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Concrete Roof Gutters and Aprons

In Indonesia there are endless instances where people are unaware of good practice, prevailing standards and even the law.

Bad practice all too often becomes adopted as standard practice. Perhaps you might have noticed that there is an unwritten local rule that dictates that traffic entering a roundabout has right of way over traffic trying to get out of it. Common sense says that this is a recipe for rotary chaos and yet still the practice prevails.

In the building industry we find many similar examples quite a number of which can compromise both building design and construction. People blindly follow generally accepted norms regardless of whether they are a) common sense or b) sound engineering or c) legally required.

Lying in the rubble of your former palace with ten tons of roof on your head is probably an appropriate time to consider the aesthetic appeal and clean lines that architects are able to achieve by designing support columns as pathetic skinny things that can be hidden within the walls, a very dodgy practice made even more dangerous when roof drainage pipes are installed down the centres of the columns.

The ability to properly support weight is the single most important aspect of the design of any building. Structural engineering design is a complex field and requires specialist training. Unfortunately sound engineering design may be in conflict with other factors such as architectural creativity, poverty avoidance or established bad practice.

You may recall the famous case of the Sydney opera house. The original design was beautiful but, due to engineering limitations, proved impossible to build. Changes had to be made, the architect had a tantrum, took his bat and ball and left in high dudgeon.

You may also recall the case of the collapse of the 5 storey Sampooang Department Store in Korea, the deadliest building collapse since the Circus Maximus collapse in 140AD. A whole series of intentional and unintentional strayings from sound engineering practice that killed 502 people.

Back to a more mundane level many older houses in Bali have gutters built along the top of house walls, it is a common practice. These gutters are a common cause of problems such as dampness in internal walls. If you are buying or renting a house it is a good idea to note whether there are any concrete gutters and check to make sure that there is no water damage in the walls beneath. You may find stained patches or blistering on the walls or signs of water running down the wall.

You are most likely to see such gutters where a house is built right up to a property line or butting against another building. Typically they will be 30 cms wide and 30 cms deep and made from concrete. They are generally heavy (and more so when full of water), are often built on fairly weak wall structures and are usually overhanging the wall on one side.

Why do we have such gutters? Well the answer is fairly obvious where two sloping roofs come together creating a valley between them and water has to be got away. There are, however, other situations where the reason is perhaps not as obvious. The most common reason is because local people can be very sensitive about water running off roofs and where it lands. To make the most of land area buildings may be built right up to the boundary of a property. Allowing the water to run off your roof and onto your neighbour's land is considered very bad form so the roof may be cut back a few cms from the property line and a concrete gutter cast along the top of the wall to collect the water and carry it away.

In recent times this arrangement has evolved into much larger constructions. On modern buildings you will note a broad concrete apron is built around the tops of the outer walls under the edges of sloping roofs to collect water and carry it away. These are, in effect, concrete gutters although they may be as much as a metre wide.

Both gutters and roof aprons may suffer problems caused by either poor design or poor construction.

Concrete is heavy, very heavy, it is even heavier than Britney Spears' breathing. The earlier concrete gutters were heavy enough but the modern version with a metre width of reinforced concrete apron is putting a huge weight up there and requires specialist structural engineering design using reinforced columns and beams to support all that weight. This weight is transferred to the ground of course and so foundations have also got to be designed to take the substantial loads involved.

The major threat to concrete gutters and aprons is ground movement. All buildings have a distinct tendency to move particularly when first built and such movement results in cracks in walls which, in turn, results in cracks through the concrete gutters which allow water to leak through these cracks and into the body of the wall. Other than rising damp this is one of the most common reasons why houses have dampness and water damage in walls.

How can you avoid settlement problems when building from new?

When you are building any building you are progressively adding a huge amount of weight onto the ground below which will gradually subside and settle under the increased load. This is why you nearly always get settlement cracks in new buildings. This problem becomes far more serious if the land has been filled and the ground not fully compacted. It is a good idea to avoid building directly on newly filled land.

Sensible builders will wait for land to settle after it has been filled before they start building. 6 months is barely sufficient and better to be at least 2 years. You can avoid the time delay by building foundation piers (columns) in the original land and then filling to the height you want afterwards.

Even a well designed and constructed building is very likely to have some settlement and movement which will crack concrete gutters and roof aprons. The best way of avoiding problems is to not use concrete gutters at all. The old Dutch houses had it right, let the water run directly off the edge of the roof onto the ground and deal with it there. If you do have to build your building up to a boundary wall avoid using a hipped roof (a hipped roof is one with "half pyramids" at the ends) instead use a gabled roof and set the slope of the roof at right angles to the wall and not towards it. Yes I know all this talk of hips and gables makes about as much sense as a dyslexic crossword, perhaps this article will help:

To resist settlement it is important that concrete gutters and aprons are designed and constructed properly. Repairing cracks and sealing leaks is not easy and something we can perhaps discuss another time.

Copyright © Phil Wilson 2014
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