Cutting purlins

[Morninglory]

Hi,

I've got an attic room in my small end terrace where I want to put in a Velux window. Each pitch of rafters has two purlins, where I want to chop a lower one through to fit the Velux. I will prop each end of the cut purlin down to the floor, with timber to spread the load across all the joists. Will I need to notify building control and if so, do I need proper calcs from an engineer? The insulation is in between the rafters, but is old and thin. Will building control want to see it upgraded to more modern standards?

Thanks in advance

 

[RandomGrinch]

Hi,

I'm only a DIY'er so maybe wrong, but I have looked into installing a roof window myself.

Building control will be interested in a Velux install. However, some specialist window installers can self-certify.

The rafters on each side of the window will need to be doubled, and they will usually run right the way from the wall plate, to the ridge.

I have a very strong suspicion that cutting a purlin will require an SE!

It will be a lot simpler to avoid cutting it; perhaps you could have two smaller windows, one above and the other below the purlin?

I don't believe BC will be interested in upgrading the insulation, if it is just a storage space. But again, I may be wrong.

If it is going to be anything more than a storage space, BC will be very interested to see if it is a habitable space.
And with a raft of additional regulations to follow, that could be a whole new ball game!

 

[noseall]

Hi,

I've got an attic room in my small end terrace where I want to put in a Velux window. Each pitch of rafters has two purlins, where I want to chop a lower one through to fit the Velux. I will prop each end of the cut purlin down to the floor, with timber to spread the load across all the joists. Will I need to notify building control and if so, do I need proper calcs from an engineer? The insulation is in between the rafters, but is old and thin. Will building control want to see it upgraded to more modern standards?

Thanks in advance

It's not just cutting the purlin and hope. A single purlin will be stronger than two individuals. You need someone competent to look at what the purlin is resisting before you do any cutting.

Interesting you ask about BC and the Velux. Most will be thinking about what the window is gaining light for...?

 

[garyo]

Do the purlins resist a download load, or is the load at 45 degrees? In my old house the purlin struts were all at 45 degrees and met each other at the base in the middle of the loft floor.

 

[Morninglory]

Thanks for the input. The attic has always been a proper bedroom, and it looks like I will need at least building control in to sign it off. As they need to be involved, I might as well ask about the insulation. I can't see they would want the whole roof bringing up to modern standards, but maybe. Good point about the props being at 45 degrees. That would be really annoying in a small attic bedroom. I'm getting an engineer in to design steels for some other bits, so might see if some additional steel rafter design of some sort can avoid props.

 

[^woody^]

Do the purlins resist a download load, or is the load at 45 degrees? In my old house the purlin struts were all at 45 degrees and met each other at the base in the middle of the loft floor.

The load is at 90° to the rafters and strictly, that's how the purlin should be as well as any struts.

 

[^woody^]

Cutting the purlin is not an issue for building control, but the loads from the cut ends need to go on to something, not just spread randomly sideways, and that will be an issue.

The position of the new props will depend on the purlin size and the allowable span between supports.

Rafters will typically need doubling up to each side of the velux

 

[catlad]

Velux do at least 20 different sizes of skylights of which I guess several will fit between your purlins, I strongly recommend taking that option.
Have you considered the 3-in-solution FFKF06