HPH163 : How to create an effective basement – with Andrew McInulty
Andrew McInulty from Ready Made Basements explains the main reasons people install basements. He also shares the challenges of building below ground and how they are best addressed.
Interview with Andrew McInulty
Andrew’s entire working life has been in the construction industry, from tradesman up to contracts management. His work has involved basements since 1975, and he understands how modern methods have helped reduce the problems previously associated with basement construction.
Basements offer a way to gain consent for increased habitable space
Planning mainly concerns itself with the impact of a structure on its surroundings above ground. Many factors will risk your planning application being rejected, such as: ridge heights, privacy issues, or building in green belt and areas of outstanding natural beauty.
However, many planning departments take a much more liberal view of basements as the visual appearance of them has no impact; they don’t increase the height of buildings yet allow property owners to maximise the use of their plots.
So with new builds, if you can only get planning permission for a bungalow, a basement will double the habitable space that’s available. However, an initial application for a new build may be rejected if you mention your basement, as the planners might say you don’t need all the space proposed above ground. Andrew suggests you first apply for the maximum footprint obtainable above ground, and then make a subsequent application for the basement.
Building a watertight basement demands intricate detailing
Basements became very common in Georgian and Victorian times when labour was cheap and the increase of urban populations meant space in cities became more valuable. After the First World War, basement building decreased as the population began to move out to outlying towns where land was plentiful. As basements became less common, so did the expertise required to design and build them. Andrew likes to help architects and structural engineers with proper detailing and waterproofing methods.
In principle, basements of one or more storeys can be built anywhere and used for storage or habitable space. The method of construction depends on surrounding areas and boundaries. A proper interpretative ground investigation report is invaluable and will tell you:
- Soil characteristics
- Soil type
- Plasticity levels
- Ground bearing pressures (how much the ground will carry per square metre)
- Level of ground water
- Anticipated movement of soils (e.g. a clay soil will rise more than a sandy soil once the hole is dug)
Using this report, structural engineers can then design the most economic structure to suit the ground conditions, detailing the junctions, the joints and where the structure sits on top.
The severity of ground water level can impact on cost
Ground water level is of course a major consideration for basement construction. Levels vary seasonally and geographically; in the southeast it’s not uncommon to find natural water levels only a metre or two below the ground.
The art of basement construction includes preparing the ground and keeping the water at bay, so that once it’s backfilled and finished, you can walk around inside without giving a thought to the water that could be above your head height!
Concrete is the best material for building basements
Based on Andrew’s forty-plus years of experience, he believes concrete to be the most robust, reliable, consistent product for basement construction. The many reasons for choosing this material include:
- It has the minimum number of joints, which is required for working underground
- Concrete itself can be made watertight
- It’s the most enduring material
- If it does fail, it is totally and permanently repairable
Even with waterproof concrete, vapour can be an issue. Either natural or mechanical ventilation is always necessary in order to prevent build up of moisture internally, as vapour can still be transmitted across the wall.
Passive basements are possible but require specific insulation
From the point of view of passive basement construction, Andrew says there are no additional technical requirements for the structure itself, just more insulation.
Specifically, to bring the whole basement structure to Passivhaus standard, 250-300mm of insulation is required underneath the basement plus 200-250mm for the walls. Specialist extruded insulation is needed to take the load of the building, and is therefore more expensive.
Quality workmanship is needed to realise the design
Andrew says the most important thing in basement construction is quality control; it’s all about detail, supervision and the quality of workmanship. For example, the exact concrete mix is crucial and the spacing of the reinforcement needs to be precise to prevent cracking. Most defects are caused due to issues with workmanship, although design flaws can of course also lead to ingress of water.
A guarantee is no guarantee!
Andrew strongly advises clients to take professional legal advice regarding the small print of basement guarantees, commenting: “It’s a fact that many of the guarantees have a limited value”.
One of the problems with guarantees offered by the contractor is they often rest on a particular product guaranteed by a supplier; however, the liability of the supplier only stretches to the cost of their material. Andrew adds: “There can be any number of contractors involved; there can be a long line of people who won’t accept responsibility for it”.
Another issue is due to the recent surge in basement construction, which has resulted in many people in the industry not having the experience required. Hundreds of owners of basements constructed in the last ten years have pursued claims worth millions of pounds. These days, Andrew is aware of only one third-party warranty provider who will actually insure against the ingress of water.
Building underground is more expensive but adds value
The additional costs of building underground include more excavation, ensuring safety and the price of concrete. However, the basement gives you a habitable space as well a foundation for the house above. So the cost of foundations for a conventional house should really be deducted from the overall cost of the basement. This means the basement actually offers excellent value for money.
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Transcript
Download a transcript of the interview with Andrew McInulty.
