Should I add one roof light or two to my flat roof? A structural question....

[WillH83]

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

[Iceverge]

One window would get my vote. 

 

From a non SE I would say so long as you have at least as many joists per kg of roof including window as previously was installed the whole thing shouldn't fall down. Triple joists sounds fine.

 

 

The ply should provide plenty of racking strength too. 

 

 

Fewer holes in a roof are always better.

 

[nod]

I installed a 2000x 1500 at our last build 9” joists doubled up Plenty strong enough 

[JohnMo]

I would do two small ones that fitted between the existing rafters. Big one loads of heat gain when you don't want, loads of heat loss when you don't want it.

 

Then with the money saved do triple glazed.

[Kelvin]

We did 1 fixed rooflight that is 1m x 1m over the hallway. It’s 3G with a protective glass cap. Ours is over the entrance hallway on the North elevation. Without it this would have been a dark corridor so glad we did it. It wasn’t too dear. 

[SteamyTea]

3 hours ago, WillH83 said:

easier in that the EPDM could be a challenge with two

GRP it.

 

[Iceverge]

You could always just leave the joists in place and install a roof light over the top of them. 

 

Sanded and painted I'm not sure anyone would ever give them a second glance. 

[JohnMo]

4 hours ago, WillH83 said:

EPDM could be a challenge with two roof lights

Just get the installer to heat weld all joints for the upstand. 

 

Upstands - form out of Compacfoam. Zero cold bridge, quick and easy

[Mr Punter]

1 hour ago, JohnMo said:

Upstands - form out of Compacfoam. Zero cold bridge, quick and easy

 

I just had a look at this stuff.  Very expensive.  At 75mm high it would be £186 to do a 1m2 rooflight.

[Alan Ambrose]

I didn't know that you could heat weld epdm, very interesting.

 

I think the choice boils down to aesthetics, which only you can judge as we don't know the location, sun direction, shading etc.

 

I'm sure the engineering can be made to work, but do you have an SE you can buy a few hours of work from to double check as the rooflight(s) will be heavyish? Will you use the roof as a terrace too?

[JohnMo]

1 hour ago, Mr Punter said:

 

I just had a look at this stuff.  Very expensive.  At 75mm high it would be £186 to do a 1m2 rooflight.

My two roof lights took approx 30 mins to do by the joiner, to fabricate a rubbish one X2 a couple of hours - bridge free doubt it. Bargain.

[JohnMo]

1 hour ago, Alan Ambrose said:

didn't know that you could heat weld epdm, very interesting

No all EPDM but some.

[WillH83]

Thank you all for your input so far - it is reassuring to hear that both options do seem viable!

 

To answer a couple of the questions raised:-

 

The location does not get that much sun and the room has always been quite cold and dark so any thermal gain would be appreciated!

 

The roof lights that I am planning to buy are triple glazed - the 2x1 is approx 100KG so not too ridiculous! There would be no other foot traffic aside from occasional access for maintenance.

 

I was planning to build the upstands using 40mm Celotex between 18mm ply which should avoid any thermal bridging. The manufacturer sells these for around £500 but I think I should be able to make for approx £80.

 

As I am going to be doing all the work myself, I had settled on EPDM as a covering as I have worked with it a couple of times before and it seems fairly idiot proof - GRP does also look fairly straight forward but it would be a first for me so I may stick with what I know!

 

Any further thoughts on the design of the roof structure, in particular the use of trebled joists in the case of the larger roof light would be greatly appreciated. Joist calculators imply that 150x50mm joists at 400mm centres over a 3m span should be fine to accommodate the additional dead load (approx 200kg total) but it becomes less clear once things are rearranged….

 

Many thanks!

[Alan Ambrose]

If not an SE, you can test before putting the window in by, say, having 4 or 5 people add their combined weight around where the window will bear.

 

The roof load is, say, 150Kg + occasional maintenance load, say 100Kg. For a safety factor of, say, 2, you can apply a test load of double that i.e. 500Kg, - i.e. however many people that is, in your case.

[WillH83]

Thank you all for your advice - I have decided to play safe and install two roof lights rather than one large one which leaves at least one joist intact between them. After a little more reading around I have decided that this would be a good time to learn how to install GRP as it seems better suited to the layout than EPDM. Now I just need to wait for summer to start!