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DRAINAGE UNDER NEW FOUNDATIONS

Building an extension out from the main house wall by 3 meters, 4 meters or 5 meters? Getting plans approved and passed by the council and then ready for that build? well in some cases there are no signs of manholes or records from Thames water of drain run locations until the builder starts excavating. Very rarely the drain run is located at the exact offset from the house as the proposed extension is, for example, the proposed single-storey extension has been designed to be offset 3 meters from the rear house wall, the builder digs the footings and finds a drain run right in that location crossing the whole length of the rear foundation extension wall parallel to the houses. This article will explain the ways around this obstacle as no foundations can be laided on top of any drain run.

MY DRAIN RUN IS RIGHT UNDERNEATH MY PROPOSED FOOTINGS WHAT DO I DO?

When carrying out surveys it is not possible in some cases not to see the drainage arrangement such as manhole locations of the drainage existing run. This could be because of a number of things, the drainage system may be hidden under decking or covered over by a patio or fake grass etc. New homeowners have moved in and they do not know the drainage arrangement until it's exposed by digging out.

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Buckhurst Hill - Single Storey Rear Extension 3 Meters

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Yellow Arrows indicating drain pipe location

The client was unaware that a manhole and drain run was located under the decking parallel to the house at a distance of 3 meters which was the same depth as the proposed extension. Thames Water did not even have records of the existing drain run on their system. It was only when the builder lifted up the decking to find a manhole and dug a trial hole to identify the angle of the drain pipe that the problem was raised.

THAMES WATER DO NOT ALLOW ANY FOUNDATIONS TO BE BUILT OVER ANY PUBLIC SEWER RUN

We advised the client to apply for a build over agreement even if Thames water has no records, it's compulsory to get this done to get the approval from the development team as they will need to see the alternative construction methods involved in agreeing on the foundation methods proposed. An Engineer was introduced to design a Pad and Steel Beam arrangement that involved steel beams sitting on mass concrete pad foundations to accommodate the external extension cavity walls as a means of support. This method was introduced to avoid any loading from the extension away from the drainage system. New drawings were designed showing connection detailing and a full set of calculations were produced for building control to approve. Thames water agreed on the design and issued a certificate of acceptance and the project started. There was a slight delay in getting these elements resolved due to various parties being involved, causing a pause on the build until all was accepted by Thames water and building control departments. The Engineer needed to be paid for his services for designing the pad and beam design so additional money was spent on engineering input, this design was unavoidable and needed to be done anyway regardless of spotting drainage issues at any given time. The Architect when designing extensions designs the foundations to be a standard strip footing type being 1000mm deep by 600mm wide. It is never the case where a Pad and beam design is designed by the Architects as the Engineer needs to design the size of the beams and arrangement as this is his area of expertise. It is also not the job of the Architect to carry out a drainage survey to establish the drainage layout, this is the job of a drainage company that carry out CCTV drainage reports which will be of cost to the homeowner.

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Yellow arrow indicating drain pipe

Thames Water Build over approval

needs to be in place

Building Control Approval

Plans Check Certificate

Engineering Design in place

with supporting Calculations

PLAN OF PAD AND BEAM ARRANGEMENT

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PAD AND BEAM SECTION

SECTION SHOWING DRAIN PIPE

AREAS THAT NEED ADDRESSING WHEN 
DEALING WITH DRAINAGE MATTERS:
 

  • DRAINAGE CCTV SURVEY CARRIED OUT 

  • THAMES WATER BUILD OVER-APPLICATION

  • ENGINEERING DESIGN CREATED

  • BUILDING CONTROL APPROVAL

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NO BUILD CAN START WITHOUT ALL THE  ABOVE PASSED AND APPROVED

Remember that if you are choosing more than one design you will be offered a further discount on any other second design.

We will not be beaten on price and if we are we will offer you a price match service in design* subject to agreement.

Please note that you will require structural calculations if any steel beams are designed within your scope of works, this will be charged separately to the engineer.

EACH PROJECT WE TAKE ON IS STARTED FROM SCRATCH SO EVERY NEW JOB IS DESIGNED WITH CARE AND EXPERTISE TO GET 100% SATISFACTION

I hope you understood this article and find it useful for your proposals we just want you to get the right results and not to make a mistake in doing the build the wrong way around as this could lead to further unnecessary delays. Regards John.

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