Ballpark cost to convert flat roof to roof garden?

Spanky_Deluxe

Hi all,

Thinking ahead a while, my wife and I thought it might be nice to convert the flat roof of our three story townhouse (about 440 sq ft of roof) into a roof garden. We're quite high up anyway so the views would likely be pretty good. Plus it would get plenty of sun and would be a perfect place for a (dream) hot tub.

The only access to the roof at the moment is a small ceiling window in the bathroom but there is the space above the existing stairs and if losing a small part of one room to add a staircase up there.

How much do these kinds of things usually cost? New set of stairs, possibly reinforcing roof etc?

I haven't been up there yet but as it's a flat roof it will need recovering at some point in the next few years so if we can budget in advance for turning it into a roof garden, it might be a good idea to just do that instead.

Thanks!

phill99

Probably more than you think. You will need a structural engineers report, especially in terms of roof loading given that you wish to put a hot tub on it.

You will highly likely need steel work on the roof, so be prepared for more expense than you envisaged.

alex.kemsley

I think the roof garden itself wont be a problem, apart from checking that the flat roof can be walked on. Might need some waterproofing. Hot tubs weigh about 1 tonne - 3 tonnes depending on their size. To work out add dry weight of hot tub to number of litres in kg.
e.g dry weight = 350kg
capacity - 1500L = 1500kg
total weight = 1850kg
plus people x 5 (75kg av) = 375kg
total weight = 2225kg (2.225 tonnes [metric])

Normally spread over an area of 2m x 2m (4m sq)
556.25kg per m2

Then take that to a structual engineer!

For size and weight of tubs see: hottubs2buy.co.uk

rxbren

needs to be assesed by a structural engineer and more than likely you would have to get planning permission for this conversion
how much stuff are you planning on putting up there
the roof may not be suitable for the added weight so at worst case a new flat roof with new joists/rsj's
on top of that you would have to have a railing/barrier around the roof
then the breakthrough from an internal room to the roof

Spanky_Deluxe

Thanks all for the feedback. Yeah, I guessed a structural engineer would need to be involved - we're working with one at the moment to take out a ground floor wall. I imagine the engineer's plans would be a very small part of the rest of the conversion. Any ideas how much putting a new staircase in would be and how much new joists/rsjs and preparing the roof for tiles would be? I mean are we talking thousands or tens of thousands? Stuff like actual tiles and railing/barriers are quite easy to work out.

I'm just wondering if this kind of thing is a £5k, £10k or £20k+ kind of job - i.e. whether it's a save-up-as-we-go kind of idea or a borrow-more-come-mortgage-renewal idea. If it's towards the lower end then we might do it sooner, if it's the latter then it would maybe get lumped into our House 2.0 plans in a few years when instead of moving home, we'll dig out and convert the cellar! It's always good to have goals!