the proposed development shall be incorporated where appropriate.
- No soakaways shall be constructed such that they penetrate the water table, and they shall not in any event exceed 3 metres in depth below existing ground level.

1) I have attached an image (groundfloor.jpg) of the cross section for the proposed ground floor. Would this be acceptable for an extension in flood zone 2? What determines whether the building contractor uses Damp proof membrane of polythene at least 1200 gauge or something like mastic ashalt?

2) I have attached another image (Drainage.jpg) of the drainage layout for the property, my house coloured in grey. This image was obtained from old local microfilm library records. There is a surface water drain in my neighbours front drive which we get to via the foul sewer. It shows no manholes are located on my land. What determines whether we continue to use the surface drain running into their property or maybe put in a new soakaway(s) within my property?

3) I have been reading "Cavity insulation should preferably incorporate rigid closed cell materials as these retain integrity andhave low moisture take-up. Other common types, such as mineral fibre batts, are not generally recommended as they can remain wet several months after exposure to flood water which slows down the wall drying process".
So what would be a good cavity wall insulation to use? Something like a closed cell polyurethane cavity wall insulation foam?



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1) Probably, though you'd be better having the concrete above the insulation (not sure how well tiling onto Kingspan would work- anywhere from rubbish to impossible would be my guess). Has that drawing come from one of the Approved Documents by any chance? Good luck with stone skirting boards, PVC might be easier to fit.
2) Don't rely too much on your microfilm- they might be the proposed layout rather than as fitted. Interested to see your north neighbour has a soakaway in your garden. If you aren't altering the drainage significantly then status quo. If you're adding or changing a load of stuff then Building Control and water board would far rather you put a soakaway in for your surface water drainage- if soil conditions don't permit that then it'll go into the foul drainage system. Don't understand how your surface water drainage links to neighbours' soakaway via foul drains.
3) If this is for new build then Kingspan/Celotex/whatever rigid closed cell insulation boards in the cavity. If for existing then no idea, be wary of retrofit installs, they can cause problems. Might be better (if you need it) to look at external wall insulation in that case- again that uses rigid closed cell boards
Thanks for the reply.
That drawing came from a government released document;

Its interesting you mention the ordering of the insulation, I'm not quite sure what is best for flood measures i.e.
- Insulation below concrete slab (Insulation, DPM, Concrete Slab)
- Insulation above concrete slab (DPM, Concrete Slab, Insulation)
What would be best in this case?

We are looking at double storey front extension (to square off the property) taking off the existing roof and adding a new loft room. Also demolishing the side garage conversion and creating a single storey to go back the length of house and then wrap around 5 metres at the back of the house. So I imagine there will be an increased amount of surface water and the drainage will be changed considerably.
I was also looking at the possibility of putting in wooden oak flooring instead of tiles / stone for most of the new section to the ground floor. (This now sounds like a bad idea!) The new hallway will have stone slabs of some sort.
I've had a look around the property, and have encountered no manholes as yet. I have yet to lift some of the decking though. I'm thinking we will have to look at installing one if we don't have any manholes, for rodding purposes.
One of the planning constraints was to keep the new sections of the house to match existing i.e. London Heather Brick. So I'm assuming this rules out any possible external flood resistant renderings / insulation. I would like to use engineering bricks (as I've read they are much more water resistant than traditional bricks) as far up as possible, is there a limit to how high this could go?
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Concrete won't be damaged by floodwater. Insulation boards might be- not sure how well they perform under total immersion. This whole insulation under concrete thing seems odd and unnatural to me but it seems to work and it does give you a solid base to tile to- with insulation as your 'floor' you can put laminate or chipboard straight down but both those floor coverings will fail under flooding. That document goes into some detail about insulation above/below the slab- they can't make their minds up either. What they do say is that tiled floor (their best covering) shouldn't be laid on thin screed above insulation since the thin screed will almost certainly fail (due to uplift pressure). But insulation above slab does allow for easier replacement of insulation if necessary.

My temptation would be to go with insulation above concrete and use a sacrificial floor covering (cheap laminate). But ultimately it is up to you, either technique is acceptable

Putting a manhole in will certainly put less strain on your relationship with your neighbours- all your new bits of tube can then go into your manhole instead of digging up their driveway. Soakaway is still up for discussion though.

Engineering bricks tend to look different to normal face bricks so may not be acceptable to your planners- may want to talk to them. Structurally as high as you like, cost will be the issue. Also be aware of the notes in that document about water ingress from under the floor, into the cavity through blockwork and thus into the rooms- you might want to use engineering bricks for the first metre of the inner course as well.

But this is all specialist stuff and I'm no expert- hopefully one will be along sometime soon.

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