Damp Proofing a Cellar

The how and why of damp proofing a cellar

by David Morton, Senior Surveyor, CSSW, CSRT

Firstly, as a cellar is generally going to be a below ground area within a building, we would treat this as a “waterproofing” job, as opposed to a “damp proofing” job.

The key difference between basements and cellars is the original, intended purpose of each. For instance, basements can be finished off to act as living spaces, whilst cellars, due to their size and location within a property, are designed to be storage spaces. As a rule of thumb, windows will only be found in basements and not in cellars.

Damp Proofing work is usually attributed to remedial treatments above ground level, where the problem exists as a result of rising damp from the ground, into the above ground wall structure.

When dealing with below ground structures, we would treat this as waterproofing work, as the structure is subject to the effects of laterally penetrating ground water.

Waterproofing of a structure below ground level is designed to prevent groundwater, under hydrostatic pressure, from penetrating into the fabric, which will lead to dampness on internal finishes.  By comparison, damp proofing is usually designed to control water vapor from soil moisture.  It won’t stop the influx of liquid water, or groundwater under hydrostatic pressure.

When waterproofing a cellar, which were often built into properties as coal storage area, there are a number of considerations to be take into account.  These include:

• Whereabouts within the property is the cellar? Usually they will be underneath the main entrance hall in Victorian properties, or within the central area of the property.
• What work, if any, has been done previously within the cellar?
• Are there gas and electricity meters within the cellar which need to be removed, to facilitate the proposed waterproofing work?
• What is the existing floor make-up of the cellar? If it is all original and no remedial work has been carried out, the floor could be almost compacted soil, or a loose laid brick floor.
• What is the floor to ceiling (or underside of the ground floor joists) height? The head-height of cellars is often no more than circa. 1700mm.
• What is the expected use of the cellar once waterproofing work has been completed?
• Does water penetrate into the cellar, resulting in standing water / flooding?
• Will it be possible for our operatives to access and work in the cellar? This is dependant on the location of the cellar, the size of the access into the cellar and the floor to ceiling height.

It really is important to fully consider how the cellar will be used, once any waterproofing work has been completed as these are difficult spaces to ventilate.  If it is to be used as storage, consideration will need to be given to providing some low-level heating, which will help to reduce the likelihood of condensation within the cellar. If it is to be used as a small utility room, which may house a washing machine, tumble drier etc. then it is really important to include some form of mechanical ventilation, along with low-level heating, to prevent condensation occurring.

Water can penetrate a structure below ground level in two ways

Where the ground is not saturated it will move through the capillaries of the ground and structure by capillary attraction (capillarity).  When this form of moisture comes to the internal surface of the building, surface dampness is evident but there will be no free flow (flooding) of water.  Capillary moisture can enter a structure laterally, such as when a wall is earth retaining.

Where the ground is saturated and hydrostatic pressure occurs, water will be pushed through the capillaries of the ground and the structural walls. Where the water comes to the surface the pressure behind it will force it into the property in the form of liquid water and flooding may occur.

Questions we are asked, when considering damp proofing, or waterproofing in a cellar, include:

• Is a damp cellar a problem?
• Are all cellars damp?
• What is the best way to waterproof a cellar to make it dry and useable?

Is a damp cellar a problem?

This question really comes down to how the cellar is being used and how the owner of the property would like to use it.  If the cellar is not being used for anything in particular and there are no plans to change this; and if the cellar is similar to how it has always been since its construction i.e. bare brick walls, brick, or compacted soil floor, is affected by damp but does not flood, then there is a good argument to leave it as it is.

However, if the plans are to use it for a storage area, small utility room etc. then dampness would be an issue, which would need to be resolved.

It is also important to be sure that the ground floor timbers above the cellar, such as timber wall-plates, built-in (to the walls) timber joist ends etc. are not being affected by the damp within the cellar.

Are all cellars damp?

Because of the way cellars were usually constructed and what they were originally used for i.e. below ground, coal storage areas in older properties, most cellars, in their original form and condition are likely to suffer from dampness to a greater, or lesser degree.

What is the best way to waterproof a cellar to make it dry and useable?

Firstly, nothing is absolutely dry.  Water and water vapour will always exist bound up in the building fabric and in the air (atmospheric humidity).  With a cellar, there is often little or no ventilation and so a higher risk of condensation within the cellar space.

When considering the most appropriate form of waterproofing, based on the general requirements of BS8102:2022, the installation of suitable heating and ventilation, de-humidification etc. may also be required, dependent on the intended usage of the cellar.  The type of systems to consider, would be:

1. A Type A – barrier – system, which is based on the use of waterproof render to the walls and a waterproof screed over the floor.
2. A Type C – cavity drained – system which is based on the installation of a cavity membrane system to the walls and the floor, along with perimeter drainage into a sump / pump chamber.

Either option could be right, based on existing and intended use, size and depth of the cellar and, of course, the price.