water proofing a concrete shed roof

by Steve
(United Kingdom)

HiJohn,


Our 1950s ex-council house has sheds in the back garden that have a reinforced-concrete flat & horizontal roof and it badly leaks.

What would be the best way for an amateur to reproof it? One problem is that whilst the roof overlaps the brick verticals all around, the concrete is impossible to nail or drill into.

Answer:

Hi Steve,

Thanks for your question. This is a fairly common situation.

I have seen a few of the type of roofs that you are talking about and you are right they are very level. If they had a bit of a greater fall they would be simpler to waterproof by painting on some concrete waterproofing type paint. However the roofs are so level that water does pond on them and finds the smallest crack or imperfection to leak through and make the whole of the slab (and inside of the shed damp).

To waterproof the shed I would go for an over-roofing system. This is fairly straightforward and I have made a sketch for you at the end of this reply to help you a bit more. (I will add this tonight)

Whatever roof covering you use you will need to fix into the concrete roof slab

To do this you will need masonry bits and a hammer drill. The idea is to drill into the slab and fix down a couple of 6x2 timbers to the roof surface. Fix these timber a bit back from the edge using 8mm diameter resin anchors. Use resin anchors rather than expanding anchors as with the expanding anchors there is a risk that you can spall off a section of the edge concrete when you are drilling and fixing close to the edge of the roof slab.

The timbers that you have fixed to the roof will form the basis for forming a new roof over the leaking concrete slab. To form the roof deck use 4x2 or 3x2 timbers depending on the span. Space these new timbers at 400mm centres and screw them down to the timbers that you fixed to the slab. A roof deck of 12mm exterior grade OSB (Oriented Strand Board) is then fixed to these new rafters.

The roof covering that you choose here is a bit down to personal choice

The simplest would be to use an EPDM Rubber roofing membrane. You will most likely be able to get this in a size that will cover the whole roof without any seams (that could potentially leak. If you get a bit more ambitious then you could convert this into a green shed roof which could have more benefits for your garden.

Other alternatives would be traditional shed felt (though this has a fairly short life span), metal or plastic corrugated sheeting. Onduline roofing is also a common choice for over-roofing systems.

Final things that you will need to consider

You will be changing the height of your shed and so you should check the local planning laws in relation to height of shed and how close you are to the boundary. Just because you are improving the condition of your shed does not mean that you can ignore these rules.

Let me know how you get on and of course any follow up questions that you have.

Best regards

John

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