Polycarbonate rooflight modification
Posted: June 24th, 2021, 4:46 pm
The small block of (collectively enfranchised and self managed) flats I live in has a skylight at the top of the stairwell, which opens to give access to the (flat) roof. The previous skylight was a single skin dome that had turned very brown over its 40 years, and had developed some cracks and was also, as a result, not very well fitting on its frame, and would not very often but sometimes let some rain in if it was windy and slanted at a particular angle.
So, a couple of years back when we were having the roof re-felted we asked the roofer to also replace the dome. Well, of course it wasn't a standard size, and having a custom dome made was going to cost £lots, and changing the size of the hole to fit a standard size skylight (including upstand) was going to cost even £more.
However, as the existing metal frame for the dome was only a few inches smaller than one standard sized dome, it was decided to extend the existing frame by a few inches all round by bolting some 3/4" wooden planks to it and then bolting a standard sized dome to that, all for just a few hundred £.
Photos of the end result are at https://i.imgur.com/fnvJnGx.jpg and https://i.imgur.com/1DuNmXe.jpg, and if that's not clear the cross section looks like this (wood in brown, original metal frame in black, upstand in dark green, not to scale, and of course it actually fits on the upstand tightly):
All well and good ... except that on certain days in winter we now get masses of condensation on the inside of the dome, which drips heavily down the stairwell -- and I mean masses: once I tried to sponge it off and half filled a washing up bowl!. We'd only put in a single skin dome 'cos that's what was there before, and we didn't have the condensation problem previously (although that may be 'cos it was cracked and badly fitting), and in any case as it was just an unheated stairwell we weren't worried about heat loss.
So, trying to figure a reasonable way of dealing with that (without the cost of throwing away the current dome and replacing it with a double or triple skinned one) and am thinking of adding a second "skin" myself in the form of a celled polycarbonate roofing sheet, e.g. https://www.roofingmegastore.co.uk/poly ... therm.html.
My first thought is to attach it to the inside of the existing structure, below the dome, like so (I can easily add some flanges for it to rest on and then silicone it all tight).
That would be easiest, but then one of the problems with the wooden plank extended frame is that it's pretty heavy and makes opening the skylight quite difficult, so I thought, well, what if I just throw away the planks, get a slightly bigger polycarbonate sheet and attach it to the frame and then the dome to the sheet, so replacing the planks with a single sheet that covers the whole area, like so:
Now, the thing is I've not worked with these polycarbonate roofing sheets before (or any polycarbonate material for that matter). All the (sellers) blurb about them says they're pretty much indestructible, but I wonder about the possibility of a very high wind ripping the dome off of the sheet, or the sheet off of the frame, or some other problem that my inexperience with the material fails to foresee ...
Anyone got an relevant experience and would like to comment, please?
So, a couple of years back when we were having the roof re-felted we asked the roofer to also replace the dome. Well, of course it wasn't a standard size, and having a custom dome made was going to cost £lots, and changing the size of the hole to fit a standard size skylight (including upstand) was going to cost even £more.
However, as the existing metal frame for the dome was only a few inches smaller than one standard sized dome, it was decided to extend the existing frame by a few inches all round by bolting some 3/4" wooden planks to it and then bolting a standard sized dome to that, all for just a few hundred £.
Photos of the end result are at https://i.imgur.com/fnvJnGx.jpg and https://i.imgur.com/1DuNmXe.jpg, and if that's not clear the cross section looks like this (wood in brown, original metal frame in black, upstand in dark green, not to scale, and of course it actually fits on the upstand tightly):
All well and good ... except that on certain days in winter we now get masses of condensation on the inside of the dome, which drips heavily down the stairwell -- and I mean masses: once I tried to sponge it off and half filled a washing up bowl!. We'd only put in a single skin dome 'cos that's what was there before, and we didn't have the condensation problem previously (although that may be 'cos it was cracked and badly fitting), and in any case as it was just an unheated stairwell we weren't worried about heat loss.
So, trying to figure a reasonable way of dealing with that (without the cost of throwing away the current dome and replacing it with a double or triple skinned one) and am thinking of adding a second "skin" myself in the form of a celled polycarbonate roofing sheet, e.g. https://www.roofingmegastore.co.uk/poly ... therm.html.
My first thought is to attach it to the inside of the existing structure, below the dome, like so (I can easily add some flanges for it to rest on and then silicone it all tight).
That would be easiest, but then one of the problems with the wooden plank extended frame is that it's pretty heavy and makes opening the skylight quite difficult, so I thought, well, what if I just throw away the planks, get a slightly bigger polycarbonate sheet and attach it to the frame and then the dome to the sheet, so replacing the planks with a single sheet that covers the whole area, like so:
Now, the thing is I've not worked with these polycarbonate roofing sheets before (or any polycarbonate material for that matter). All the (sellers) blurb about them says they're pretty much indestructible, but I wonder about the possibility of a very high wind ripping the dome off of the sheet, or the sheet off of the frame, or some other problem that my inexperience with the material fails to foresee ...
Anyone got an relevant experience and would like to comment, please?