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External Membranes.
Sticky-Back Bituminous Sheets and Sodium Bentonite Hydrophylic Clay-Filled Carpets.

Membrane Essentials:
  1. Good Workmanship Everywhere, especially at joins and corners;

  2. Being Dry during application and Staying Dry until they are backfilled;

  3. No Damage by those working and carrying nearby;

  4. No Damage when backfilled.
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Sticky-back membranes stick immediately to themselves and they cannot easily be unstuck, so great care is needed during their application because an inadvertant fold could leave an area unprotected.

But sticky-back membrane only sticks to itself when it is dry AND WHEN THE ATMOSPHERE IS DRY AS WELL. So if it rained recently or it is about to rain, even new membrane straight out the packet can refuse to stick. Will your operatives wait days, if necessary, to do a good job?

Unfortunately waiting is unlikely to be enough. When a basement is excavated all the rain that falls over a wide area nearby finds its way through the soil until it seeps out the side of the excavation. Once the slab is cast there is usually a very narrow strip where all that water collects. In that narrow strip is the sticky-back membrane from under the slab currently getting trampled to nothing as the walls are built and getting, and staying, very wet and dirty.

At some point the strips of membrane need to be stuck together up the side of the slab and then to the membrane on the walls. The manufacturers' instructions all say 'completely dry', 'completely clean', 'no debris between the membrane and the concrete to be waterproofed'.

 

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One side of your basement remains in permanent shadow once the walls go up. You can blast the membrane with compressed air - but compressed air condenses water and makes the membrane wet. You can erect a tent and set up a gas heater - but a gas heater creates water vapour and the membrane will not stick in a damp atmosphere. You can mop away with rags and tissues - but you cannot get into the ends of the joins before they disappear under the slab.

One has to wonder if they ever worked anywhere? Who would have the patience and time to go to the degree of care necessary, and only during a prolonged completely dry and hot spell of weather, to make them work? Presumably only the desert! (And who would know if they might leak there anyway).

There can be no certainty that sticky-back membrane would protect a basement against any pressure of water outside.


Sodium Bentonite Clay Filled Carpet.

This is really designed to be nailed to the inside of traditional shutters. Then, after the concrete pour, the carpet is gripped by the concrete and remains in place even as the shuttering is torn away.

I have not used it since a big school project for Interserve in Southampton about 11 years ago. The carpenters would go and get some and it would lay on the ground a day or two till it was used. By the time they fixed it to their shutters it would be wet and very, very heavy because the clay readily soaks up water and swells the clay. The clay would fall through the carpet in some places so there were always patches unprotected.

I remember a Grand Designs programme where Kevin McCloud crept down beside a basement and filmed one of these clay carpets. It was a mess and he indicated, in his delicate way, that it might not work!

And that was before any damage backfilling it.



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