Basement frequently asked questions

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

Client Frequently Asked Questions.

  1. Feasibility?

  2. Excavation? Structure? Electrical and Mechanical? Finishing?

  3. Cost?

  4. Where will we work?

  5. How are we certain the basement will not leak?

  6. How do clients insulate their basements?

  7. Can I have a basement in the water table, for instance, down at the same level as the Thames?

1.  Feasibility.

Will go ahead Will often go ahead Might go ahead
Those prospective clients who call to say they included a basement in their planning permission and are going ahead probably have the budget. Those who would like a basement beneath a new extension behind their detached property, at least 3m from their boundaries, will often go ahead. Underpinning your back wall and piling along the boundaries to build a basement in your terraced house back garden is expensive. It only makes economic sense despite the cost in Central London. But with stamp duty very high over a million pounds, even there could be in doubt.

2.  Design, Structural Calculations, Excavation, Construction, Electrical and Mechanical, Finishing.

Would you like your contractor to just build your basement structure or take on your whole basement project?

We are waterproof basement construction specialist civil engineers. We are best suited as just your structure contractor.

We can project manage for you as well should you wish, from inception to completion. But we would usually only be your contractor for the basement structure. This is because your building contractor needs to build the walls, put on the roof and put in the windows before the basement can be pumped free of rain water and fitted out.

3.  Cost.

We do not find extras to bill you for at the end.

You choose from the list above. We give you our price. If you accept it, that is what you pay.

We are the experts. We will include all that is usual whether it has occurred to you or not.

4.  Where?

We operate more or less throughout London and the Home Counties. But without going inside the North or South Circular Roads.

where we will build your basement in the home counties and London

5.  How are we certain your basement will not leak?
  • First, we will get evidence that the concrete used in your project is completely impermeable.

    The only proper test for this is to BS EN 12390 section 8. Our waterproof concrete web site has sample certificates
    a little way down this page.

  • We will do all the concreting work to BS 8007, which basically means we will make sure that
    1. the concrete is properly reinforced to restrict cracking,
    2. We deliver the concrete properly,
    3. We get the concrete past all the steel properly,
    4. We compact the concrete properly and
    5. We cure the concrete properly.
    6. We scabble and clean all joints.

  • We have learned, because we have taken responsibility for leaks for so many years, how to build formwork that makes the 5 points above very easy to achieve.

    Please do not be impressed by anyone who will fill a wall in a single day. That saves them time and money but leaves you a load of problems later.

6.  How do clients insulate their basements?

We are often asked how people insulate a basement.

There is no clear winner. Every way is used.

However, you might bear in mind
  • A basement is buried, almost a cave, surrounded by ground always about 10°C.

    Therefore a basement will not absorb much heat from the ground if it is cooler, and neither will it lose much heat to the ground if it is warmer.

  • Ground is insulation because it takes excess energy a very long time to find its way to the open air.

  • Basements only benefit greatly from solar gain if the sun gets in, not if the basement has little window area.

  • If you want to maximise thermal mass, why not use the ground surrounding the basement?

    Or, why not use the basement structure to even greater effect by storing heat from the roof in the base slab concrete?

In 2012 we built a basement near Southampton, in damp sticky ground and only two thirds buried.

These clients initially thought they had achieved zero energy bills for their new home with U Values around 0.20 rather than 0.10.

See a video, made in 2014, explaining this basement and its heating here.
This BRE information paper is over 20 years old and may no longer be considered accurate. However it suggests that an average size basement could have a U Value of 0.16 without any insulation.

BRE U Values
   Page 1    Page 2    Page 3

You might consider thick insulation around the outside of your basement only from ground level down 1 metre and rely on the cave effect for the rest of your insulation.

But when I went back to see them in Spring 2017 they reported annual energy bills of around £1,000. For just two people this is more than the national average, I think. What could have gone wrong?

6 months later I am updating this piece again.

Mainly, I expect that the Ground Source Heat Pump went wrong. In the same way that the ground insulates a basement, the ground also prevents the GSHP coils, radiators or borehole from warming up again. Therefore, (unless your heat source includes running water to bring replacement heat quickly) the more heat you take from the ground the colder the earth around the coils will become and you will need to use more and more electricity to get any more heat.

In the worst of winter they may be using 5kW of electricity for every 3kW of heat they get from the ground.

It seems remarkably difficult to get a promise from any heat pump company that you will save much money when it is cold outside. So I tend to study their choice of words very carefully:

I read an article on page 11 in the Sept/Oct 2017 issue of Selfbuilder and Homemaker magazine hopefully still here by a Mitsubishi air source renewable heating expert. I was struck when I read "While this will need slightly more electricity in the colder months, the seasonal efficiency throughout the year will average out at 3.5 or more"

These articles never seem to go as far as to say that in the coldest weather heat pumps will still save you any money. That is why I suspect that claims of 3.5 times efficiency over the course of a whole year hide their failure to be as cost-effective as gas when you need heat most.

However, the choice by my clients to have an air source heat pump indoors that heats water from the energy still left in the air that their heat recovery ventilation expels might be inspired. But I think that using ICF throughout with insulation both inside and outside their walls is the other major blunder.

The Government's position, I read, is that thermal mass stores a lot of valuable heat on cooler days to release at night when windows are shut and thermal mass soaks up excess heat from solar gain on hot days to release at night when windows are open.

Lightweight timber frames and insulation on the inside of a dwelling will make a home easier to heat on the 10 or so coldest days of the year and easier to cool on the 10 or so hottest days of the year. But thermal mass will save money on almost 345 more usual days of the year and save money overall.

I can reveal that this couple finds their bedroom upstairs too hot at night sometimes even with money spent on air conditioning.

In practice there is no favourite combination of insulation. But I would have external insulation only over solid, air tight concrete walls and floors in a single, monolithic RC structure with a single, continuous covering of insulation.

7.  Can I have a basement in the water table, for instance, down at the same level as the Thames?

See a 19 second video showing what happens down at water level if you dig or auger , such as CFA piling, into or through the water bearing layer here.

The short answer is Yes You Can If Your Pockets Are Deep Enough.

It will swallow money because arrangements have to be made, such as a perimeter diaphragm wall, steel sheet or secant piling or pile casings, to stop the ground collapsing in sideways as you extract gravel from water.

if you allow any material at all to come in from the sides then the ground around is going to have to settle and you might cause subsidence to neighbouring properties.

river gravel basement


We have a number of other pages with information.

waterproof basement construction   You Must Have a Soil Report Before Design Starts   Please Ask Your Architect To Read This Page   Please Ask Your Structural Engineer To Read This Page   construction contract

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