Roof garden planters ....

Hi, I am putting together a roof garden on an existing decking patio, and would like to border the garden with plants at about 24 inches from the deck floor level.

I intend to build a wooden frame like structure to do this with a tray like inner, with which I will fill with soil etc, and top of with pebbles and stones etc.

Q1. How deep do I need the soil to be in order to allow plants to grow satisfactorily.

The trade off is that I do not want to fill the borders to the full 24inches due to weight.

Q2. Do i need to have some sort of drainage in the trays as well, how do you do this effectively, i.e. do i need a porous liner ?

Hope soneone can help.

kind regards, Simon.

[ May 13, 2005, 01:40 PM: Message edited by: simonwar ]

 

Hi Simon
Have you had the roof checked to see if it will bear the load? Soil (especially when wet) + plants + pebbles/stones + the containers themselves are going to be very heavy. You need a structural engineer to have a look especially if the "underneath" is occupied.
Having said that, you can reduce some of the weight by part filling the planters with polystyrene. How deep to have the growing medium (use soil-less medium -its lighter)depends on what you want to grow. For shrubs & the like you will need a minimum of 18 inches. Perennials, grasses, annuals, bulbs, & especially alpines will do with less. Providing the flooring of your roof/deck is sound, and you have facility for water run off (a channel/gutter) then you should always have holes in the base of your planters to enable good drainage. Alternatively, drill holes about 3cm from bottom along the sides and fill the base with charcoal to this level.
what material are you using for your inner tray? - plastic is light, waterproof & doesn't leach water from the medium (& would be hidden presumably by your wooden shell) What sort of "look" are you trying to achieve? Let us know, so you can have some plant suggestions

 

Sorry, just seen you are an engineer, so you've probably sussed the weight bearing business!

 

Hi Bayleaf, thanks for the reply.

Contemporary and Continental
The look I'm after is contemporary with a bit of continental, (therefore clean lines, straight edges, white walls, using lights, water features, and a pergola over the top).

Loadings
Loadings are a major consideration, only because i did not assemble the decking myself and therefore can make only rational assumptions about its loading capability.

I am thinking of making the unit about 18 inches to 2 foot high and about a foot deep. It will run along a wall about 8 foot into a corner and then out along the corresponding wall about another 8 foot.

(This works out at about 0.72m3 in volume, so if soil sodden with water is approx 1 tonne per cubic metre, this is given me a guideline loading of about 10 people in weight).

Wall Mounted Planting Unit
To this end I am intending to mount the planting unit off the wall as opposed to sitting it on the deck, (a bit of engineering involved, but not too difficult really).

Planter and Bench Seats in One
Again, due to loading i am intending to make a structure that acts as a support frame for the wooden planting box and a set of seats, so this corner arrangement will be bench like seating with plants and things behind.... if you can picture it ?

The Look
The walls making the corner are rendered, and at present are light ash, (magnolia), in colour, so i was thinking of trellis and climbers to create a warm backdrop, leading into the corner, which I am thinking of cascading up a tier to create a focal point and create platforms

We do like pebbles and stones as opposed to soil, but are not sure if this is possible if we have 'soil and planting' as opposed to 'pebbles and plants in pots'.

In amongst all this and possible as part of the corner section a water feature providing some soothing burbling would be perfect.

Materials
The structural framework will be a 3" x 3" treated timber, giving me the rigidity and strength.
The planting frame will be 2"x2" treated timber lined externally with treated ply (12mm thick).
I have not determined the internal liner, so any recommendations would be much appreciated.
The Bench will be decking style timber that wraps around the structural framework.

Extras
I will add lighting; amongst the plants, under the bench, hanging from the pergola, in the deck boards, etc.
I will no doubnt have a few free standing plants in pots, some seating, tables, etc.

Thanks for your details, and thanks for any further do's and don'ts.

Simon.

P.S. What does the charcoal do?