Waterproof membrane under concrete slab

[Crystal]

Hi Forumites,

I have a granny flat I'd like to get council approved. My concern is that there may not be a waterproof membrane under the concrete slab.
Is there any way to add a waterproof membrane? Can I keep the granny flat and jackhammer the concrete and pour new concrete over a membrane?
Or do I have to knock down and rebuild?

Thanks,
Crystal

 

[vbplease]

You really need to prove this for council approval? Is a certifier asking for photos during construction?

If the issue is a black plastic mat - although it's BCA required, IMO shouldn't be relied upon as a water proof membrane. It's main purpose is to act smooth layer for concrete to be poured on and adequately cure - so the ground doesn't absorb the moisture too quickly causing shrinkage cracks. The mat usually ends up with countless tears/holes from traffic from the steel fixeres/concreters..

If it's for your own peace of mind (not council approval) you'd need to add a water proof membrane to the top surface.

 

[Scott No Mates]

sorry vb, it is a vapour proof membrane to minimise the amount of contact between the slab & the soil. A few holes will not greatly affect the effectiveness of the plastic but there may be the odd damp spot.

If you need proof, dig around the edge/under the slab in a couple of places & try to find some plastic or other membrane.

 

[Crystal]

So what do I need to do to obtain council approval if the membrane is not there?
Thanks,
Crystal

 

[DaveM]

So what do I need to do to obtain council approval if the membrane is not there?

A suitcase full of $100 notes may work

 

[Crystal]

I'm happy to throw plenty of money at this problem to fix it because the alternative (knockdown and rebuild) is very expensive but I don't think that is what you meant!!

 

[BMan]

dig all around under the granny flat edge some plastic under the slab and cover it up then theres proof its infact there ....

 

[maza]

Hi Forumites,

I have a granny flat I'd like to get council approved. My concern is that there may not be a waterproof membrane under the concrete slab.
Is there any way to add a waterproof membrane? Can I keep the granny flat and jackhammer the concrete and pour new concrete over a membrane?
Or do I have to knock down and rebuild?

Thanks,
Crystal

why do you need a waterproof membrane under the slab?

Did the engineer specify you need it?

 

[Crystal]

I think it is a requirement for a habitable dwelling and my Surveyor asked me whether there was a "membrane under the concrete slab to stop damp rising."

 

[maza]

Not to sure what your surveyor is smoking but find a new one.

Its none of his business wheather there is damp course etc or not, his job is only to survey the area and peg out the area etc.

Speak to your engineer.

It all depends what footing/slab you have.Sometimes you have damp course under the slab, sometimes on top.

 

[ozsupra]

What council is the granny flat in?

 

[Crystal]

Gosford council.
Thanks,
Crystal

 

[meant2be]

NCC-2011 (National Construction Code) NSW 3.2.2.6 Damp-proofing membrane

A damp-proofing membrane must be installed under slab-on-ground construction for all Class 1 buildings and for Class 10 buildings where the slab is continuous with the slab of a Class 1 building as follows—
(a) Materials
A damp-proofing membrane must be—
(i) 0.2 mm nominal thickness polyethylene film; and
(ii) high impact resistant, determined in accordance with criteria specified in clause
5.3.3.2(c) of AS 2870; and
(iii) be branded continuously "AS 2870 Concrete underlay, 0.2 mm High impact
resistance".
(b) Installation
A damp-proofing membrane must be installed as follows—
(i) lap not less than 200 mm at all joints; and
(ii) tape or seal with a close fitting sleeve around all service penetrations; and
(iii) fully seal where punctured (unless for service penetrations) with additional
polyethylene film and tape.
(c) The damp-proofing membrane must be placed beneath the slab so that the bottom
surface of the slab is entirely underlaid and extends under edge beams to finish at
ground level in accordance with Figure 3.2.2.3 (also see Figure 3.3.4.9 for single skin masonry details).
Note:
A range of polyethylene films can be used, including black film and orange film, provided they satisfy the requirements for high impact resistance in accordance with the criteria specified in clause 5.3.3.2(c) of AS 2870.

The "Damp-proofing membrane" is there to stop rising damp.

Your PCA had to verify the form-work, re-enforcement etc before the concrete pour. If it so, don't worry about it.

Also, you said "I have a ..." not sure about this "I have" bit

 

[Crystal]

Thanks for your information.
I believe the building was originally approved as a double garage with a bathroom and rumpus room.
Thanks,
Crystal

 

[meant2be]

... building was originally approved as a double garage with a bathroom and rumpus room.
Thanks,
Crystal

To answer your question, the Damp-proofing membrane have to cover all of the underside of the slab, up to your finish soil level. You can't retro-fit and comply with the code.

On the bright side, no one will make an issue of this, just because it not there. Its the huge cost to fix a "may be problem"

There is one instance where you have to deal with this;
You can have a look around the building to see if there is evident of rising damp. The bottom of the building have green stuff growing on it. If so, they may ask you to put in a drainage system around the building. ie, pvc pipe in geo-sock, sitting on sand around 400-600mm deep.

Cheer up, don't be too concern

 

[jsoe]

WE are in the same situation now. PCA (private certifier) is asking us to be compliant with BCA (building code of Australia). Existing concrete slab has to be waterproofed (lay a waterproof membrane on top and pour 75mm concrete on top).
Otherwise, if moisture seeps through and the concrete cracks/weakens, you're in trouble, especially when there are tenants involved.
Such, a bummer in terms of costs now, as I heard it could be pretty expensive. We're still calling around concrete guys. But I guess, it has to be done.

 

[nev]

Hi Forumites,

I have a granny flat I'd like to get council approved. My concern is that there may not be a waterproof membrane under the concrete slab.
Is there any way to add a waterproof membrane? Can I keep the granny flat and jackhammer the concrete and pour new concrete over a membrane?
Or do I have to knock down and rebuild?

Thanks,
Crystal

I'm confused
Why do you need approval if the building is already there??
I'm missing something obviously

 

[jsoe]

Nev, I asked the same question before and was told that the existing structure is just a garage or work shed and not regarded as a habitable place that's compliant with BCA. So if you want to turn that garage/shed into a dwelling (habitable place), then you have all these rules to follow. Blehhhh...
Another thing was the termite barrier we were told to put around the concrete slab. Luckily, ours was about 150-200mm thick so it wasn't necessary. Otherwise, it would've involved digging a trench around the slab and putting pipes, etc and would've set us back about $3,000.