Monday, June 09, 2008

Shed soakway

Shedworking reader Andy writes:
"Just read your "Is your garden office a flood risk?". I was thinking of using the shed roof to collect water for the garden and wondered if you had any thoughts on that? I was thinking that I might need to put in a "soakway" next to it as there would be nowhere for the rest of the water to go once the waterbutt was full?"
Could you roofing experts out there give us your opinion please?


  1. Anonymous3:22 PM

    Hi Alex,
    I've sent you a picture of one shed-builders answer to this problem. Add another barrel...then another etc.

    Seriously however the answer depends on the size of the shed, the permeability of the surrounding ground and local topography.

    The larger the shed, obviously the larger the volume of run-off.

    When designing a soak-away for any building project how quickly the rain is absorbed into the surrounding ground is a critical factor. Sandy and gravelly sub-soils absorb water quite quickly whereas clays are almost impermeable (they are used in the core of for dams). In a residential project typically a hole would be dug and filled with a specific volume of water and the time recorded for the water level after specific time periods. From this data and a knowledge of the expected volume and duration of rainwater run-off (from statistical charts) the size of the soak-away can be determined.

    The location of the soak-away itself must also be considered. Generally a soak-away would not be built within 5 metres of any construction due to concerns that it could ‘soften’ the foundations and cause subsidence.

    Having said all of that though for a building the size of a typical small garden shed an overflow into a nearby flower bed should be enough.


  2. John, thanks for the update, given that my soil is clay and that I've a few flowerbeds and trees nearby I think I'll just send the "spare" water in the direction of those via some drainage pipe? I don't want to build what is effectively a large hole that my workshop might fall into.

  3. John, no pic has come through - could you try again please?

  4. Anonymous4:29 PM

    I don't seem to be able to post an image into the comments box? Instructions please.

  5. A couple of comments from soggy Seattle:

    1) What you guys are calling a "soak-away" is probably what we call a "drain field." Our experience is that they rarely work well, especially when installed in clay soil areas. The problem is that they are needed most when rainfall is really heavy, and unless they are unusually (i.e. expensive) large, they are commonly overhwhelmed just as the upstream container is. The only sure solution is to mechanically pump the excess runoff to a major storm drain.

    2) For a small structure like a shed, the simplest solution may be to distribute the excess by hand with a bucket. This has the advantage (well, I guess it depends on your perspective) of providing a little exercise as well.

    Cheers.....Bill Kratz

    P.S. This is exactly the sort of post and responses that I would like to see more of on Shedworking....discussions of design and construction problems and solutions, which do indeed mirror those of larger buildings.