Shed living roof

I'd have thought the problem was the weight of the wet soil and plants on the poor old shed, unless you are going to use concrete posts all around it to take the weight, and something very strong as the base to stop it sagging in the middle. Brian

I have seen a few of these around on extensions and frankly they all seem to end of as a mossy mess and for most of the time looking anything like living or green

Richard

Yes. Let's say the roof holds just water. Water weighs 1 tonne per cubic metre, so a 6" depth would weigh 150kg per square metre. Then there's gravel and soil up there as well. Density of rock is 2-3 tonnes per cubic metre, so let's guestimate 2 (there will be pores filled with water, and plant material has density <1). So we need to design for 300kg/m2 + margin. And then if somebody might be standing on it they might weigh 150kg, so that's SWL 450kg/m2 (which probably covers the margin).

If the roof is about 5m2 that needs to hold 2.5 tonnes, not including the weight of the roof structure itself.

You can build a deck to handle that (do the beam loading calcs - probably need to space the joists quite close together) but if this is a timber shed I would question how strong the walls are?

(Without doing the calcs I think you can do it with timber framing, but would need chunky timber posts not typical garden shed battens)

One resource that might be worth trying is 'snow loading' calculators, because this is roughly the same problem as having a load of snow dumped on the roof in the winter. And that's something many roofs have to be designed to cope with.

Theo

Adnams bottling building at Southwold has this. Never seen it anything other than khaki/brown:-(

Unless it is very very strong your shed will fall down then. Do the load calculations! This isn't a bad guide:

More or less perfectly wrong. (unless your shed roof is incredibly well reinforced)

The living roof plants that will work reliably tend to be semi arid desert or alpines that need very little soil ~1" and will grow in pumice or perlite/perlag with a trace of topsoil or humus.

Sempervivums, sedums and that sort of thing.

Beware that some of them will set seed and will also colonise your gutters and house roof in addition to where you want it to grow. eg.

Thanks - I'll do some calculations and see what comes up. The shed itself will be strong in compression - it's more the 'foundations' that'll be the unknown (at the moment large bits of postcrete left on the site).

This is a shed roof on an allotment.

Thanks - I'll take a look

I've yet to see a living roof live up to all the hype. The local council built a "green" building in one of the local parks with one of the living roofs. The problem here in Essex is possibly the lack of rain, or more importantly enough rain in a timely manner. We had no rain for around 3 months which included the record temperature summer days. The roof on the building isn't green and now that we have had some rain seems to be germinated blown in weed seeds.

As for Brains comments, it is a factor to be considered even on a traditionally build house roof or balcony. It's the weight of a fully saturated living roof bed that needs to be considered and not just the weight of what you put up there.

I had a green flat roof. Half an inch of gravel that hadn't been touched for years. It was enough to collect windblown soil and moss to grow. Of course the moss absolutely loved the rain in the winter. In summer it dried out, given the lack of rain.

Roof loadings weren't too bad, and it reduced gutter overflows in heavy downpours.

You don't want to do this if you have leaky felt, though.

Theo

Question for which I don't have a definitive answer...

Could something like a living roof that may remain wet/damp (or frozen) for months in the winter actually cause condensation on the underside of a un-insulated wooden shed roof?

I occasionally see some of those make-over garden shows and wonder how long some of that wooden decking, replacement fencing and pergolas are going to last - especially when seeing cut timber being inserted into holes and after the postcrete being backfilled with earth. As for earth retaining walls and raised being built of softwood sleepers....

Possibly. It would depend on a source of warm air from inside the shed. Warm moist air hitting the underside of the cold roof would condense. To make moist air you'd need some source of dampness inside.

Possible sources:

- people

- heaters

- appliances (eg freezer, tumble dryer)

- hot power tools

- sunlight through walls/windows

- soil

- plants

If this is just a tool shed without power, you wouldn't really have the first four (nobody will stay in there for long).

You would get sunlight, but it would only evaporate moisture that's already in there. For example, if you put in a lawnmower covered in damp grass, that would contribute moisture. If the shed is well ventilated then the moisture should disperse. Although if the shed is bedded on soil or is not watertight you could get moisture ingress that way.

If it's a potting shed and full of plants in damp soil, that would be a problem I agree.

However this is easily solved by insulating the roof, because you get exactly the same problem in a house: people breathe out moisture which hits the cold ceiling, causing condensation and mould.

Theo

Can't see it staying frozen for months in the UK unless its very high and quite northern.

Only if the underside surface is below the dew point which is unlikely.

Actually isolated from the ground it will probably warm up pretty quickly although being damp it will always be marginally colder than ambient. It depends how close to saturated the air is whether or not there will be condensation on the underside of the roof.

A comparatively thin layer ~3mm of polystyrene insulation under the DPM would be more than enough to prevent it being a problem. I used the thin foil backed stuff on the inside of my S facing garage door to make the interior cool enough to work in during the summer. Without it the whole space would end up like an oven by nightfall. The only downside was that the metal door would be too hot to touch at midday in mid summer.

It didn't make much of a difference in winter.

Show gardens are designed to survive for about 6 weeks.

As are some of the very expensive garden makeover with buy and die very expensive large trees that really don't like being moved and probably won't be looked after adequately by their new owners.

That is because they need a lot of maintaining, since the depth is poor the soil needs to have its nutrients constantly renewed. The way forward is hydroponics. This does work, but does or can get clogged and is prone to damage from the elements as there is little to hold the plants solidly. Brian

One idea is a kind of terracing, effectively making the roof into ascending gutters draining into each other. Trouble is the more complex it is the more things to go wrong and more heavy things get. Maybe in the end you would just use it to generate power instead? Brian

You know what they say, the difference between plants are weeds is that weeds are plants in the wrong place. I was kind of thinking about grass and wild flowers during the year. Still heavy though. I recall the hanging strawberry farms of Guernsey inside a greenhouse and you could sit and eat below the growing containers. So maybe the answer is a shed with an upstairs greenhouse. Brian

did this long ago. As folk have said, soil, water etc are very heavy, so one uses as thin a layer as possible. That neans it needs very frequent watering unless you're only growing desert plants. Which is hardly ideal for something away from home. In the end I found a climber grown from the ground made a better roof covering.