Cellar Conversion Topics - Which Basement Sump And Pump System? Part One - The Sump Liner

This is the first is a series of article that I have written on each aspect of this, the most critical part of any cellar conversion project - the waterproofing.  This first article is on the sump liner. The sump liner sounds like such a mundane item, I hardly imagine the average person getting passionate about which one they have, but it can make the difference between enjoying a reliably dry basement and the misery of dealing with a flooded one.

The sump liner...

The biggest decision is whether to have a perforated liner or a non-perforated one. This is an easy one, 99.5% of cases benefit from having a perforated liner.

A perforated sump liner will allow water in from under the floor and in so doing will actually deplete the water from around the outside of the basement, creating an environment for the basement structure that is less water logged. It takes pressure off the waterproofing system. If you have a drained cavity system you will no longer have water penetrating the primary structure of the basement walls and floor and having to find its way to the sump through the narrow cavity created by the membrane, the water will be disposed of at a much deeper level reducing reliance on the membrane to function as the primary barrier against water ingress.

Objections to perforated sump liners are mostly ill founded and include the fear of undermining the floor slab or even the foundations.  Millions of perforated sump liners have been installed in the USA and many thousands in the UK over the last decade and a half without any such problems.

There are exceptional circumstances where a non-perforated liner is preferable these are: -

Where the ground is very silty and unstable and silt freely runs into the sump chamber

Where the ground is so waterlogged and free draining that a never ending supply of water enters the sump chamber.

Both these conditions are relatively rare and should be spotted by an experienced and knowledgeable expert at the very latest during the installation process if not during investigative work beforehand.

Opting for a non-perforated liner to 'play safe' actually costs you the many advantages that a perforated liner can bring. In fact using a non-perforated liner can be playing dangerously as it encourages the build up of water pressure above the floor level outside the basement, making every aspect of the rest of the installation (of the waterproofing system) being more critical.

Even when the ground is found to be silty a geotextile filter fabric can be installed to line the excavation before the sump liner is fitted. In fifteen years of installing such systems (at a rate of about one a week) I have only come across the need to do this on two occasions - that is how rare it is.

Now the disadvantage of most filters is that they clog - that is what they are supposed to do but when they clog they cease to work efficiently. In this case your sump would progressively behave more and more like it has a solid liner when the filter clogs which is no worse that if you fitted a solid liner in the first place. 

Another type of liner has a grating rather than perforations. The grating is far more open than the small perforations that you normally get in a standard perforated liner. This  works well with a different kind of filter - a plastic gause, a bit like a pan scourer and that goes on the outside of the grating. The filter stops silt from moving into the sump chamber but is also accessible for cleaning - a simple hosepipe applied through the inside of the grating will do it. With this kind of grating - which allows a greater flow rate of water the sump chamber - can also shallower than most thus allaying fears of undermining the footings. 

To find out more click the anchor text link to ask the author a question about any aspect of basement waterproofing and cellar conversion projects.