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Compost weights for a roof terrace

Hello - I'm hoping someone might be able to help.

I am currently specifying planters for a roof terrace and need to give loadings to the structural engineer.

I have established that I can use a substrate mix in a roof terrace planter of roughly 1/3 Hydroleca to 2/3 compost with a drainage layer of Hydroleca in the bottom of the planters.  I have been given the weight of the Hydroleca per cubic metre but am struggling to find information on the saturated weight of the compost (taking into account that the planters will have good drainage).  Ideally I would like the planters to have an integral irrigation system, so again I'm not sure how I find out the weight implications of this.

Any information would be great.  The planters will be constructed from powder coated aluminium, so should not add a huge amount to the overall weight of the planter.

Thanks in advance!

melissa

Garden Designer

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Replies

  • Can you not ask the technical department of the manufacturer? They should know that sort of information I would have thought.
  • I would suggest using a scaled down container/model and filling it with quantified amounts of soil/planting medium and adding water then extrapolate from there.
  • soak a bag of compost and weigh it. very heavy!

  • PRO

    I don't the exact answer to your question Melissa.

    I agree with Kieran, maybe a call to the manufacturer or even buy a bag of what you are using and conduct an experiment.

    Just a quick calculation. If, for example, your containers are 75 litres in capacity. 33% (1/3) of this volume (Hydroleca) is 25 litres, which means the remaining volume would equate to 50 litres, or 50 kilograms if filled only with water.
    Taking a view that the Hydroleca will have some pore space then you might at 10-15% to the 33% volume/weight.
    Taking another view, the compost will displace some of the 50 litre/kilogram and conductivity will reduce the water quantity even further.

    In summary, the 66% value of volume cannot exceed the equivalent if converted to kilograms.

    It's worth buying a bag or two and trialling this.

    In respect of info available of the internet...I can't find much.

    I found this - measuring the water content of compost

  • PRO

    Hi Melissa, I can't help myself but I will share on twitter as I think I know one Garden designer that specialises in roof terraces who may be able to help. Susan

    • Thanks very much for your help everyone.  It seems to be very difficult to find out and I've spoken to a few manufactures and they have all sent me detailed specifications but none give the weight per m3.  I'm also speaking the structural engineer about it, so hopefully together we can make sense of it.

  • just weigh a known volume dry and then wet and interpolate it up

  • PRO

    A structural engineer or good Surveryor would be my first port of call, especially if i needed that fact confirmed formally in case of future liability(ies).

    Otherwise, dig out the little blue "Architect's Pocket Book" or the little green "Landscape Architect's Pocket Book" - which, if I remember correctly, has pages of  'material weights' in various situations. I am also sure Spon's details this aspect.

    As example, I remember that damp soil has a weight of just over 2T per M3.

    • Thanks Gary, 

      The structural engineer on the project can hopeful help.  Thanks for the book info, that will be really helpful.

  • Hi Melissa, having had to do this in the past, I have used the following method:-

    Take an empty 1 litre carton (such as milk or juice) and weigh it to find out the empty weight.  Then fill it full of the compost or planting medium you are going to use.  Pour water into the filled carton until no more water can be absorbed and the compost is fully saturated.  Then weigh again and subtract the initial weight of the carton to find out the weight of 1 litre of fully saturated compost.

    1000 litres = 1 metre cubed, so you can now work out the weight of 1 metre cubed of fully saturated soil.

    This may seem a bit Blue Peter, but it is the only method I found of making the necessary calculations.

    The structural engineer should be able to tell you what the maximum weight loadings allowed are.  Also we used planters constructed with a false internal bottom to minimise the amount of planting medium used.

    Hope this helps!

    Sarah 

    Hampstead Garden Design

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