It is possible for the works to the drainage to be carried out by the groundworkers at the same time as – or following straight on from – the foundations. After all, diggers will be on site and the same lorries that remove the spoil from the foundations can take away the excess from the drainage trenches. But there are some drawbacks.

Firstly, the work to the drains may interrupt or preclude any works to the superstructure. Secondly, the likelihood is that the newly excavated and backfilled trenches will find themselves beneath scaffolding, and that could present bearing problems for the uprights. It could also mean that the point loading and compaction of the ground beneath the scaffolding will adversely affect newly laid drains.

On the other hand, those building with a timber frame, where there is a long period between order and delivery, might choose to use the intervening time getting the drainage done and then take the necessary precautions to prevent damage.

In most cases, however, drainage is best left until the scaffolding is down, and often the groundworkers can move on from the drainage works to the landscaping and driveways, which can’t really be done until everything else is practically complete.

Works to connect the site to the sewer in the road would already have been accounted for, as these works can only be carried out by approved contractors and will vary. Below, however, is an example of typical connection costs for a home in an average village street with all mains services to hand.

Water and sewerage £2,112.00
VAT (payable only on building water supply and recoverable) £15.85

Laying Drains

Drains must be laid in trenches to a fall of 1:40, surrounded by and covered over with clean pea shingle before the trenches are finally backfilled with spoil. Soakaways for surface water drainage from guttering and downpipes etc. are usually 1m³ holes, which are filled with clean hardcore to within 100mm of ground level and then covered over with a square of membrane with topsoil over. All pipes, fittings etc. are assumed to be 110mm plastic type, suitable for underground use.

The Costs

Excavate 88m drains (max 900mm depth): ganger and labourer for 3 days @ £221.04 per day £663.12
Excavate 5 soakaways: ganger and labourer for half a day @ £221.04 per day £110.52
Load lorries: ganger and labourer for one-and-a-half days @ £221.04 per day £331.56
Digger hire for 5 days @ £200 per day £1,000.00
VAT (not recoverable) @ 20% £200.00
3 loads of muck away @ £200 each £600.00
VAT (not recoverable) @ 20% £120.00
30 3m x 110mm pipes @ £14.90 each £447.00
VAT @ 20% £89.40
9 tons Type 1 hardcore for soakaways @ £13 per ton £117.00
VAT @ 20% £23.40
5 gulleys @ £38.29 each £191.45
VAT @ 20% £38.29
3 rest bends for SVP (soil and vent pipe) @ £12.36 each £37.08
VAT @ 20% £7.42
2 branches @ £18.23 each £36.46
VAT @ 20% £7.29
24 110mm drainage collars @ £4.88 each £117.12
VAT @ 20% £23.42
24 110mm drainage bends @ £14.50 each £348.00
VAT @ 20% £69.60
5 manhole inspection chambers @ £147.27 each £736.35
VAT @ 20% £147.27
5 manhole covers @ £48.79 each £243.95
VAT @ 20% £48.79
Ganger, semi-skilled labourer and labourer for 8 days @ £328 per day £2,624.00
Drainage total £8,378.49
(of which recoverable VAT) £454.88

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How to reduce these costs

  • Dealing with plastic-type drainage isn’t that difficult and, although it’s dirty work, you could carry out all or most of it.
  • If you know how to use a level it’s easier to correctly lay the pipes, but alternatively you can work back from the connection to the main sewer using pegs and calculating the rise at 1:40 (i.e. for every 40m of drainage run, the drains must rise or fall 1m). At a pinch, drainage can run at 1:60.
  • Pipe comes in 6m lengths, and as they are laid on 150mm pea shingle it is relatively easy to make sure that there are no dips. The pipe must also be surrounded and covered over with 150mm pea shingle before backfilling. This is labour intensive but can often be done by the self-builder.
  • With a beam and block ground floor, it is relatively easy for the drains to run beneath the building and that may save on not only drainage runs, but manholes.

Our Benchmark House

The floorplan for the house costed within this series measures up at 227m² — much larger than the average family home. Four bedrooms on the smaller first floor share three bathrooms. Downstairs, the living spaces flow easily onto one another, with an open plan family kitchen being the hub of home life.

All prices correct at time of writing, November 2011

Special thanks to Estimators Limited (estimators-online.com 0161 286 8601) and Design & Materials Limited (designandmaterials.uk.com 0845 404 0400) for their help in the preparation of this series of articles.

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