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Waterproofing Flat Concrete Roofs

"I Love Mum"

Now that the rain is starting and, of course, all our roofs are leaking in true Balinese style. The Ubud branch of Drips Anonymous (the support group for leaking roof sufferers) has started having meetings again as leaking roofs make peoples lives unmanageable.

But did you know that roofs here are supposed to leak? it is a cultural thing. The spirit of leaking roofs has to be appeased. When it rains the people must remember to honour the spirits or they might become mischievous and everyone will get really grumpy so, to maintain harmony and balance in the face of precipitous frustration an upacara is held during which buckets and bowls are blessed and then placed around the house in strategic positions while chanting spiritual words like “it's pissing in over here, quick get me a bowl.” No one is allowed to think that they are above their station in life and so these days building design demands that everyone has to have a leaking roof.

There are certain aspects of standard building practice in Bali that simply don't work I'm afraid. One such thing is the way in which flat concrete roofs are finished. Most of us have never seen the top of our flat concrete roofs, they are sort of up there, out of sight and out of mind aren't they - until of course one day when it starts to rain.

Concrete roofs tend to have a low parapet around the edge to stop water flowing off the sides and they have drains to carry the water away. If the drains block we end up with a swimming pool on our roof.

All too often the drains on flat concrete roofs are too small. They also tend to be rather stupidly built into the structure of a building often being installed down the centre of structural columns. This creates two problems, firstly it reduces the strength of the column and secondly you can't get to the pipe should you need to unblock it. Modern design fashion demands an uncluttered look and architects don't want downpipes hanging on the outside of their pretty buildings to carry the water away. Add to this that natural tendency for trees to drop leaves which block drains and we start to understand why we have problems.

Perhaps this latter point explains why it is that most developers have a finely tuned scorched earth policy, they hate trees and will chop them down if they can possibly find an excuse, come to think of it they chop down trees even if they don't have an excuse. A beautiful row of tall palm trees were cut down just up my street a couple of weeks ago. They were on the roadside and outside the boundary of the property being developed, it made no difference, down they came. Sorry but my soapbox needed a bit of an airing, now where was I?

To build a concrete roof we start with strong reinforced columns to support it. A wooden formwork is then built as a mould in which the flat roof slab will be cast. The roof will need a good thickness of concrete and reinforcing steel bars through it to give it strength. The thickness of the concrete and the thickness and placement of the steel bars are dependent on the span (the distance between the supporting columns) we have to bridge. The longer the roof span, the thicker the roof slab and any supporting beams and columns will have to be, the heavier the house, the stronger the foundations have to be and so it goes on. Most concrete roofs are kept down to a maximum span of around 5 metres to keep things reasonable.

When everything is ready the concrete is poured into the formwork and left to cure before the formwork is removed. It is very important that the concrete slab is poured in one piece and it is also very important that the concrete mixture is correct with the correct ratio of sand, cement and small stones (usually 3 parts stones:2 parts sand:1 part cement). The amount of water and the degree of mixing when poured are also important to ensure that the chemical reaction of the curing process works correctly.

Now we come to the part that doesn't make sense. In western countries the upper surface of the concrete slab is finished off while the concrete is still wet, a flat concrete roof is shaped so that it will drain to the drainage points. Here in Bali this is not standard practice. Here a concrete roof may be very roughly shaped. After the concrete is set another thin layer of concrete (this is known as a screed) is put down and carefully troweled to shape so that rainwater will drain properly. This method has one major drawback. In the hot sun the thin concrete screed gets very hot while the dense concrete slab underneath stays cooler. The hot concrete expands more than the concrete underneath which makes the concrete screed crack away from the concrete underneath. If you climb onto a concrete roof in Bali you will usually find the surface is cracked over most of its area. Once the screed is cracked water can enter and travel under the thin concrete layer. This is not a problem as long as the concrete slab underneath is in good condition. Problems start if the concrete slab cracks (which may happen as a building settles), if there are joints between slabs in the concrete underneath or when some tradesman with brains in his boots starts drilling holes through it for some reason.

We are now facing a difficult waterproofing situation. Innocent Joe Bloggs doesn't like water dripping out of his light fittings and into his beautiful flambed crepe suzette so he calls his tukang (that nice young man who spends much of his time on the beach putting pictures of skulls or words like “I love mum” on people's skin) and asks him to fix the leaking roof.

As we can easily imagine tattooing is a slightly different skill set to waterproofing concrete roofs and so he is highly likely to fail. Not to worry, the pembantu knows a man who sells mobile phones - he is bound to know what to do. The third attempt could well be a wood carver from Tegallalang who takes a day off carving giraffes for the African export market.

By now weeks have passed, the ceiling has fallen down and the need for a hairdresser is long gone. Hair restorer that works is, of course, as easily found as George W. Bush's brain cell so what steps can we take to preserve our sanity and keep our hair.

If you are building check the drawings and take note of how the building will cope with heavy rain. Large roofs will need plenty of drainage.

Make sure there is a backup so that if one drain blocks the water can still get away.

Make sure that the roof is designed and constructed properly. Concrete roofs should always be designed by a structural engineer in spite of what your architect or builder might tell you. Concrete roofs are very heavy and kill people in earthquakes.

Do NOT install drains in the columns or other structural elements of the building, it weakens the structure.

Try to persuade your builder to shape the upper surface of the roof slab in its final profile.

Make sure the roof drains are a large diameter (100mmm is good) and without too many bends so that rods can be used to unblock them.

If a flat concrete roof is leaking get it checked out. If there are cracks in the concrete surface it is probably that these are cracks in a screed. The first thing to do is get the screed removed so that the point of leakage can be found.

Carefully select who will carry out waterproofing work. Waterproofing is not easy and leaks should be repaired by someone who knows what he is doing.

You may then choose to apply a waterproofing membrane to the roof. Depending on the roof and the condition of the concrete the membrane may be a thick coat of a thick polyester paint or a thicker bituminous layer. Bitumen or tar has been used for centuries for sealing roofs and wooden boats. It is still considered one of the best materials for this purpose.

If the roof needs to be shaped to get it to drain properly you may use thick asphalt as a waterproofing membrane or you may need to replace the screed. If you do replace the screed it is a good idea to keep it cool until it cures properly and then protect it from direct sunlight. You may paint it white to reflect heat or a common protection method is to put a layer of stones on it to shade it from direct sunlight.

A common problem with concrete roofs is that the small parapet around the edge is of lightweight concrete and usually has been added onto the top of the roof slab. Often the parapet cracks away from the solid concrete underneath which, when it rains, allows water to enter the structure. This can then work its way down through the building and into your crepe suzette. Particular care must be taken to repair the parapet.

Water coming through light fittings can be particularly detrimental to health and may have a tendency to prevent further life experiences. If water comes through your light fittings it is advisable to switch off the power and seek some help as soon as possible.

Alternatively you can always do what the locals do, put out a bucket, put your feet up and enjoy the rain.

Copyright © Phil Wilson 2009
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