New extension

[Willa]

I am new to this so excuses if my format is not up to speed.
I am building a new extension and will be pestering you guys in the coming weeks. My first query is....ground floor construction. Hardcore, blinding, dpm, concrete, insulation, screed as per regs. On some unrelated building plans I have noticed that this can be rearranged, ie. insulation under the conc. then screed. Can someone tell me what is the best arrangement please. Thanks in advance.

 

[noseall]

100-150mm compacted sulphate free hard core.

30-50mm sand blinding.

d.p.m turned up into d.p.c.

insulation layer.

second layer of d.p.c. (not turned int d.p.c.)

100mm min concrete.

50-75mm min screed.

 

[^woody^]

I can't see any reason to have a second DPM. The one above the blinding is enough.

Don't forget you normally need to have some insulation around the slab to prevent a cold bridge from the external walls.

The problem with insulation under the screed, is that it can cause the screed to crack if it is not deep enough, or if the insulation is not dead flat on the concrete and so could flex when walked on. I would add chicken wire to any screed directly on insulation

 

[noseall]

I can't see any reason to have a second DPM. The one above the blinding is enough.

building control in staffs are adamant that the second layer is fitted. though, they say, it does not need to be 1200gauge.

there are at least two architects that i receive drawings from too that always insist the polystyrene is protected from the concrete.

it may be possible to concrete directly upon some denser ore more tolerant insulation slabs though.

 

[Fluffster]

I have just done my extension floor and was advised of to use 2 membranes the second one being a vapor control shield as Noseal said 1000 guage.
Pete

 

[Willa]

Thanks for the advice. Please understand that I will be getting profesionals in to do the work and that I would like to have some knowledge of what is required to give the impression I can follow the project.

 

[^woody^]

building control in staffs are adamant that the second layer is fitted. though, they say, it does not need to be 1200gauge.

Well they can't.

Approved Document C gives some examples of acceptable floor construction, and none of them have 2 DPMs

Therefore your BCO cannot refuse to accept these as they are already accepted methods.

And there is no need for a vapour check in a floor

 

[noseall]

if it's any consolation ^woody^ i agree with you.

you try telling 'tricky trevor' and his merry men the rules, they'll make you go down another two feet

 

[Thermo]

have to say i cant see the point of it

 

[masona]

have to say i cant see the point of it

I suppose as noseall pointed it out the protecting of the polystyrene when concrete is pour plus also I would think it would make it easier to push the concrete around on top of the dpm.