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HI all, I have used this forum for years but finally have to pose my own question rather than simply leech off others… I am replacing the existing felt flat roof on my 1970s kitchen extension with a warm roof build up of (existing) 18mm ply deck / Alutrix vapour barrier / 150mm PIR / 18mm ply / EPDM. I would like to add a flat roof light while I am at it which brings me to my question(s)… The roof dimensions (exterior) are 3.8m x 4.2m and the joists are 150 x 50mm at 40cm centres spanning 3m. I have currently narrowed down to two options which are as follows: 1. Install a 1m x 2m rooflight which would mean cutting 4 joists - I was planning to treble the joists at both ends and double the headers on the long sides (sistering joists and using hangers/fixings etc as outlined in building regs). 2. Install 2 x 1m x 1m roof lights which would mean cutting two joists for each - in this case I would only double the joists at either end but still double the headers. This design would leave two whole joists between the two roof lights. Option 1 is both cheaper and easier in that the EPDM could be a challenge with two roof lights to work around and clearly one window is cheaper than two! However, i feel that option 2 is better structurally but I would really appreciate some input as to the proposed design for option 1. One corner will also sit above a window lintel which is unavoidable in either lay out! Any advice would be greatly appreciated! Many thanks in advance, Will

Hi I have a large (c. 5.8 x 1.6m) skylight, on a low pitched roof (I'd guess c. 15 degrees). This appears to be made of two sheets of 10 or 12mm polycarbonate. I suspect it was built in around 2006/2007. The polycarbonate has since turned somewhat cloudy – so a little translucent now, rather than clear (clear would be preferable). The roof is an unusual and lightweight construction – see Since it’s a single sheet, thermal performance is terrible – there a lot of heat loss, and a lot of condensation in winter months. So it either needs to be: 1) Replaced with a glass double glazed unit (quotes seem to be c. £12k-20k), and a structural engineer would be needed to confirm whether the loft can bear the weight of a glass unit, so quite apart from the huge cost, it might not even be viable. On the plus side, this would be professionally installed, stay clear, provide decent u values, and could be done in UV control glass, etc. 2) Secondary glazed with acyrlic. I suggest with a sheet of 12mm Perspex (clear cast acrylic), either on the inside or outside, with a between the two sheets. It might make sense to replace the existing polycarbonate at the same time with new, clear sheets. Vastly cheaper (perhaps £1k-2k, including labour and materials). Thermally not quite as good as a proper double glazed unit with inert gas between the layers, but should make an appreciable different for a much lesser outlay. I’ve had a few thoughts on how this might be done by building a frame on top, or a frame underneath, but ideas would be much appreciated; also on how best to re-seal the outside of the skylight to the roof (flashband would probably work … though it seems a bit temporary!). I’m a bit concern that 2 might be a false economy in the long term. But since we may ultimately need to replace the Decra roof, I’m equally concerned that replacing this with a glass unit (1) should only really be done once the roof’s (re)done …. Step 1 of course is getting a structural engineer in, but in the meantime, thoughts welcome – as would suggestions of inexpensive glazing companies for sky lights.

About to order a rather large glazing unit that is 3900mm by 820mm in size. It is double glazed with no joins (so each pane of glass will be 3900 by 820). Weight is about 180kg. Asked about the warranty and the vendor said "well obviously I can't guarantee it won't spontaneously fail". I asked what he meant and he said that all glass can in theory spontaneously fail. On probing further, he tells me the inner pane will be heat treated so that if it fails it will shatter into thousands of pieces, each piece no bigger than 1cm by 1cm. I asked if heat treatment is in accordance with BS EN 14179-1 and he is going to find out. The outer pane however will just be regular glass. With heat treated glass in accordance with BS EN 14179-1the rate of failure is 1 in 400 tonnes of glass. So for a skylight this weight, there is something like a 1 in 4444 chance of failure of the inner pane (assuming both panes are of equal weight, which I think is likely as they are both are same thickness). So not too likely, though more likely than I'd like. But failure of the outer pane is much more likely and if that fails it won't shatter into tiny little pieces, it will shatter into shards. Surely that would then cause the heat soak tested inner pane to also break? This unit is going in an upstand we've built to make a skylight directly above our dining table. Not so enthusiastic about this now. Window company said laminated glass would be about £800 to £1000 more. Not sure why that's so expensive. Do people worry about this sh!t, or am I being paranoid?