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Basement Conversions – from a Structural Engineers View

Logo smallTHINK BASEMENT specialises in basement conversions in the private residential sector, offering amazing transformations to existing underground spaces.

We get many enquiries from homeowners keen to take advantage of the fantastic potential that their basement offers, but who may be worried about the structural effects that a conversion could have on the rest of their property.

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Keen to dispel some myths and answer some of the most common concerns, THINK BASEMENT has teamed up with Dean Miller, Structural Engineer and founder of Signature Structures, to offer some valuable information to those thinking about undertaking a basement project.

My basement has very low ceilings, what does ‘digging down’ entail?

Knowing the nature of the ground beneath the existing floor and foundations is fundamental to a basement conversion involving excavating. Naturally, if the ground is extremely hard e.g. sandstone / limestone then excavating will be more intensive than for clay or sandy ground. Digging trial holes is a vital method to establish the existing ground conditions and foundation depths. This should be carried out well before any works commence on site and forms part of a ground investigation report which an engineer would carry out.

Excavation requires good access or a practical method for debris removal by the contractor, as there can be sizeable amounts of material to take away from site via skips or other means.

A common error on sites without proper design work can be excavating the ground to a depth which provides insufficient head-height. Signature Structures works with Think Basement and the Architect to ensure that the correct head-height is achieved.

I can stand up in my basement, but the floor is damp. What can be done?

If adequate headroom exists currently, then you may want to ensure that the area is properly damp-proofed and insulated. The design team would specify the means of damp-proofing for the floor and walls in contact with the external ground. We would work with Think Basement to ensure that no break lines occur across this layer, as this would totally defeat the purpose of the damp-proofing installation.

Will my house need underpinning? What does this mean?

Underpinning means deepening of foundations. This can be necessary for basement conversions to allow for the desired head-height. Often the depth of a new concrete slab, insulation and floor finishes can lead to excavating for the underpinning of the walls and also the existing floor. A good engineer will detail the sequence for underpinning, striking a balance between not removing too much or too little ground at intervals e.g. one would not remove all of the ground beneath a wall in one go, as this would create an unstable wall.

Will structural work on my basement affect the rest of my house?

Without sounding too alarming, some form of settlement will occur if underpinning is carried out, as the existing building will be supported off a new foundation layer. This settlement may be a few millimetres but can lead to visible cracks in walls at the floor above. This is to be expected especially with plaster finished walls or tiled areas. This should not be a long-term occurrence and an allowance for the repair of these cracked areas should form part of the works.

Do I have to inform my neighbours of the intended work?

The owner of a terraced or semi-detached building undergoing underpinning will need to notify their neighbours of the proposed works. The underpinned party wall will after all be theirs also, so they will need to be kept in the loop. We advise to maintain a dialogue with the neighbour from the offset. A party wall notice can be served whereby all details and proposals are submitted to the neighbour. Always communicate with your neighbour, as the converse can cause friction and in some cases objections to works being carried out.

It is the duty of all involved to safe-guard the well-being of those carrying out the excavation works, as the basement of the property poses a large degree of risk to those on site without proper details.


I have walls dividing my basement into separate rooms, do these need to be supported?

When excavating adjacent to internal partitions, which tend to be brick, care needs to be taken not to rely on a sliver of ground to support the wall during the process. A large number of stoppages and abortive works concerning basement conversion can be down to the lack of satisfactory support for the internal partitions during the excavation works. During works, sites can get water-logged and so a worst case scenario would be the collapse of a partition resulting from insufficient ground beneath it. We detail a method from the outset to avoid this regrettable scenario.

What affect does a structural engineers report have on a building inspector approving the works upon completion?images

In addition to the ground investigation report, an engineer should provide structural calculations and drawings for the proposed work. These structural details are submitted to building control for approval. Whilst works are on site, the construction is checked against these details. If the two match then the owner is awarded an approval certificate. Make no mistake about it, this certificate is extremely important as it verifies that the works were carried out satisfactorily and that the end result is safe for living. In future when it comes to selling your property, the difference between having and not having this certificate is vast. The solicitor for the purchaser would point out if these works were not approved, if so, they cannot be deemed satisfactory or even in some instances – safe.

Clarity is vital for informing the contractor of the proposal, so we have evolved our drawings over recent years to achieve such clarity.

What if I need steel support beams installed?

If any partitions are to be removed in the proposed scheme, these can often be difficult to get into the basement in one length. An engineer can provide details so that a beam can arrive at site in sections and then bolted into the finished length on site.

Steel reinforcement is often required either in the new ground slab, underpinning or both. British standards and Eurocodes give guidance of how much gap (cover) to provide between the edge of the concrete and the reinforcement. An engineer should be proficient in specifying such detailing. Relying on the judgment of an untrained person can result in steelwork with insufficient cover and therefore increase the risk of corrosion. Remedial works to combat corrosion of steel reinforcement can be very expensive.

Up or Down?

Loft conversions have been very popular over the past few decades. We believe basement conversions, with the correct design & build team, can be just as popular if not more so. Choose Think Basement to get your conversion to the finish line!

You can find THINK BASEMENT online: or contact us: or 01142 583500.

You can find Signature Structures online: or contact us: or 0114 3450381.


Relatived Posts
Going Underground… ( 8 Jan,2014 )
Welcome to our new website! ( 13 Dec,2013 )
Basement Conversions: from the Structural Waterproofing Consultants view ( 23 May,2014 )
Basement Conversions: from an Architects view ( 21 Feb,2014 )
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