Flat roof

[woodstalker]

So this is linked to my post about removing snapped off taps from a steel I beam. I have a GRP roof that is leaking in a few places. I have patched as best I can for now so the work to resolve this will be in the spring.

There are too many penetrations in the roof to facilitate it being a roof terrace so I am hoping to cover it in epdm and design the terrace in such a way that the balustrade and privacy screens are on a frame resting on supports on the roof and the tiles on pedestals. I’ve done a few shed epdm roofs but not one this big nor this non rectangular shape. My big issue is that the roof joins the wall under the french doors leading onto it with only a very small gap for an upstand so I am trying to figure out a way to address this detail or a product I can use to go under the door threshold and floor to ceiling windows that look onto the roof terrace. The GRP installer made a proper pigs ear of this detail and I have a leak in the room below (plus a few other places).

I am thinking I might sign up for one of those one day epdm installer courses and see if they can suggest a way forward but would appreciate any advice or suggestions, as ever….

 

[Mike G]

A drawing of a section through the door cill/ wall/ roof junction would be really useful. Otherwise, a photo. How high is the possible upstand?

 

[woodstalker]

Hi Mike,

Pic1 shows the general layout. The gap between the bottom of the door threshold and the top of the roof structure is about 25mm I think. Will measure when I get home tonight.

Pic 2 is in a bit closer. I think I need some sort of tray that I can cut in under the window/door thresholds to come down the face of the wall, over any upstand I can squeeze in there, and over the roof covering material. I could then adhere a piece of EPDM or GRP over top of it to prevent water flowing under it.

Essentially if we had our time again we would have completely demolished the existing raised deck to build the garden room rather than making use of the existing steel substructure. This would have been quicker, cheaper and easier with a much better interface between the roof and the wall. Unfortunately neither our architect or builder gave us advice that this would be the best way to tackle the job and just designed and built using our initial sketches as the start point.

 

[Mike G]

If you can take the door frame out, the very best answer is to dress the EPDM under and then up the back of the threshold. It's actually fairly easily done once the frame is out of the way. So imagine the red line in your second image has another vertical line added to its left hand end. You can do this in multiple pieces of EPDM, because the adhesion of the material to itself is so excellent.......or, you can try to do it in a single piece.

 

[woodstalker]

Hi Mike, this is an old photo when the raised deck was in place so taking the doors and windows out would be impossible for me. It’s why I think I need something like a solid profile I can cut in under the thresholds/cills.

I think EPDM is the answer but again it’s a very irregular shape which I think you can see in this old photo:




I want to remove the support posts for the privacy screen and balustrade that currently penetrate the grp roof and do something like this:

 

[Mike G]

Oh, I see. It's going to be the junction between any profile and the EPDM which is going to be the issue. Do you think it would be possible to clear a gap underneath the whole length of the threshold? If so, I'd be trying to slide some EPDM underneath and up the back of the cill, and adhere it to the EPDM on the flat roof, with a lap of say 250 or 300mm. Otherwise you're going to be reliant on a sealant of some description.

 

[woodstalker]

I am not sure how could cut under the threshold/cill all the way along in order to get the EPDM under the windows/doors. Ultimate when the timber on the doors and windows fail and the need replacement that’s what I’ll do but right now I suppose I’ll have to install the EPDM roof with as much of an upstanding as I can get and use a sealant sandwiched between the EPDM and some sort of solid profile to prevent water getting behind?

 

[Mike G]

If there's enough of a gap to get a solid profile in, then surely there's enough of a gap to feed EPDM though, isn't there? The stuff is less than 2mm thick.

 

[woodstalker]

It would be like pushing wet spaghetti up a wild dog’s bum I think. I can’t get my hand in there and I’m not sure how I would get a tool in there to cut the channel for the rubber/profile. The only thing I can think of is an L shaped profile adhered to the roof and up to the threshold and the the EPDM on top of that?

 

[AJB Temple]

EPDM is stiffer than you might think - I had a similar problem when adjoining a flat roof to our main barn and I was able to cut a gap and feed EPDM through with the assistance of Mrs AJB T and some metal spatulas. Admittedly it was only across about 2 metres. Although your shape may be awkward you can join EPDM very effectively with special primer and adhesive. This may actually do a better job as the stuff is jolly heavy when laid over a large area.

 

[Alasdair]

It would be like pushing wet spaghetti up a wild dog’s bum I think.

I do hope that is not a voice of experience talking there although I understand that there may be websites where one could view this type of contest.

 

[woodstalker]

I do hope that is not a voice of experience talking there although I understand that there may be websites where one could view this type of contest.

It’s a helluva workout that’s for sure

 

[woodstalker]

EPDM is stiffer than you might think - I had a similar problem when adjoining a flat roof to our main barn and I was able to cut a gap and feed EPDM through with the assistance of Mrs AJB T and some metal spatulas. Admittedly it was only across about 2 metres. Although your shape may be awkward you can join EPDM very effectively with special primer and adhesive. This may actually do a better job as the stuff is jolly heavy when laid over a large area.

Yes it would be over about 5 metres I think and involve me cutting all the way under the threshold and cills without destroying the windows in the process. I think I’d have to do the roof in multiple panels anyway and have done a few square/rectangular shed roofs before. I’m just not confident with the space available of getting a good seal in that tight gap.

 

[Mike G]

I’m just not confident with the space available of getting a good seal in that tight gap.

The join wouldn't be in the gap. You'd cut off a strip of EPDM 2 or 3 feet wide, and work this in under the door/ windows. Fold it up out of the way, and then lay your roof. Glue the strip down onto the top of the flat roof rubber, lapping a foot or more onto the flat bit. That's where your join would be, even if you've drssed the flat roofing up the upstand a little. Then a bead of silicon between the rubber and the threshold.

 

[RogerS]

I sympathise with your problem. In my own experience a GRP roof is a recipe for constant water leaks. I continue to fight a battle with our orangerie. Even Acrypol is finding it hard.

 

[woodstalker]

I sympathise with your problem. In my own experience a GRP roof is a recipe for constant water leaks. I continue to fight a battle with our orangerie. Even Acrypol is finding it hard.

Yes GRP chosen due to the number of penetrations to the roof surface but I’m planning to remove all of them onto a frame resting on support held up above the EPDM. Mike has given me the confidence I just hope I can do it. I have a small leaded roof on the bay window in the kitchen I will do first. The lead has totally failed annoyingly.