Cutting rafters next to the purlin for Velux window

[OKOK]

Installing a new Velux roof windows in the loft.
Need to cut two 2x4 rafters for a 1000mm opening, rafters are app. 300mm apart. Purlin is 7ft down from the top ridge board. Rafters are appr 14ft long. Standard flat clay tiles.
Question - as I need to double up on the side rafters around the window opening and can't place a full length rafter due to the purlin being in the way, can I cut the new rafter from the ridge just past the purlin? If yes, what distance can I go past the purlin, 1ft/2ft? What would be the best place to cut the existing rafters, 1ft/2ft up from the purlin? I would like to place the window as close to the purlin as possible.
Please see the picture, the window is to go between the strut and the board.
Thank you

 

[Doggit]

Some would say that with that perlin being there, you could leave the joists as is, and others would say just fit a new piece from the perlin upwards, but you could also fit it in two stages, one part going downwards, and the other part going upwards. As long as it's well secured, the Perlin being there would negate the need for it being in one continuous piece.

 

[bobsuerita]

get some one around that knows what there doing you will need to cut out 3 number rafters to be able to form a 1000 opening
can you not get a couple of standard size veluxs you only need one rafter cutting out so no need for doubles just trimmers

 

[OKOK]

Thanks doggit, that's what I thought. it's probably not needed but I want to be 100% safe. How far do the rafters need to overhang the purlin?
Bobsuerita, The rafters are not equally spaced, some are 35cm, 30, 25, etc. There are couple of spaces to fit the 94cm wide windows with two rafters being removed. The one I've chosen is next to a couple of middle rafters at 23 and 25 and 700mm from the chimney stack.

 

[Doggit]

I didn't say it wasn't needed, but I was warning that some might; I think you're being sensible. In your case, it's not going to be a case of how far do the rafters need to overhang the purlin, but how best you manage to get them in. Measure above and below the perlin, and that'll give you the longest length that can be slotted in, and it'll be a bit of a wood puzzle job really. If the Purlins in the middle, then just put in 2 7ft sections, and trim a bit to get them under the purlin. I don't think it's essential that they go top to bottom, but work it out as you go. Put a support across the rafters you're going to cut to keep them in place, and then fit a double trimmer above and below the window opening.

 

[OKOK]

Here is a better picture, 700mm away from the chimney is important BTW as there is a wood burner chimney present and must be greater than 600mm according to the J Reg

 

[Doggit]

Except that you've still got other rafters closer than 700, so it's not actually that important. The distance is only relevant by the wood burner, not at that height.

 

[noseall]

Your purlin is acting as your bottom trimmer so doubling up in this instance will only be necessary to supplement any roof elements that are above the head trimmer of the Velux window. It may just mean fitting a deeper pair of rafters either side of the Velux which may be a wise move in any case. Are you converting the loft?

The trimmer rafters do not need to go beyond the purlin.

 

[OKOK]

Thanks noseall, would 4x4 new rafters from the top ridge to the purlin be sufficient in this case or is that an overkill?
Yes, planning to convert the loft. Just trying to understand all the work that needs to be done before Building Control inspector comes for the initial assessment.
I found this link if anyone finds this thread useful in the future https://www.planningportal.co.uk/info/200130/common_projects/47/roof/5
and this picture explains the terminology http://www.tpub.com/engbas/6.htm49.gif

 

[^woody^]

Yes double up the rafters from the purlin to the ridge.

It will most likely leak with no sarking felt or membrane though

 

[catlad]

Doubt it at 35°

 

[OKOK]

38.2 the only thing that's leaking is the ridge that was fixed by a reputable local roofing company that was supposed to know what they were doing 3 years ago. Fitting a dry ridge is just another job on the to do list now
But seriously, is it worth all the hassle getting the membrane in? I can see that in the 90 years that this house had existed the only leaks were from all the flashing that were replaced and the ridge. I'll be putting an inch batten under the tile battens to have the 50mm air gap and then filling the rafters with 75mm PIR insulation followed by Superfoil SF19 moisture barrier plus plasterboard, etc.

 

[Doggit]

You'll want another 55mm of pir across the rafters, then the moisture barrier, and then the plasterboard.

 

[noseall]

You are best off stripping the roof and introducing breathable membrane then fixing the insulation wholly between and below the rafters, especially if you are a semi or terrace.

 

[^woody^]

If you are doing a conversion under building control, then they may well insist on a membrane below the tiles.

Velux also require a membrane up the outside of the frame and below the metal gutters. I'm not sure if this is a belt and braces thing, or if the gutters will perform ok on their own. This is not related to the performance of the tiles, as yes, this was a common and proven method in the old days.