Concrete Flat Roofs

[cheeryble]

Hi there

I have a theory.

It's that pitched roofs are shaped as they are because of the materials which used to be available long ago. We used to be limited.

Nowadays we have reinforced concrete and we have heat insulation which can be put underneath on the ceiling. You build a roof.....and you get a roof terrace to hang out and enjoy the view from on high.

So the number one thing is.......how waterproof are they?

How can they be made better?

The number two thing is.......what is the cost compared to a tile roof or better still per sq metre?

Thanks!

[PattayaParent]

Flat roofs are waterproof if built properly, same as pitched roofs.

Flat roofs are more expensive due to the renforced concrete (or pre-cast conncrete) and the insulation / waterproofing system used.

[trogers]

Doing a term paper?

[doggie888888]

My own experience:

I have an existing concrete flat roof over the car porch. Even though there is a slight incline, I do find rainwater puddling in spots up there, which indicates that it's not as flat as it should be, which in turn may indicate not many builders here are able to do it properly. In my previous house, there was also an older concrete flat roof. It was only when stalactites were forming on the ceiling did I realise there was a problem on the roof - again rainwater was collecting due to it being not entirely flat and the incline was inadequate for rainwater to run off it completely. I am not a builder and can only relate my own observations. And I think I won't have flat roofs again, eventhough aesthetically it can look good in some situations.

[trogers]

My own experience:

I have an existing concrete flat roof over the car porch. Even though there is a slight incline, I do find rainwater puddling in spots up there, which indicates that it's not as flat as it should be, which in turn may indicate not many builders here are able to do it properly. In my previous house, there was also an older concrete flat roof. It was only when stalactites were forming on the ceiling did I realise there was a problem on the roof - again rainwater was collecting due to it being not entirely flat and the incline was inadequate for rainwater to run off it completely. I am not a builder and can only relate my own observations. And I think I won't have flat roofs again, eventhough aesthetically it can look good in some situations.

Zero tolerance in flatness? We are talking about concrete and not polished steel right? Slopes for draining a concrete flat roof (or a car park floor) is not built into the structure. It has to be from a separate cement-sand screed laid to slope to gutters.

[katabeachbum]

I have flat concrete roofed buildings in addition to ordinary bricks on steel structure buildings, and buildings with both roofs.

Flat roof, minimum 10cm 280 Ganzum (waterpoof) readymixed Cpac, can be used on concrete planks. Must be poored all at once, and of course vibrated. Rebars according to spams. Slope 1:100 polished shiny cement, 1:50 tiles or sandwash. Lots of 3" drains.

Cost on concrete plank(no formwork), 600-1200 baht/sqm, pluss draining

No leaks

No leaks in upper floor swimmingpools either

[dave111223]

As long as you have proper drainage so that you don't get pools of water on the roof then flat concrete roofs work fine.

Even if you get cracks/leaks in the roof you can just fill them with concrete then paint roof with "roof seal" (readily available at any hardware store).

But if you let your drainage pipes get blocked up water is going to seep though eventually, rusting the rebar, causing major cracks and falling chunks of concrete...etc

[ProThaiExpat]

Due to the triangular shape of my house, flat concrete portions were required, largest 4 meters by 10 meters. Bedroom leaked as developer didn't seal the concrete. Sent specialist out who had a product that seemed to be a rubberized compound, quite expensive. Developer went for a white paint like substance with an additive and ten years later no leaks. 4 inch pvc drains in each sloped segment takes care of water accumulation and I removed the strainers they had installed as they gathered debris and blocked up the drains. No strainer and the 4 inch pvc pipe is large enough to debris does not accumulate.

In Palm Springs, California flat roof construction is quite popular and a white cement type paint/compound is used to seal and reflect summer sun.

[trogers]

...ordinary bricks on steel structure buildings,

I surmise that bricks are laid on top for a steel beam. What method was used to prevent cracking of plaster?

[katabeachbum]

...ordinary bricks on steel structure buildings,

I surmise that bricks are laid on top for a steel beam. What method was used to prevent cracking of plaster?

sorry, lousy english, it should have said: ordinary bricks on steel structure roof.

thai style, all joins sealed with Sikaflex mat 0,3x10 metrs rolls, and then covered with cement/plastering decoration. CPAC has a similar waterpoofing, but I find Sika better

I notice most above recommend flat concrete roofs sealed with waterpoof surface treatment. I find waterproof 280 readymixed CPAC a better a solution, not depending on surface lifetime and maintance. Requires a solid structure and foundation tho. No cracks, no leaks. Roofs and pools

[Pacificperson]

Flat roof, minimum 10cm 280 Ganzum (waterpoof) readymixed Cpac, can be used on concrete planks.

I am not familiar with the term concrete planks. Can you explain?

[katabeachbum]

Flat roof, minimum 10cm 280 Ganzum (waterpoof) readymixed Cpac, can be used on concrete planks.

I am not familiar with the term concrete planks. Can you explain?

made and sold by most concrete manufactorers, 5x30x??? cm on top of beams or load carrying wall, used to replace formworks to poor concrete in. can be painted underneath as ceiling in garages etc

ordinary floors 5-7,5cm concrete is needed on top of planks, wet floors or slabs 10cm waterpoof concrete is needed on top of planks

I use concrete planks even at ground floor, as it creates an airgap 30-50cm under floor lowering floors heat pickup from ground

[bobbin]

Does anyone recommend the white cement paint/sealing compound mentioned by ProThaiExpat for use here in Thailand?

My condo is on the top floor and has a flat concrete roof. The reason why I am asking for input is that another resident did paint his roof white. Basic outdoor concrete paint, I believe.

I have asked him several times if the painting resulted in lower temps in his condo below but he is unable to offer an opinion. As in, he can't tell me if there is a difference.

I have gone up to the roof and felt the painted/unpainted concrete. There is a substantial difference. The unpainted concrete is hot to the touch while the painted surface is almost cool. I would think this should stop the radiant heat from the roof but someone who has done it can't tell the difference.

Electric bill should show a reduction but he is not a big user of a/c so can't show a big reduction.

If it is worth (3-4000Bt) it, I would like to do it before the hot season starts in earnest.

[trogers]

Does anyone recommend the white cement paint/sealing compound mentioned by ProThaiExpat for use here in Thailand?

I would not for two reasons: the white will turn grey due to the amount of air pollution, and your portion of the roof is but a segment of the total roof of the condo building and will still be heated up via conduction from the non-painted mass.

But the main question is - what class of condo did you buy into that has no heat insulation already built onto the roof slab?

A better solution for you is to attach foiled fibreglass insulation on to the underside of the roof slab, which together with the air void above the false ceiling will insulate your condo unit.

Edited February 27, 2011 by trogers

[bobbin]

I can't really see the white paint turning gray from pollution but possible I suppose. Regular rains would be the same as washing off the dirt.

My condo building is not a cereal box design with only one roof. It is a stepped design, so although I am on a high floor, as my condo is quite large my roof is mine alone. That is, if I were to paint it, there would be no adjacent unpainted concrete.

The class of condo regarding heat insulation? It's a flat roof! Where would the insulation go? I don't have a dropped ceiling. I did not want to give up the room height.

[trogers]

I can't really see the white paint turning gray from pollution but possible I suppose. Regular rains would be the same as washing off the dirt.

My condo building is not a cereal box design with only one roof. It is a stepped design, so although I am on a high floor, as my condo is quite large my roof is mine alone. That is, if I were to paint it, there would be no adjacent unpainted concrete.

The class of condo regarding heat insulation? It's a flat roof! Where would the insulation go? I don't have a dropped ceiling. I did not want to give up the room height.

Solar slab is the most common form of heat insulation for concrete flat roof in Thailand. If the roof is generally not accessible to the condo occupants, they are left with their joints filled with bitumen. In your case, there may be a screed laid over it. To check if there is already a solar slab on the roof, checkout the temperature of the underside of the roof slab compared to the surface exposed to the sun. The underside temp would be at least 15C cooler then the surface that is hot to touch.

http://www.cecrete.c...h/tsolar03.html

Edited February 28, 2011 by trogers

[trogers]

I can't really see the white paint turning gray from pollution but possible I suppose. Regular rains would be the same as washing off the dirt.

How I wish this is true but we know otherwise by looking at those dirt streaks on the paintwork of buildings, esp the shophouses fronting roads.

[cheeryble]

First thanks for all your answers....

Does anyone recommend the white cement paint/sealing compound mentioned by ProThaiExpat for use here in Thailand?

Back in the UK they us a sort of high grade roofing bitumastic roll. It is a backing layer with bitumastic tar on it. This tar is itself covered with a layer of fine shiny chips of who-knows-what......looks like flint.....like very coarse sand. That stuff is there not only to coat the tar, but more importantly to reflect heat.

My point being that unless the roof is leaking just finding coarser reflective chips like this and spreading it across the roof might do the job.

Talking about insulation.....

Don't they have plasterboard here backed with expanded polystyrene....or even better polyurethane foam?

Cheeryble

[bobbin]

Ok, I had to find out what screed and solar slabs are before replying...

My building has neither, i believe. Just a waterproof covering, which doesn't appear to be weathering the climate all that well. At least there are no leaks...so far.

Doing some additional Googling shows some good claims for these UV-resistant, heat-reflecting paints though. The ones in HomeWorks etc are warranted from 10-15 years depending on price. Seems about 4000+Bt. for my approx. 100 sq. meter roof.