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  Kieran Ray December 12, 2014 at 4:06pm

  Can you not ask the technical department of the manufacturer? They should know that sort of information I would have thought.
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  Green Hand December 12, 2014 at 5:12pm

  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.
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  Thermo December 12, 2014 at 7:58pm

  soak a bag of compost and weigh it. very heavy!
• PRO

  Phil Voice December 13, 2014 at 8:56am

  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
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  Ayegardening Ltd December 13, 2014 at 10:34am

  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
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    Melissa Jolly > Ayegardening Ltd December 13, 2014 at 2:43pm

    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.
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  Thermo December 13, 2014 at 4:47pm

  just weigh a known volume dry and then wet and interpolate it up
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  Gary RK December 13, 2014 at 5:09pm

  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.
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    Melissa Jolly > Gary RK December 14, 2014 at 4:57pm

    Thanks Gary, 

    The structural engineer on the project can hopeful help.  Thanks for the book info, that will be really helpful.
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  Sarah Oxby December 15, 2014 at 9:20am

  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