5 Pitfalls of Basement Drainage - Ben Norman

For those wanting to extend a property in an area of prime real-estate, where land is at a premium and extensions have already gone back, front and up, they have only one course left ... down.

Basement extensions are popular to add further dimensions to valuable urban property, adding a study, reading room, music room, perhaps a playroom or bedroom.

There are effectively three types of basement: converting an existing basement into a useable, ventilated, well lit and drained room(s), retrofitting a basement, excavating one below an existing property; and adding one to the plans of a new build.

We’re looking particularly at the middle option here.

Before embarking on such a retrofit basement project, please, please seek professional advice. Basement excavations do need Planning Permission, mainly relating to the installation of a means of escape or light wells as these affect the exterior appearance of the property.

As well as the obvious considerations: access, party walls, existing infrastructure etc, two major considerations are waterproofing and drainage.

 In assessing the viability of the project, consideration must be made for the following:

• Accessibility and affects on neighbouring properties.
• Geology, topography and the water table.
• The proposed use of the basement.
• Access from existing the property.
• Ventilation and light.
• Existing drainage.

There are different ways to make basements dry which is one of the most critical aspects of a successful basement. The waterproofing falls into 2 main areas, cementitious tanking and cavity drain membranes.

Tanking, layers waterproof material directly to basement walls, ceiling and floor, encapsulating the space in a waterproof ceil. A cementitious waterproof render system is applied in several layers, linked to a waterproof screed on the floor.

Alternatively membranes are used to create an inner waterproof structure with a cavity (created by the membranes studded profile) behind it that is fully drained. The constant draining reduces any pressure build up on the exterior structure.

The pitfalls:

1. Failure to check the location of existing drainage and services - Quite often existing sewers run across the rear of properties, particularly terrace housing.  Typically these sewers are around 1.5m deep, just about head height in the proposed basement.

2. Following on from number 1, owners naturally want to maximise basement space and extend right up to the plot boundary. This offers a bigger new 'family room' but leaves no space to divert the sewer. Manholes and drainage would now have to be located outside the plot boundary, perhaps even on neighbouring land.

3. Following on from 2 regarding sewers. The sewer may already be adopted by the water authority.  Therefore any works on or close to it would require a formal legal agreement of acceptance to be put in place. Without it, building control may not sign it off.

4. Make sure the basement is fully waterproofed with a membrane and sump pump system to drain anything low level including light wells.

5. More of a tip to avoid a pit in which to fall, bathrooms and WC's in a new basement require pumping. Installing pumping systems further reduces usable space, so avoid using the basement for this purpose. Keep them above ground level and with some careful planning they can discharge via gravity. 

A bonus pitfall:  There is sometimes a risk of sewers surcharging. A non-return valve is sometime a good idea to prevent flooding. 

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To avoid these pitfalls and many more, take professional advice, 
contact Ben Norman and the team at
 JMS Engineers on 01473 487047 

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are, and will be equally proud to support.