Flat roof with roof windows/lights

dellymull6

Hi Folks
We are in the process of building an outdoor room in a garden. Long story but we have decided to try and put some flat windows in the flat roof. The building is 8m by 4m. We have made contact with Fakro windows and they have suggested a DWG at 120 by 220 which weighs 180 kg's. The question that cannot be answered by them as we dont have a structural engineer : do we need steel support in the roof to support this weight? We may get a window weighing up to 250kg . We are using 9by2 timber and will double up beside the window. Is this strong enough? Any advice on your experience of windows in a flat roof would be greatly appreciated. Thanks

Fine Cheers

I don't believe structural advice permitted here. I presume 9×2 are spanning the 4m. What is outer wall construction ie what are 9x2 bearing on ?
Would 2 smaller square ones would be just as good ? Are you thinking openable which I don't think is needed. I presume those are double glazed ? Have you considered coxdome and or polycarbonate type ? What is roof finish ? Sorry for all questions but this info will help inform replies.

dellymull6

Great thanks for replying: it’s a block
Construction so 9 by 2 sitting on block work: we have 6m of sliding doors at the front of the building . We are planning on having good insulation in the cavity walls roof and glass is triple glazed. The roof finish is Fibre glass. We thought we could put up one big roof window initially 4m by 1m until someone suggested we may need steel beams for that which is adding a lot to the cost. So now we are considering either one farro 2 m by 1m which weighs approx 200 kg or maybe 2 smaller ones. How would that look and would we need steel beams to support? Any thoughts on this?

Arbuckle

dellymull6 wrote: »

Hi Folks
We are in the process of building an outdoor room in a garden. Long story but we have decided to try and put some flat windows in the flat roof. The building is 8m by 4m. We have made contact with Fakro windows and they have suggested a DWG at 120 by 220 which weighs 180 kg's. The question that cannot be answered by them as we dont have a structural engineer : do we need steel support in the roof to support this weight? We may get a window weighing up to 250kg . We are using 9by2 timber and will double up beside the window. Is this strong enough? Any advice on your experience of windows in a flat roof would be greatly appreciated. Thanks

I'd not be putting in the weight you have quoted above. It may initially hold, what about cabin peak windows in small lighting but open a packaged velux on roof to maximise

Lumen

You don't say which way the window will be oriented. Presumably so that the longer dimension is parallel to the rafters?

If so, the hole means you're missing 1 or 2 joists, so your sistering plan will only deal with taking the normal roof load, not that of a heavy window.

Dead load from a 2.64m2 250kg window is about 0.95kN/m2 which exceeds that in normal span tables which don't deal with gaping holes.

e.g.





Unless you're going to massively overbuild it to cover any possible error I'd get an engineer.

When I was building the mezzanine floor in my shed I just kept on adding timber and measuring the deflection with a laser until I was in spec, but I can't see this approach being practical unless you start with timbers deep enough to cover all possibilities (e.g 12", which are hard to get).

...Ghost...

A bit off topic, but I recommend going for the laminate bottom layer for your roof window(s) especially if going for 4SQM. With these large windows, there is always the possibility of them spontaneously shattering into thousands of pieces and raining down on anyone below. The slightest imperfection can result in them shattering years later from a tap, or even a temperature change. I was told about this but scoffed at the advice I was given because it was extra money and so unlikely to happen.

I went with a 5x5 rear extension and put three 2x1 quad glaze skylights in it, which gives us great light. 2 years after the installation I heard a very strange and constant cracking sound in the extension and I went to investigate. One of the panels was shattering. My 4 year old was right below it when this was happening and I cleared the kitchen. Within seconds, the bottom panel had come down, showering the kitchen with glass which caused minor damage to the table and wood floor below. I was lucky it was a slow shatter, because I discovered it's often much faster and sometimes explosive in nature. It's rare, but tempered glass sometime does this. See this video =>

I still have 3 layers in that rooflight, but I contacted the company I dealt with and organised a free replacement after proving there were no installation problems. I upgraded to a laminate sheet on the bottom layer. I've yet to have it installed, there being no rush at all, but when it is installed, I at least know that if it does suffer the same faith, the laminate will hold the glass in place.

Fine Cheers

If you have 6m sliding doors to front of this 8m x 4m building surely that is enough daylight ? What is the structural head detail over the sliding doors ? To omit rooflights altogether would be one less major headache and also cost saving. Maybe another window if you think its necessary but for me I'd be avoiding puncturing your fibreglass roof.

...Ghost...

Fine Cheers wrote: »

If you have 6m sliding doors to front of this 8m x 4m building surely that is enough daylight ? What is the structural head detail over the sliding doors ? To omit rooflights altogether would be one less major headache and also cost saving. Maybe another window if you think its necessary but for me I'd be avoiding puncturing your fibreglass roof.

For the floor area and depending upon orientation, I don't think it will be too bright. My roof is also fibreglass. You apply the fibreglass first, then install the rooflights.

dellymull6

Gosh you were lucky to get your child out of there. Yes we did hear of that happening before. Thanks for your advice

dellymull6

Yes thinking of leaving out the roof lights - certainly be less of a headache. Although I love light coming into a room and the way the sun sets in the evening it would be nice. From reading the advice here we may be safer going for two smaller fakro or velux windows 🀔🀔. Thanks for your advice

dellymull6

Arbuckle wrote: »

I'd not be putting in the weight you have quoted above. It may initially hold, what about cabin peak windows in small lighting but open a packaged velux on roof to maximise

Thanks: what do you mean by cabin peak windows in small lighting and then a velux on roof?