Hello

[Anna V]

We are new to this forum so we wanted to say hello and we also need some advice. We are planning to do a 5m long by 5.90m wide rear extension. We want an open plan L shaped  kitchen space. Originally we wanted to do a pitched roof but out surveyor recommended a flat roof with a lantern window (dimensions of the lantern window 3m by 2.5m). We are not convinced it is the best solution. We are worried that such a big lantern window will make the room very cold during winter time. Could you please recommend what is better in your opinion: a flat roof or pitched one, a lantern window or a rooflight? If a lantern window is good what size would you recommend to still keep the room warm?  Does anyone know what the percentage of extensions are done with a flat roof with a lantern window compared to a pitched one? Thank you for all your feedback.

[Nickfromwales]

Hi.

If it's triple glazed then it should be reasonably high performance so maybe not such an issue. You can get 'the numbers' off each proposal and compare them side-by-side to gauge the impact of either choice.

You could also choose to offset the losses of a larger window by beefing up the insulation level in the rest of the roof structure. Adding 30mm of PIR over the face of the joists before plaster-boarding for eg would be one way. Conversely, consider the adventitious heat gains from solar gain with the larger window and weigh up the pro's and con's  

 

And welcome! 

[Russell griffiths]

I would be more concerned about it being too hot in the summer, unless it is shaded. 

Flat versus pitch is really down to the look you want. 

Have you priced up a roof lantern that big. And the weight of it. 

I bet that’s over 200kg, you won’t be picking it up on your shoulder and carrying it up a ladder. 

Architects like to draw things without much thought on how to build it. 

[dnb]

1 hour ago, Russell griffiths said:

Architects like to draw things without much thought on how to build it.

So true!

 

Like everyone else, I would suspect that summer comfort is the main worry these days. The extension will need to be pretty well insulated to pass building control so should stay reasonably warm. Also, plenty of solar gain from the lantern window will help even under winter sun. (I expect it will look pretty good too.) I suggest doing a bit of maths to look at heat losses through the walls. There's a handy guide and a spreadsheet on Buildhub for helping with this.

When all said and done most things can be made to work, but some of them might end up cheaper than others. And of course, welcome!

[Dreadnaught]

There is a rule of thumb about the ratio of glazing (whether windows or rooflights) to floor area. A figure of 10 - 15% is typically quoted. Having too much light can be problematic for causing fading of materials and so on, as well as the problems you mention for heat gain and loss. I wonder what the percentage is for your proposed L-shaped kitchen.

 

Furthermore, for a given area of glazing, a roof light is generally far more effective at letting in light than a window, particularly when mullions and frame are accounted for, which tend to be larger for windows than rooflights.

[nod]

9 hours ago, Russell griffiths said:

I would be more concerned about it being too hot in the summer, unless it is shaded. 

Flat versus pitch is really down to the look you want. 

Have you priced up a roof lantern that big. And the weight of it. 

I bet that’s over 200kg, you won’t be picking it up on your shoulder and carrying it up a ladder. 

Architects like to draw things without much thought on how to build it. 

That was our fear As part of our kitchen we thought it would be like having a conservatory on the kitchen

So we drastically reduced the size 

[joe90]

Welcome, it also depends if you are stealing light from the room(S) behind and need the light in the roof to make up fir it. It also depends which way it faces. We have a large south facing conservatory which everyone said would overheat but we love it, enough windows to allow plenty of air during hot weather.