Roof lanterns versus flat glass roof windows

[ey143]

Why have a roof lantern when you can have the same if not more light come through a flat roof window which will also keep the room warmer by not having a void space to heat?

Are there advantages of one or the other (apart from height concerns relating to planning permission)?

 

[theprinceofdarkness]

A plastic roof lantern is like a mini conservatory roof, the water just drips off on to the rest of the roof. If you had a flat roof you would have to put a slope on it to shed water and it would pick up moss, so would not be self cleaning.
There would also be a problem with the "flatness" of it. A sheet of glass held around the edge will bow downwards in the centre so would have a puddle in it.

Frank

 

[ey143]

I guess some of the flat glass roofs from Companies like roof maker.com will have a slight slope of some sort for the concern you raise. But leaving that aside, I'm trying to establish any other reasons. For example, the light entry into the room space will be the same if not more compare to a lantern I would have thought? they also do self cleaning glass.

 

[theprinceofdarkness]

If you consider what happens when a beam of light hits a horizontal piece of glass. If the beam hits it at right angles (mid day summer) it goes through the thickness ~4mm . If the angle is very low like winters sun, the beam goes through 10 or more mm. So in summer its brighter but its dimmer in winter compared with a vertical window glass. The lantern angle is in between . I reckon the closer to the vertical the more useful light you will get through the glass. This is because you do not need maximum light in midsummer or by definition, the room will be dim in winter.
Frank