Floors over a basement can be a cause of leaks

waterproof guarantee basement construction build manage
  waterproof guarantee basement construction build manage

What will the floor over your basement be constructed with?

The ground floor of most houses is beam and block over dirt with a space ventilated to the open air between.

A house with a basement beneath has a ground floor not over dirt and not ventilated to the open air.

However these two differences are not the reason why beam and block is a bad choice over a basement. The reason is waterproofing.

If house construction was repeated for a beam and block floor over a basement. the cavity tray and DPC would prevent water rising up into the wall above from the wall below.

But you can see that there will be water ingress under the cavity tray.

This is allowed under a house where the water goes into the ventilated void over the ground.

But clearly it creates an issue of water ingress into a basement.
  The way round this problem is to incorporate an upstand in the waterproof concrete.

Sticky back membrane over the area outside will not work because the membrane will not stick where it is wet, it will be difficult to seal over all the odd shapes at doors and so on, and the bricklayer or scaffolder is certain to puncture it.

But look at the complexity necessary to accommodate the external brickwork and an upstand with beam and block.

Corbels cost money.
  However, if an engineered timber floor is chosen, the ledge is fixed to the upstand that is the width of the inner leaf of construction above.

There is room for the cavity insulation and the brickwork on the ledge outside.
waterproof concrete basement construction no upstand   waterproof concrete basement construction upstand   waterproof concrete basement construction I beam upstand

beam and block  

A suspended ground floor over soil has to be concrete, obviously. But the floor over a basement is indoors.

beam and block on YouTube a 30 second video to put you off beam and block.

Years ago our founder was paid as a consultant to try and stop water getting into the basement behind this wall.

It was built with a concrete retaining wall and corbel at the top to make the top wider, cast with a flat top and covered in sticky back membrane.

The backfill contained all the site rubbish that punctured the membrane in thousands of places. So we waterproofed the concrete all over the inside. But water still got in.

It turned out to be because the brick and block cavity wall was built on top using normal house construction: with a DPC and a cavity tray and the floor beams sat on the rest of the top of the wall inside.

The outside ground level was just above the top of the retaining wall so that the membrane was hidden, and whenever it rained water soaked through the brick mortar, flowed between the floor beams and ran down the inside of the basement.
external waterproof membrane for basement

To stop water getting in under the beam and block around the edge you need an upstand to stop the water that gets through the bottom course of brick. The beam and block needs a ledge and your exterior brickwork needs a foundation as well as somewhere for any cavity insulation to also reach to the bottom. It all adds up to wider than your basement wall, so you have to form a corbel, an upstand and a ledge to cope with it all.

waterproof basement construction upstand and ledges     beam and block sketch waterproof basement construction floor ledge upstand

The other half of the sketch shows engineered timber joists instead of concrete beams.
  1. Overall thinner construction - so you aren't digging deeper and making walls taller.

  2. Works with just the wall width - so no corbel.

  3. No screed necessary.

  4. No separate service void needed.

  5. Joists can be supported by joist hangers or the top chord can sit on the ledge board.

  6. They can have an OSB web, steel web, ply both sides for massive strength, be 200mm deep, 205, 210, 215 ..... 450, 455

  7. Casting and pouring the upstand flush with one side of your wall is very much easier than a central upstand that will have to be a separate operation later.

  8. The floor ledge plate (the timber to support the floor joists) can be part of the wall formwork so cast solidly to the concrete and just left in place. It doesn't even need fixing as a separate operation.

    There are a few more photos on the page for your architect to read.
floor over waterproof concrete basement cellar floor joist over waterproof concrete basement cellar

Another common objection to using anything other than beam and block is underfloor heating. Some people think underfloor heating has to be in a concrete screed and timber is not strong enough.

But there is an alternative as these photos show.

underfloor heating underfloor heating

We have a number of other pages with information.

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