Plantool’s "How to" - Damp proofing your basement

The basement within any form of building is one of the most common areas for damp owing to its placement beneath the ground and the tendency for the exterior to suffer from some amount of moisture soaking in from the earth that immediately surrounds it. This may be particularly noticeable if you’ve not damp proofed your basement for some time, or have suffered from saturated ground from ongoing rain, snow or storms. Staying on top of your damp proofing is vital if you’re to ensure that the remainder of your home is to stay damp free, as rising damp can lead upwards from the basement and cause untold damage in a relatively short period of time.

Damp proofing your basement: step by step

Tackling the floor

First of all you should decide upon the best approach to take, as there are several different ways of damp proofing your basement. Here’s a summary:

Method One

1. Begin by painting the floor with between 3 and 4 coats of water based rubberised bitumen emulsion.
2. Place the sand or cement screed on top of the paint, this should be built up to around a depth of 50mm.

Note: If you have flagstones or bricks down as a floor then you’ll need to repair any rotted, cracked, or crumbling mortar joints.

Method Two

1. Place a layer of sand down, and then cover this with a polyethylene damp proof membrane. This sheet can then be tacked to the walls and finally secured down with flashing tape.
2. Cover the sheet with the sand/cement screed, again to a depth of around 50mm.

Tackling the walls

1. If your walls are concrete or brickwork that remains in reasonably good condition then you can apply diluted RBE (Rubberised Bitumen Emulsion) as a primer, followed with 3 coats of undiluted RBE.
2. If you’ve chosen to use a plastic sheet then you can use a sand/cement render to cover the RBE to a thickness of around 50mm.

Note: If your walls are not in good order, with cracked or crumbling pointing then you’ll need to add another layer of sand/cement rendering with around 20mm depth, applied directly onto your walls, and then followed up with the steps above.

Troubleshooting your damp proofing

If the RBE comes off the wall

If your RBE comes off the wall then it may be that the wall is already damp. In this event your rendering is going to need wire meshing in order to help with the way the rendering takes. This mesh should be tacked firmly onto the wall, with squares of flashing tape used upon the places where the nails will tack the mesh onto the wall.

Once you’re ready to apply the rendering you should take care to apply right up to the RBE coating with absolutely no gaps.

If the RBE doesn’t dry out

If your RBE doesn’t dry out, or your solvent based products won’t stick to the wall then it may be that your walls are too wet. One of the easiest fixes for this may be to use a thick, studded plastic sheet which should be nailed to the walls. This can provide for an essential air flow between the wall and the sheet, allowing for ventilation. The rendering can then be applied to the sheet.

Considering expert option

Whilst the steps within this article are fairly straight forward it’s worth considering when you may need an expert’s opinion. Above all else if you’re at all unsure as to whether your basement is suffering from damp, and whether this may have travelled or affected other areas, then you should consult a building professional. Oftentimes the symptoms of rising damp can be all too subtle for a novice to spot without a professional helping hand.

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