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Could someone kindly help me with some advise. I have 25 mtr square roof on my workshop,I would like to know is it possible to build a 8x4 feet raised bed , i intend to put glass on top of the raised bed,I would like to make it about 4 feet high,so as i dont have to bend down to much,What way would i have to fill my bed,ie,for draingage ,what materials to use,I need the full rundown on everything that i need to complete this project,my roof will take the weight that is involed in this project,all help is greatly appreciated.

                                    Yours Kindly Paul


  • I’m not sure I understand what you are building if it will have glass on top, do you mean a set up similar to a cold frame? Also when you say your roof can take the weight, presumably the calculation was based on certain assumptions on materials or total weight and the weight distribution. The builder/supplier should be able to advise on the calculations and weight tolerance/spread which you can then use to help decide on materials. 

    I did help plan a roof garden some years ago. The above information dictated the layout as certain areas could take more weight than others. Other factors we had to consider were how the drainage from our containers would work, drainage within the containers is the same approach as at ground level. Do remember to factor in your own weight as well (and be sure to know if you can have anyone else up there with you or not, for construction and afterwards)

    Also remember to factor in the weight when the soil/composts are saturated as it’s significantly higher. 

    There are composts and drainage mixes designed for roof gardens which are lightweight. And the general advice fro containers is to go for lightweight and non porous

     If you have a garden and a library, you have everything you need.”—Marcus Tullius Cicero
    East facing, top of a hill clay-loam, cultivated for centuries (7 years by me). Birmingham
  • NollieNollie Posts: 7,511
    The weight of a solid, raised bed 8x4x4 foot filled with damp soil and plants will be phenomenal, I’m not sure any roof could take the load of that. Or do you mean something like the photo below, a bed/cold frame with glass on legs, but bigger?

    It also depends what you want to grow in it. Are we talking raising seeds, growing annual vegetables and salads or permanent planting?

    Apart from raising seedlings and young, tender plants, glass should not be necessary. If frost is predicted a horticultural fleece or plastic cloches would suffice and help keep the overall weight down.

    Most veg need a depth of around 18” and the raised container would need to be sturdily constructed and lined with plastic (old compost bags stapled to the inside work well). You would need drainage holes drilled through the timber/plastic liner and possibly a drip tray below. This depth would be fine for most perennials and some dwarf shrubs too.

    If you want a deep, solid construction, consider breaking the volume/weight up by splitting the desired planting area into four separate constructions, say four beds of 3x3’ or 4x2’ and placing them strategically over supporting roof joists, spaced out over the roof to spread the load. A series of smaller beds also have the advantage of being accessible from all sides, easier if you have difficulty bending and stretching.

    Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
  • In your weight calculations before starting this, have you taken into account the weight of rain water and a dump of snow as well as timber, soil, glass etc.
    Perthshire. SCOTLAND .
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