Basement external waterproofing.

What are our choices?

Sticky-back membrane.

Clay filled carpet that expands when wet.

Waterproof paint, bitumen or asphalt applied hot or cold all over outside.

Render.

Rubbing in a cement slurry to densify the concrete surface to reduce porosity.

External drainage. Gravity or pumped.

I don't think you should use any of them, except external drainage if it will always drain away under gravity even during a fierce storm.

If you need to pump rainwater up from basement level, try to keep it to a minimum. The fewer times your pump switches on and off the longer it will last before it, or your discharge pipework, fails.

Sticky-back membrane.

I have a lot of relevant experience of sticky-backed membrane, and none of it is good.

Any external membrane is easily damaged during backfilling.

Sticky-back membranes do not stick in the damp conditions found down the north side of a basement excavation where the sun does not shine once the basement walls are built.

When the sticky-back membrane is laid flat and joined together over the blinding, the structural floor slab steel reinforcement is very likely to puncture it in dozens of places as the steel is carried in and tied. Not every tear will be repaired.

Then, when the floor slab edge formwork is removed and the membrane folded up the side of the slab the membrane will be damaged and wet. The wall membrane will not stick to it. A video on YouTube demonstrating the problems.

In a court ruling Outwing v Weatherald the judge said "overlapping self-adhesive membranes cannot be expected to achieve a total or absolute watertight bond capable of resisting penetration by water pressure."

The new edition of the Standard for waterproofing a basement, BS 8102 : 2022 says this

"3.7 fully bonded.

bonded, preventing water from tracking from a defect in the membrane to a crack/joint in the structure

NOTE This can provide enhanced protection where applied to a suitably impermeable structure because for ingress to occur, a defect in the membrane has to align with a pathway through the structure."

It is absolutely clear. Sticky-back membrane should not be used on a structure that might generally leak. For instance, the concrete was not sufficiently dense (reduced water and with sufficient extra cement to be waterproof ), or the waterproof concrete was not thoroughly vibrated.

"6.2.5

The need for continuity in the waterproofing protection should also be determined when selecting a type of protection. In most circumstances, the protection should be continuous (typically from DPC level or 150 mm above ground throughout the below ground structure)."

I have explained above that sticky-back membrane is not going to survive or stick everywhere.

Clay filled carpet that expands when wet.

I have one relevant experience of a clay-filled hydrophylic carpet, and it was not good.

The carpenters got the carpet out of the shed and in the rain the box split and the carpet started to get wet and expand.

They fixed it to their formwork but in place, waiting for the rest of the formwork to go up, it rained and the clay split the stitching and instead of being a thin film of clay ready to expand and become a waterproof membrane, a huge lump of earth at the bottom offered most of the wall no protection whatsoever but at the bottom the important starter bars were not surrounded in concrete.

Waterproof paint, bitumen or asphalt applied hot or cold all over outside.

I am sure that hot applied bitumen or asphalt would be far too expensive for a domestic basement.

Far more likely would be waterproof bitumen emulsion paint.

I have a lot of relevant experience of waterproof bitumen emulsion paint.

It only worked well when I applied it. I take care not to spread it on thick, because otherwise the surface dries but the emulsion against the substrate neither dries nor bonds.

I find you need to keep going over it looking for tinier and tinier bits missing. Little bubbles burst and spring back. What was complete when you put your brush down needs another coat a few hours later.

But if you give it to a labourer and tell him to follow the instructions it will be terrible. When I worked on big sites, the foreman always gave this job to the labourer he wanted to sack, because after he went home covered in black goo his wife couldn't wash out, she didn't let him go back.

It is not nice stuff. But it works if properly applied.

Render.

Render will fail to meet the criterion in the Standard "the protection should be continuous (typically from DPC level or 150 mm above ground throughout the below ground structure)." because you cannot render under a floor slab.

If you use waterproof concrete you will find that it is too dense for render to stick to it.

Rubbing in a cement slurry to densify the concrete surface to reduce porosity.

I don't have any experience, but it seems to me that if you were to buy Penetron Crystalline Surface Applied Waterproof Coating you would have to make sure that your basement concrete was truly dense and waterproof for the floor, because you could not post-apply this product under the concrete floor, but your wall concrete would need to be poor and porous to get this product to penetrate.

They sell an admixture to make waterproof concrete as well. But, like so many others, I expect it is likely to be mostly some extra cement powder for a lot of money. Mine is better and I expect cheaper because I have no overheads to speak of.

External drainage. Gravity or pumped.

Gravity.

If you can surround your basement in free draining material so that all rain water will quickly fall by gravity to your substantial perforated drainage pipe surrounding your basement, then downhill in brown drainage pipe to discharge into a ditch, then it is so certain to always work and never allow your basement structure to need to withstand any pressure of water against it that it should be a no-brainer to include it.

But if you have any doubt it could sometimes be over-whelmed then don't use it. Find something else more reliable to waterproof your basement.

Pumped. I would fear a pump failing one day but if you have a sunken patio or window well you are going to have to pump rainwater out.

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