Building regs on timberwork for roof windows

[thesled]

Hello,

I'm currently looking at putting some velux-type roof windows in my loft. It's not 'habitable' but used for a bit of storage so with the cost of electricity going through the roof (sorry) I'd thought longer term I'd be better off with some free light.

It would only be one or two small windows but I'd probably need to break into at least one rafter on each window (they're pretty narrow in my roof anyway). For building regs., everything I read talks about the trimmers and enclosing rafters needing to be doubled up. I've been scouring the regs and can't find any reference to this, maybe it is in a British Standard ? Or just some long held practice that's just become adopted ?

Any pointers as to where this originates from much appreciated. Then I can see what actually applies in my situation.

Many Thanks. Simon

 

[Bonni]

If you're taking out a rafter (trimming it), then you double up the side rafters. If it's new build, double rafters would be inserted at the start.

A roof on a new build comes under regs, but it's normally following the manufactures plans and building regs have a look when the timbers are fitted and braced.

Fitting a velux to existing, I don't know if you need regs or not. A quick phone call to your council would determine that. If you're trimming a rafter, is that classed as structural? A lintel is, but not experienced fitting a velux to existing.

 

[clifford1]

What does a window weigh compared with that of the slates you have taken out? Is it a lot higher?

 

[thesled]

Thanks for the responses.
I think 'regs' approval will definitely be needed because it's a structural alteration to the roof. If you could perhaps fit a very thin window between existing rafters I guess you could get away with it because there would be no structural changes to the roof. Now there's a thought.
The window I'm currently looking at is a Fakro access window, WGT/WGI type. These are about as basic (and cheap) as it gets so probably quite light. They've don't give a weight but I reckon it wouldn't be much different to slates. Possibly less. It's not very wide but I think I'm still looking at one rafter cut though unfortunately. I'll work on the assumption that I'm going to have to double up until I find out any different.
Once I know where it's all headed, I will indeed have a word with the council folks.

 

[cdbe]

If you're removing a rafter you have transfer the load it was carrying to the ones either side - if you look round at older houses you will often see where this hasn't been done - the roof visibily dips where the windows are. In the grand scheme of fitting roof windows this is surely the easiest part anyway. I've never understood the need to double up the trimmer which people seem to do.

Your payback from electricity savings will be about 1,000 years.

 

[^woody^]

If you can prove by calculation that single rafters at that increased spacing can support the load of the window and trimmed rafters, you wont need to double the rafters.

 

[Dereekoo]

I had the same issue 30 years ago where I needed to cut one rafter and insert trimmers and 2 X 3 velux so just did it and never went through regs. Roof is still OK but then I suppose its never had that 75kg/m2 snow loading on it that it would have been originally designed for. He who dares and all that.

 

[thesled]

"Your payback from electricity savings will be about 1,000 years"
Sounds plausible. But then it's not the ONLY reason. . . . . .

"If you can prove by calculation that single rafters at that increased spacing can support the load of the window and trimmed rafters, you wont need to double the rafters"
Nice idea, but I'm guessing the Building Regs people would only accept this sort of calculation signed off by a structural engineer. so probably easier (and mainly cheaper) just to do the doubling. . . . . .

"so just did it and never went through regs"
Yes, you and a lot of other people I strongly suspect. However the window would unfortunately be very visible from the street so it would be just my luck for someone to dob me in.

 

[noseall]

It's not a massive job to double up the rafter and probably less hassle than getting an SE to prove otherwise.

 

[thesled]

Nope, could be worse. Looking at it this afternoon. Wondering if I could get away with only going as far as the purlin. . . . . .

 

[Dereekoo]

Nope, could be worse. Looking at it this afternoon. Wondering if I could get away with only going as far as the purlin. . . . . .

Don't see why not, the purlin is there so the rafter can be treated as 2 separate lengths for TRADA table purposes so it serves no purpose strengthening the bit without the window insertion

 

[Designs.3DR]

This is all i could find regarding installing rooflights into existing structure

• ROOF LIGHTS
  Min U-value of 1.6 W/m²K.
  Roof-lights to be double glazed with16mm argon gap and soft low-E glass. Window Energy Rating to be Band C or better. Roof lights to be fitted in accordance with manufacturer's instructions with rafters doubled up to sides and suitable flashings etc.

 

[^woody^]

This is all i could find regarding installing rooflights into existing structure

• ROOF LIGHTS
  Min U-value of 1.6 W/m²K.
  Roof-lights to be double glazed with16mm argon gap and soft low-E glass. Window Energy Rating to be Band C or better. Roof lights to be fitted in accordance with manufacturer's instructions with rafters doubled up to sides and suitable flashings etc.

All of that applies when installed into a habitable room. Otherwise its a nonsense to have a window perform better than the tiles and rafters around it

 

[thesled]

"can be treated as 2 separate lengths for TRADA table purposes"
Excellent. So at least when the Council jobsworths tell me 'Just do it all the way up' I can at least direct them to that so they can tick their box. But I'm presuming. They may be very nice people.

This is all i could find regarding installing rooflights into existing structure

• ROOF LIGHTS
  Min U-value of 1.6 W/m²K.
  Roof-lights to be double glazed with16mm argon gap and soft low-E glass. Window Energy Rating to be Band C or better. Roof lights to be fitted in accordance with manufacturer's instructions with rafters doubled up to sides and suitable flashings etc.

Would be very interested to know where you got that from 3DR. Is that in the Building Regs somewhere? As Woody mentions the bit about the U-values wouldn't apply in my case but maybe this is where the whole 'rafter doubling' thing originally comes from.

 

[Designs.3DR]

Agreed but as mentioned

"can be treated as 2 separate lengths for TRADA table purposes"
Excellent. So at least when the Council jobsworths tell me 'Just do it all the way up' I can at least direct them to that so they can tick their box. But I'm presuming. They may be very nice people.

Would be very interested to know where you got that from 3DR. Is that in the Building Regs somewhere? As Woody mentions the bit about the U-values wouldn't apply in my case but maybe this is where the whole 'rafter doubling' thing originally comes from.

if you google regs4plans there is a website that lists all the regs out for certain types of household extensions.

If you go to the specification section and choose loft conversion and then click on roof lights, then it mentions it there. also if you go onto detailed drawings section of the website and type rooflights then it has detailed drawings on there with the doubled up rafters.