Moving purlin by 2 feet

[mikewilliams416]

Hi, new member and straight in asking for advice!

I have just removed a stud wall to expose the eaves with the intention of moving the wall outwards by 2ft to achieve approx 26sqft extra room (obviously with a reducing headroom).
The purlin, which i had thought was tight against the studwork is stood back. Now this has meant that it is at a height that is WELL in the way!
I am happy to put a new purlin in lower down, but instead of putting a second one in above the existing one I was considering removing the existing then same day doubling up the 3*2 rafters and plyboarding..

Untitled

• mikewilliams416
• 13 Aug 2012
• 1

The exisiting purlin is 10*2, I will replace like for like.. this one has held the roof straight for 60 years so no need to change I think!
Any expert views much appreciated!
Mike

 

[tony1851]

What is the span of the purlin?
Are you getting Building Control involved with this?

 

[mikewilliams416]

What is the span of the purlin?
Are you getting Building Control involved with this?

Hi thanks for the reply.
The span is 4m dead. Looking at current regs there isn't a piece of wood that will span that these days!
Not/wasn't planning on getting BC involved...
The original plan was to put a new top purlin in as well. So doubling up the current strength... but the top one will be really close to the edge of a chimney... maybe only half a brick so not much support for one end.
The current span for the rafters is 2 m. looking at adding approx 600mm to that..
cheers, Mike

 

[noseall]

How do you intend using the loft?

 

[mikewilliams416]

How do you intend using the loft?

Sorry, it's not the loft! it is the eaves of a bedroom in a dormer style house.

there are two existing purlins, one in the loft just above ceiling height and the one in the photo.

 

[noseall]

It is not just a case of doubling up the rafters i'm afraid. For instance how would you birdsmouth onto the new purlin and existing wall plate.

The existing rafters (at 3" x 2") are so small by todays' standards they are not even listed in any tables.

Have you thought about insulation and ventilation?

 

[tony1851]

On that span, you should consider doubling up the purlins, ie use two 2x10s fixed together.
Of course, also double-up the rafters on the upper slope.

 

[tony1851]

noseall - my post went in just after yours.

Would he need to carry the rafters all the way down to the wall plate?
I envisaged just stopping them short within an nch or so of the top edge of the (lowered) purlin.
As long as they are firmly fixed right along the length, they will serve to stiffen the existing rafters.
Yes, 3x2 is a bit flimsy by todays standard, If they were doubled, the bending stress should still be OK, but perhaps they might deflect a bit more.
Probably best adding 2x4.
As for insulation....????

 

[mikewilliams416]

As for insulation... what you see in the photo is what and where I found it! So in effect the room was uninsulated to the tiles.... the original wool sheet had torn from the nails on the rear of the stud wall.
I was planning kingspan type insulation of 50mm still allowing some ventilation space.. between the rafters, then maybe another sheet secured to the underside of the rafters or insulated plasterboard. Again not up to modern standards but 100% more than it had.
The old felt has now been removed, a lazy roofer had placed the breathable membrane straight onto it.
I realised by research that the rafters are no longer on any current charts... but as I said before, what is there hasn't so much as squirmed in 60 years so surely my original idea of two purlins would be better than what is there presently. I realise it may not be up to a modern build...
Birdsmouthing hadn't entered my head as the existing purlin is at 90 degrees to the rafters, that was how i intended to place the replacement(s).
I know doubling up doesn't double your span, in effect I am looking at a max increase of 30% so that is why I was wondering about double rafters plus ply.
Thanks again for input.

 

[freddiemercurystwin]

What's the point of moving the purlin anyway, just clean it up and expose it or board it. Hellova a lot of work/money to gain a bit of space. Doing it without Building Regs is just daft, again just to save a few quid. If a surveyor ever finds out on a future sale it could scupper the sale without BC approval.

 

[mikewilliams416]

Just reading through again.
My intention is to run the second set of rafters an inch above the loft purlin to an inch below the new one. Bolted across with toothed washers.
So having installed the new lower purlin "quickly" cut out the existing one and double up, insulate and ply asap.
The reason of the post was if I don't use a new upper (would in effect be middle...) purlin......... because the edge of the chimney means that the rear of the purlin set at 90 degrees is less than a brick away from no support. don't think I'm making sense. I will take a photo tomorrow.
If the wall was flat I would have ploughed ahead and stuck in a new upper and lower then cut out the old one. each span would be less than the existing so the stresses are reduced if still not up to todays standards. No doubling up of rafters at all.

 

[mikewilliams416]

What's the point of moving the purlin anyway, just clean it up and expose it or board it. Hellova a lot of work/money to gain a bit of space. Doing it without Building Regs is just daft, again just to save a few quid. If a surveyor ever finds out on a future sale it could scupper the sale without BC approval.

Thanks for the reply.
The main reason for moving it is its present height. It's at chest height.. so can't place a bed for example against the new stud wall.
As far as BR/BC as I said before, there is only one 10*2 there at the moment. would that scupper a sale?
Don't take that anyway other than a question! Please

I could change tack a little... could I place a new upper purlin vertical and birdmouth the existing rafters? I have only considered placing them at 90 degrees so far...as per the original... If it were vertical I wouldn't have the concern of the load pushing out the side of the chimney..

As far as cost, labour is normally the most of any job.. I've got two and a half weeks off now so I'm looking at materials and a good challenge!

 

[maltaron]

Just as a matter of interest, until we built roofs out of trussed rafters made out of poor quality matchwood, all rafters were 3 x 2 with one 9 x 3 purlin half way down for a standard semi. I have yet to see one collapsed!! Allthough I have seen them slightly sagging when the original rosemary tiles/slates wer replaced with concrete tiles.

 

[mikewilliams416]

Just as a matter of interest, until we built roofs out of trussed rafters made out of poor quality matchwood, all rafters were 3 x 2 with one 9 x 3 purlin half way down for a standard semi. I have yet to see one collapsed!! Allthough I have seen them slightly sagging when the original rosemary tiles/slates wer replaced with concrete tiles.

That is the way I have looked at it, the tiles are the original Acme purples, with a pitch of 45 degrees so quite a steep pitch as well

 

[indus]

Hi

For my own education can somebody please confirm which bit is the 'Purlin' in the photo? Is it the thick single piece of wood that runs horizontally across the rafters?

Thanks