Cutting a Purlin??

[george744]

I am replacing a Velux window in my converted loft area. There is a purlin below the window where I need go down with the longer new window. Can I cut the purlin and support somehow or do I need to fit a second Velux below the purlin and have it as 2 windows with a boxed/plastered in purlin between.
By the way the purlin is only 2 x 4.
Any advice would be greatly appreciated.

 

[Shytalkz]

If you chop, then you must prop. And that means you'll probably have to strengthen the joist that the prop bears onto.

Alternatively, strengthen the rafters so that they can go from eaves to ridge and not rely on the purlin - size notwithstanding - for intermediate support.

 

[george744]

Thanks shytalkz - this is really what I wanted to hear. The fact that I can "chop" at all is good.
Do I need to double up all associated rafters. What I mean is, the ones each side of window and if there is one in the middle trimmed across above and below the window, double up that one also?

 

[Shytalkz]

As a rule of thumb, the rafters trimming the sides of the velux should be increased in number to replace the ones cut above and below the velux eg if you cut out two rafters, then place an additional one either side of the opening. The chopped (trimmed) rafters aren't doubled-up, but will span onto trimmer timbers comprising two members each the same size as a single rafter, running between the trimming rafters.

 

[george744]

That's pretty clear now - thanks. I think I'll be ok to do the job like that. By the way, do I need Building Officer approval for this job - or is it already taken care of in the initial window installation?

 

[Shytalkz]

A window app only covers the actual installation, not the structural work. So it's best to stick one in for the structural work and window combined; don't think that there will be much, if any diff in fees, but check with your LABCO.