The Brew Cabin

Posted: **Tue Oct 05, 2004 9:43 pm**

We have a terraced town house. The rear is downhill of a garden and paved patio, with heavy clay soil. When the former owner added an extension on the rear he, uniquely amongst the 6 house-owners, was forced to have a soakaway. It must have been a temporary fashion, as all the neighbours later added extensions and were allowed to direct the extension roofwater down into the existing water drain.

We learned before buying that the former owner at one point had water backing up the from the soakaway, up the downpipe and cascading over the guttering above the extension. He agreed with the neighbour to re-direct the main-roof rainwater into a hopper so that rainwater from the main roof went the same way as the neighbour's extension-roof rainwater.

This worked OK, but recently after very heavy rainfall the water again overloads the soakaway. I have added a water butt so that some of the extension-roof rainwater goes into that instead, but the extension roof guttering still on occasions fills right up and only slowly drains away after the rain has stopped. Rain sometimes seeps into the extension at floor level near the patio doors.

The patio has a key-channel near the extension, separated by a 3" filler of paving that is badly laid, so soil gets in and weeds come up.

What improvements can I make? We are having new double-glazing and I should repoint the brickwork and fill in between the paving to try and keep water out. I have thought of another water butt at the other end of the guttering, but that wouldn't have an overflow and couldn't be allowed to fill up completely. Maybe a linear channel, but where would the water go, except to the gulley leading to the soakaway? Maybe a sedum roof - a nice green idea, but the extension roof would then have to cope with a 90kg per sq. metre saturation weight. I'm not sure how I find this out.

I think it is too late to do away with the soakaway and achieve connection to the original drain - it is under the extension, with no access. Why were soakaways ever mandated in heavy clay areas, clay being used to store water from ancient times?

Thanks for letting me get this off my chest - a great site as everyone says. The drainage does cause increasing worry in these rainy days. When we were buying, the solicitor warned that the former owner's agreement with the neighbour had no legal force and if ever the neighbour had his own drainage problems he could withdraw his permission to direct main-roof rainwater down his drainpipe. But I've always thought this would be cutting off his nose to spite his face, as the water cascading over my extension would also affect his property; and anyway half the main-roof rainwater is his and did used to go down the common downpipe in days before the extensions.

Any advice will be welcome.

Posted: **Wed Oct 06, 2004 12:55 pm**

The water butt idea is not going to help very much. What happens when it's full?

If you adapted the 'storage' idea, you could have what is known as an 'attenuation tank'. Basically, this is an underground water butt that stores water while it's ipssing down, and then gradually releases it into the ground, a kind of slow-release soakaway, in effect.

However, before rushing off to buy an attenuation system, you need to determine what size of system is required. This is affected by the permeability of the ground itself, and the size of the hard area to be drained (paving and roof). To explain it all in simple terms, you need to establish at what rate water can be absorbed by the existing ground, and then work out how much water you can expect to put into the system during a storm event. You add a bit extra as a 'safety factor', and the difference between how fast you can dispose and how fast it comes in tells you what size of tank is needed. You'll be relieved to know that the lovely people selling the attenuation tanks are prepared to do all the sums on your behalf.

These tanks can be quite big - several thousand litres capacity is the norm, and installation often involved MASSIVE disruption to your garden area. They are normally fitted to new-build properties, where no-one is immediately affected by the installation, but there are sites where retro-fitting has taken place.

Your only other options are either a new soakaway, and that would require you to ensure that the soakaway could function properly, or finding some way of linking up to the existing SW system. The soakaway idea could be considered a cut-down version of the attenuation system, and you'd need to give serious consideration to which would be best. Given that you are on a clay, it's quite likely that a soakaway would have to be almost as big as the attenuiation tank to be guaranteed to function properly.

Connecting to the existing SW goes against everything that we're trying to do with SUDS, but as green as I like to be (environmentally speaking) there are circumstances that for geological, structural, or economic reasons, using a SUDS-friendly option is just not feasible, and this may well be one of those. Faced with, say, a 3,000 quid bill for installing a new soakaway or 1,500 quid to install a connection to existing, which would you choose?

Posted: **Thu Nov 04, 2004 8:06 pm**

If you have the space, you might consider landscaping your garden so that part of it acts as an 'emergency storage area' in case the soakaway fails and floods. Assuming you have a bit of spare land, you could excavate out a low point in your lawn/rosebed/whatever and then set up a pipe or open channel network to direct any overflow from the soakaway to this point. It wouldn't get rid of your problem entirely, only contain it, but at least it would be in a location where it can't cause any damage until it eventually soaks into the ground and/or evaporates.

Of course, you would have to also ensure that if this also flooded then it wouldn't cause further damage to your property of anyone elses.

I am not sure if the idea in the previous post is really valid. What it suggests is, in effective, a larger soakaway. If ground conditions are not suitable for a soakaway now then there is no guarentee that simply putting in a larger one is going to work. Or it might do. It depends on the ground conditions so you ideally need to ascertain what they are before you decide the best course of action.

-Richard

The Brew Cabin

Inefficient soakaway - Problem with rainwater