[Grubbypaws]

I am just about to build a new raised bed but for the first time this one will be onto a hard surface; asphalt.

I have bought a liner for it as advised by T&M but am unsure about drainage. Should I put some gravel in the bottom or will this hurt the liner?

[sunshineband]

I would have thought some sort of drainage layer would be essential 

Gravel would not damage most liners

[Ice]

You will also need a few drainage holes in the material you build it with. 

[simonwatson]

I take it the liner is of the permeable type? You will need some gravel in the bottom, as you want to have some air gaps for the excess water to drain into. This in turn needs somewhere to escape to, so a couple of drainage holes in the bottom at the same level as the gravel.

If you leave the gravel out, it will drain, but much slower and will be prone to getting waterlogged.

[Grubbypaws]

The liner is purpose built . The RB is going to be on the patio, hence the need to keep it a bit clean, otherwise I think my permission to do this will be rescinded   

http://www.harrodhorticultural.com/HarrodSite/product/Raised_Bed_Gardening_Raised_Bed_Essentials/GDN-620.htm

I wouldn't have thought that I should be putting holes in it   

The picture shows it being filled with earth 

[simonwatson]

I see what they're saying on the site, but I suspect (I admit I haven't done this myself) that if you build it as they say on a hard non-permeable surface like a patio, that in very heavy rain it will end up waterlogged.

The trouble with doing a RB on a hard surface is that you lose all that lovely soil underneath it that acts as a natural control on the drainage. When the underlying soil is drier than your bed, it will accept more water. When the underlying soil is wetter than the bed, the flow out of the bed is much reduced.

I think it will always be either well drained and therefore dry out easily, or waterlogged, as the patio surface underneath never changes its nature no matter how wet or dry things are.

Good luck.   

[Grubbypaws]

Trouble is I have run out of space. The patio is all that is left and until now I have not had permission to use it!

Do you think that it is a waste of time then? I could grow plants that are very thirsty like peppers and courgettes.

[Ice]

Make drainage holes around the base of the raised bed before you put the liner in.  Gravel will help prevent the water at the bottom from going sour.  Take no notice of them lifting a partially full liner out saying it's easy.  When full it would take a JCB to shift it.

[simonwatson]

I don't think it's a waste of time, but I do think it will be higher maintenance than with raised beds on soil.

I think if you do it, you need to wait and see how it behaves before deciding what it's best suited to growing.

[Vit]

Get permission first. On paper. Istead of gravel i'd use rough sand or thin scoria same as landscapers using for drainage. Basically your RB should be same as huge pot  so treat same, but scale