Loft Conversion-Velux spacing-rafters

[Jitno123]

Hi

Currently having a loft conversion. After some advice regarding velux window positioning. Im planning on have 3 X 780 x 980 velux windows in one room. Is there a certain amount of space required between the rafters? As the rafters will have to be cut and then doubled up. The rafters are spaced out at 400mm approx. I will be asking a structural engineer to assess the loading. My builder has said usually 3 rafter gap between each window or else the roof may sag after a few years. Can someone elaborate on this?

Thanks

 

[tony1851]

Don't know where that three-rafter gap comes from?
If it's an older house with 75- or 100 deep rafters, you will be asked to double them up each side of the Velux.

 

[bobsuerita]

if your talking about the spacing between them
they do a 100m flashing to link them if you want them close together

 

[Doggit]

I was under the impression, that if you had to take out any of the intervening rafters, then you should double up as a matter of course, to provide extra support for the trimmers. If you do double up, then there's shouldn't be an issue with space you leave between them, but as they are 780 wide, you're only taking out one rafter in the middle but how long is the room/roof, and what spacing, no of rafters will you have between each window.

But as you're only taking out one rafter for each window, it could be argued that doubling up 6 rafters is getting a bit excessive.

 

[tony1851]

In most older houses, with 3x2 or 4x2 rafters and conventionally-spaced purlins, it is not structurally necessary to double up the rafters for a typical Velux such as the MO4; this can be proved by calculation.
However, most inspectors slavishly follow this nonsense, without being able to explain 'why'.

 

[primetime]

I have a single MK04 in the main part of my loft with dormer. Sounds like you will have loads of light but to maximise it I had the adjacent rafters spaced a bit further apart so I could create a chamfer into the room.

 

[bobsuerita]

that first picture of that velux trimming is just wrong

 

[John D v2.0]

Useless comment?. Are you refering to the fact that the rafters are not doubled up all the way to the wall plate? Presumably it's designed to not need the strength all the way down to the bottom.

 

[Doggit]

The window opening would normally be centrally between the joists, not slightly offset as this one is. Not a big issue though.

 

[bobsuerita]

theres no support to the left at all (its only supported buy the old rafter coz that header for opening is doing sod all )and when rafters are trimmed to a opening size of 1200 or greater carrying more than one rafter they should be doubles if the opening was left at 780 then no

 

[chappers]

Useless comment?. Are you refering to the fact that the rafters are not doubled up all the way to the wall plate? Presumably it's designed to not need the strength all the way down to the bottom.

wouldn't need strengthening to the bottom if the purlin wall had been made correctly and was actually supporting the rafters