Roof terrace advice

[ljn]

I have a 60 square meter flat roof which I want to use as a roof terrace with decking and large planters.

My concern is that the waterproofing capsheet layer and the insulation under it are soft and that any decking supports I put on it would press into them and eventually puncture the capsheet, resulting in a leeking roof.

I have asked the help of a roof garden specialist, ( http://www.mylandscapes.co.uk/ ) but although he seems to know about plants, planters and decking, he knows nothing about the roofs they go on. He would have just put the decking directly on the capsheet. In answer to my concerns, he has just told me to find a solution and call him when the site is ready to lay his decking.

A decking company ( http://www.citydecking.co.uk/roof.html ) advised me that it is sufficient to place rubber pads under the decking supports.

I have asked a structural engineer to look into a solution. He initially sugested laying an "egg carton type" draining board used in green roofs to spread the weight and ensure draining. He's getting back to me with a solution.

In Italy (where I grew up) and Spain, they have a lot of roof terraces. There they would cover the capsheet with plastic sheet or 3 cm thick polystirene, then ad a 10 cm light agregate screed with zink grid armour, and then tile.
The plastic sheet is for water protection, the polystirene protects the capsheet from splitting if the concreete cracks, the zink grid prevents the concrete from crackng too much. Probably efficient, but expensive and I'm not sure a brit builder would know how to prepare the screed.

My fear is that I have invested in a new roof, and then I lay decking and put a load of planters on it and after a year I have to strip the wole thing off to find a leak.

For reference, here is the exisdting roof specification built working with building control and the structural engineer, the roof has recently been reinforced with double joists.


on top of the joists are the following layers:
- 18mm WBP ply
- Marley Goldseal vapor control layer
- 150mm kingspan TR21 insulation
- Marley Gold venting underlay
- Marley Goldseal underlay
- Marley Gold capsheet

Any thoughts?

 

[Lime]

My immediate thought about your roof is "I hope it isn't flat!"
Draining off the water is much more important in our wetter and colder climate and the roof needs sufficient run-off to ensure that happens.
I wouldn't have used a soft material to cover a flat roof if I had intended to place any load upon it.
It depends on the total load you intend to place upon the roof as to whether and what you cover it with to protect it.
My concern for the roof would be the sag that is created by the load of plants etc.
Any sag produced is immediately increased by the standing water and then a failed roof is just a matter of time.

 

[ljn]

Thanks Lim

INCLINE
The roof has a 10% incline to the sides and a 10% incline to the rear where the water exits.
There is no ponding.
Doubling the juists should prevent flection.
Planters would all be around the perimiter walls, so shouldn't cause sagging. Anyway, the engineer will do that calculation- he has the size weight and position of the planters, and the roof is built to his specification.

VIET THREAT
As for the soft roof, it is a bit of a long story.
In our leasehold lease it states that we own the walls and roof and that it is our obligation to maintain them. The roof had serious leeks. The freeholder wouldn't let us put scaffolding up, saying it would be disruptive. We sought legal advice and the scaffolding went up, to the protests of 6 angry Vietnamese threatening martial art moves.
So we were pretty keen to get the job done quickly.

LIKE FOR LIKE MAINTAINANCE
On various websites, including various councils, it stated that you could resurface a flat tared roof "like for like" without the involvment of building control. But when we stripped the old tar off, the Building inspector said "aha! You've stripped the roof - I want to see engineers calculations"

I had the builders standing around waiting...

STRUCTURAL UPDATE
So I rushed off and found an engineer who'd come straight away. 1 week later the builders had done some odd jobs and I had the calculations- bringing the roof of my Victorian building up to todays structural standards. ( I told him I wanted to use the roof as a roof terrace)

It turns out that the Building inspector was wrong bless him!- a load of extra money spent for one of his many mistakes. The positive outcome is that his boss said "sorry, he thought you wanted to use the roof as a roof terrace"

So now I have a roof terrace, which is good, but as the surfacing isn't prepared for it, I need to find a remedial solution.

If you wouldn't have used a soft material, what system would you have used?

 

[Penners]

If you've asked a structural engineer to propose the best solution to your problem, I would certainly go with his recommendations above all else. After all, he's examined the roof, has the right qualifications to answer your concerns, and will presumably have to carry the can if anything goes wrong!

Which doesn't apply to anyone on this forum.

That's got you off the hook, Lime! :wink:

 

[JanieB]

I have just had to have a new roof on my kitchen extention (extention C1981, rest of house 200+ years old). No permission needed.

When the roofer stripped the old material off he commented that the ceiling joists were overkill for just a roof and were of the standard to take a floor. I assume that the owners who had the extention built in the first place had contingency plans for another story. However, I asked if I could use it in the summer as a terrace. My roofer said that was fine as long as I put boards over the felt to protect it from damage, particularly when hot and the felt is soft.

 

[Lime]

If you've asked a structural engineer to propose the best solution to your problem, I would certainly go with his recommendations above all else. After all, he's examined the roof, has the right qualifications to answer your concerns, and will presumably have to carry the can if anything goes wrong!

Which doesn't apply to anyone on this forum.

That's got you off the hook, Lime! :wink:

Yes.
My feelings are that in this climate any roof garden/whatever needs to be built as such and not attached to an otherwise ordinary roof.
The spindly modern roof truss is designed for only that, a roof.
As JanieB says their roof is constructed using floor joists which are much more able to support heavy uneven loads.
I would have used lead if it had been my roof, grade 6 or above.

 

[ljn]

the roof truss is not a "modern spindly roof`', but a solid reinforced structure specified by a structural engineer to carry aroof terrace.

Often we think of the brittish climate as hostile to roof terraces, but the ancient terrace on the 400 year old Florentene house `I used to live in, had it's fair share of rain - probably as much as you'd get in london, 40 degreed in the shade in summer and up to 20 below in the winter without failing...


Also swizerland, Germany Canada, and other cold countries manage to build historically sound roof gardens and terraces. The traditional roofs seem to last much longer than the modern ones.

I know they are'nt common in the UK, but I was wondering if anyone had knowlege of historical coverings for roof terraces.

 

[sarahj]

Many old church towers have flat roofs - I think they used zinc, copper or lead?