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Ernst Conradie 09-03-2011 08:28 AM

Basements drainage in South Africa
Hi All

Im a South African Student currently studying Civil Engineering. I have to write a paper on basement drainage solution for existing homes. The problem is there are not a lot of basements in SA! You guys in the US have a lot more first hand experience and I hope you could help me out. My project needs to include a short(5page max) guidebook in which I describe the different causes of dampness in basements, different solutions/systems and some kind of a flow diagram that a homeowner can follow to arrive at a solution. The diagram will start with a the problem and then site specific questions etc will elliminate some solution until ariving at a solution.

This is what I have so far:(only problems caused by groundwater is considered+condensation, no burste pipe or water from other floors etc)

The main sources of damp
  • Lateral penetration(seepage)
  • Rising damp
  • Condensation
  • Rising water table(flooding)
The solutions
  • External drainage(french/fin drains)+sump
  • Chemical damp proof course
  • Internal cavity drainage system+sump
  • Dampproof Membranes(interior or exterior of wall)
  • Waterproof membranes(interior or exterior of wall)
Can you guys tell me which systems are the best and cheapest for each type of problem. Any help will be greatly appreciated!:thumbsup:

Are there any similar guides available???

AllanJ 09-03-2011 09:32 AM

Lateral penetration is a problem when the soil against the foundation is not that porous, for example clay soil. A damp resistant membrane or painted on waterproofing is very helpful, preferably on the outside. Exterior application is better. Inside application is used when it is too difficult to re-excavate to apply the material on the outside. The goal is to get the water to soak downward instead of through the foundation.

Condensation on the inside happens when the interior air temperature is higher than the ground temperature. Typical methods of controlling this are insulating the basement walls with a vapor barrier on the innermost surface (requires no membrane on either the outside or inside of the foundation), and constant mechanical dehumidifying.

Internal cavity perimeter drainage or external perimeter drainage down at foundation footing level are almost always needed if the basement gets wet down at floor level. These systems protect the basement from a rising water table. The exterior system works a little better but can be more difficult to install after the house was built. Assuming the system is built and working properly (and not clogged) you should not need both inside and outside systems.

Very important is grading the land surface so rain water does not pool up against the foundation.

Ernst Conradie 09-03-2011 10:08 AM

Thanks AllanJ

Just another question, seems that you know what youre talking about.....

Lets say a basement gets flooded often by a rising water table, how do you choose between the interior cavity and exterior perimeter drain? How big is the cost difference in a typical project or what other factors will help you choose?

Must a cavity be created on the floor and walls or can just a floor cavity do the job?

From what I understand the perimeter drain should preferably go right around the house or on a site with a gradient the drain should be installed on the "upstream" perimeter.

AllanJ 09-03-2011 04:43 PM

The perimeter drain pipe, which should have perforations near or on the bottom surface, is the cavity. It is put below basement floor level. Normally it is nested in gravel. the pipe should not be wrapped directly with filter cloth but rather some gravel should be enclosed between any filter cloth and the pipe.

YOu have to dig down to the bottoms of the foundation footers. You already understand what it would take to dig down the outside earth all the way around the house down that far versus breaking up the concrete basement floor and digging down to footer depth from the inside. But do not dig below the bottom surface of the foundation footings up against the footings.

Normally the perimeter drain pipe will have little or no slope because you don't have much vertical wiggle room to work with below basement floor level. It will still work. If there is some slope, the sump pump pit should be at the lowest point.

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