self build waterproof basement formwork membrane concrete drainage
 


External Basement Drainage.

Drainage is a system that carries water downhill.

You would want External Basement Drainage to always carry water away from your basement.

Sometimes you need a pump too, for instance from a stair well. I deal with this at the end of this page.
  self build basement


self build basement   Therefore, you must make sure that wherever water beside your basement goes to, that the soakaway or whatever, will cope even during the worst storm. Otherwise your drainage will fill up and apply water pressure against your basement - which would be worse than not having that drainage.

In my experience, external basement drainage was not suitable in the majority of cases because the water would not always drain away fast enough.

The essentials are:
  1. A bobbled drainage board covered in geotextile filter membrane that is not crushed when you backfill.

  2. Water drains freely from the drainage board into a pipe.

  3. The whole system is protected from silting up.


external drainage membrane

These are photos of the drainage board I have for sale and in stock.

I only have the high density plastic drainage board. I don't have the geotextile filter membrane already attached to it because the board does not need to be overlapped whereas the filter must be overlapped. So it is far easier, and more likely to be installed properly if the filter is a wide sheet off a long roll and it comes down the wall and gets wrapped under the pipe as well.

Note also that I fold the filter over the top of the board. This is far more effective than the plastic strip others would sell because, firstly, it doesn't go out of shape when backfilling and. secondly, it is easier.

  non woven geotextile membrane sketch



I bought, and will sell to you, much tougher external drainage membrane than is sold by the famous brands. It cost me more and it might cost you a little more, but it is worth it to use a product that survives the construction process.

The workman is holding the end of the perforated pipe that goes around beside the basement floor slab because this end will become a rodding eye beside the steps down to the basement.

external basement drainage rodding eye   external basement drainage tough membrane



basement external drainage

external drainage membrane

You can buy woven filter membrane such as the black in the top photo and non woven such as the white in these photos.

The problem with the woven is that where you overlap two sheets it becomes waterproof and will not let clean water through.

So the non woven variety would be better.



My drainage board comes in rolls 20m long and 1.9m wide. You can see the flat edge for overlaps where you have one board above another. The dimpled width is 1.8m.
  external drainage membrane


I sell these for £150 each if 20m long and £75 each if 10m long. I would usually cut 20m rolls in half before dispatch because full rolls are a bit too heavy. The ends don't need overlapping. The board is not intended to be waterproof, only to provide water its easiest route down.


With a bit of planning fitting membrane can be really easy.

If you simply nail a few little bits of timber against the inside of your formwork you will leave little blocks of wood in your concrete to simply screw on membrane board and filter membrane.
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A sump and a pump.

A stairwell might need a sump and a pump to get rid of rainwater.

The pump needs to be in a chamber and it needs enough height for a float switch to operate freely. This might be a plastic drum cut to size or inspection chamber riser sections.

It needs a power supply, a discharge hose and it should have an electronic alarm as well in case the chamber fills with water and the pump has failed. Alarms need only cost £40 from electronics suppliers.

The sump might be fed just by a gully in the floor of the stairwell or a perimeter land drainage pipe from around the basement as well.

In the photos below you can see the sump being prepared and the drainage pipes that will be buried in trenches leading to it; the sump after the slab was cast and you can see the temporary outlet from the pump already installed; the opening for the patio door from the basement and the area beyond and I have also shown a suggestion for hiding the sump and the gully over it by putting paving on these stools. They can be adjusted for height and they will let rain water drain between.

sump rain water storm



This was someone else's take on the same idea.

sump rain water storm   sump rain water storm



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