For those who are doing extension for concrete roof, get waterproofing that is added to concrete mix before pouring. Those that you buy off the shelf, there are no warranties because they only guarantee the product, but they cannot monitor how you do it.

It requires proper mix ratio, especially the water content. Also, to minimize the risk further, ensure the slab has sufficient gradient to fall towards the RC gutter where the rain water down pipe is. If you are planning to utilize that area and requires it to be flat instead of with gradient, then add waterproofing to the cement screed before tiling works (assuming there are tiles).

Cementaid is one option you can opt for.
http://www.cementaid.com/product.htm#waterproof

If you want double protection, before screeding, apply a layer of Proofkote before screed on the dry concrete slab. Then ensure the screed mix is added with waterproofing. Then tile the whole area.

What you said is the most accurate reason why concrete slabs leak. Concrete in itself is porous. It will absorb water/moisture continuously. That is the nature of concrete, irregardless of concrete grade. If rain is continuous, the concrete will absorb till it is saturated and can absorb no more. Eventually, all the moisture/water it absorbs will have to follow law of gravity and it will find a way out of the soffit of the slab.

From my preferred option till the least desired option.
1. Waterproofing added to concrete ensures none or minimal leakage issue.
2. If the slab are already done, then waterproofing should be added to the cement screed. Then the screed should grade to fall towards the RC gutter.
3. Or you can apply waterproofing on the slab, followed by screed.
4. Never expose any waterproofing material to weather or human traffic. It won't last. That's why they should be added to concrete or add to screed or protected with a screed layer. Best would be to have tiles after that.

Maybe later I will post a picture of what a normal grade 25 concrete with and without waterproofing will do.

This post has been edited by matthewctj: May 11 2012, 05:46 PM

Both skill and technique is essential. You can't have one without the other, especially when it comes to avoiding leakage.



The above picture shows 2 grade 25 concrete sections after several drops of water was dripped onto it. Within minutes, the one on the left absorbs. As you can see on the right, the droplets remains. That's because that block of concrete was added with waterproofing compound.

I would not say that concrete overtime will leak. It is porous by nature. The picture only shows droplets of water. Imagine a slab which is expose to rain water and without water proofing. The concrete will absorb water bit by bit continuously. Where does it go? It goes into pores of the concrete. Now before it even has time to dry, it rains again next day. Eventually, the concrete will be so full of water, it has to go somewhere. That's when you will start seeing moisture and drips of leaking water from wherever the concrete is the weakest or have signs of shrinkage cracks.

But if your flat roof is well graded for water to fall towards the gutter or rain water down pipe, the risk for leakage is minimal. That's my experience with flat roof and also my opinion

In that picture, it was Cementaid 3CC.
http://www.cementaid.com/product.htm#waterproof