Velux type windows in RiR attic trusses

[MortarThePoint]

I was thinking of incorporating Dormer window provisions into a roof truss design to allow for future conversion, but have changed my think on that in large part because it needed too much input at the truss design stage. Another reason is that it made for big gaps between some trusses. I had thought I could simplify matters by using Velux windows instead. It normally seems to be simple to fit Velux windows by doubling up the rafters on each side, adding purlins top and bottom and then cutting out sections of any rafters in the middle. I spoke to the truss designer and he said for attic trusses it wasn't that simple. I don't understand why. Is it because attic trusses have bending moments on their members rather than just the tension and compression loads of a fink truss? Even then I still don't see why it's anything more than doubling up the rafters either side rather than the whole truss.

 

[ProDave]

I have not done it myself, but I have seen plenty of attic trusses cut to fit dormers and velux windows.  Perhaps it needs the input of a structural engineer?

 

They seem to usually put doubled up attic trusses either side of the window and frame the headers on site.

[MortarThePoint]

2 minutes ago, ProDave said:

I have not done it myself, but I have seen plenty of attic trusses cut to fit dormers and velux windows.  Perhaps it needs the input of a structural engineer?

 

They seem to usually put doubled up attic trusses either side of the window and frame the headers on site.

 

The truss designer is talking about doubling up the whole truss, but I don't understand why the whole truss needs to be doubled rather than just the rafter part. Doubling the rafter can be done on site with the benefit of time whereas doubling the whole truss requires a lot more thought

[PeterW]

At about £100-150 a truss, it is simpler to do it now and take the hit but have a design that works. You can then box in or infill with timber on site until you decide you need it. 
 

Don’t forget that any roof window needs to be below 1100mm cill if it’s to be used for escape purposes. 

[MortarThePoint]

On 28/10/2020 at 16:07, PeterW said:

At about £100-150 a truss, it is simpler to do it now and take the hit but have a design that works. You can then box in or infill with timber on site until you decide you need it. 
 

Don’t forget that any roof window needs to be below 1100mm cill if it’s to be used for escape purposes. 

 

Pretty much spot on, it adds £550 for 4. 1100mm cill may be a challenge as it's a long way down. off the roof

[PeterW]

1100 is measured from the cill to the internal floor vertically. 

[MortarThePoint]

6 minutes ago, PeterW said:

1100 is measured from the cill to the internal floor vertically. 

 

Yes I wasn't clear, the challenge is on the other side of the <1100mm cill ?

Edited October 29, 2020 by MortarThePoint

[Johnnyt]

I was initially having dormers to the rear but decided on 2 velux side by side with a sloped fixed panel beneath.

The truss manufacturer used the following design which could accommodate a dormer in the future.

 

 

MAIN ROOF PLOT 1[23064].pdf

[PeterW]

11 hours ago, MortarThePoint said:

 

Yes I wasn't clear, the challenge is on the other side of the <1100mm cill ?

 But that’s irrelevant - fire escape windows don’t need to consider that element. 

[MortarThePoint]

On 30/10/2020 at 08:20, PeterW said:

 But that’s irrelevant - fire escape windows don’t need to consider that element. 

 

Thanks Peter. Crazy really, so you die falling from the roof rather than roasting to death due to the fire. The rules are the rules.