Is this purlin up to the job?

[joewa]

I am about to complete the last stages of our attic conversion which involves mounting 6x3 sheets of 9.5mm tapered drywall on to the rafters.

I just want to check that the purlins are up to the job of taking the load. They are 6.5 x 3 inch and span the entire attic space 3.5m.

The house is a Victorian terrace.

There is some slight bowing in the roof which seems normal looking at similar houses on our road but there are a couple of cracks in one purlin, pictured. (The Purlins are partially obscured by the insulation)

Do I need to look at reinforcing them before adding the weight of the plasterboard?

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• joewa
• 21 Nov 2013
• 1

Untitled

• joewa
• 21 Nov 2013
• 1

 

[20years]

They should be fine, yes; those cracks are nothing to worry about.

Presumably you have steels installed to take the new floor? If so they should be quite close to the purlins in the vertical plane. You can consider bracing the purlins with vertical struts to the floor just to be extra safe.

 

[joewa]

New floor joists are wood, C24 6x3 in line with Trada tables.

Thanks. I want the space to be open so I will brace slightly below the purlins. I have some 3x2 C16 which should add some support.

 

[20years]

Fair enough. Yeah I suppose if you wanted to go belt and braces you could glue and bolt a timber to the purlin. It'd definitely help.

 

[joewa]

I was thinking more of putting a smaller purlin about a meter down the slope bracing the rafters with the new floor. This enables the space immediately under the existing purlin to be used for flexible storage.

Shoule I be using joist hangers to fix the additional purling to the brickwork either side or will the combined strength of the new structure be sufficient without them?

Diagram attached.

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• joewa
• 22 Nov 2013
• 1

The floor joists span from outer walls to two inner supporting walls.

 

[freddiemercurystwin]

This being done to Building Regs? Have you had to remove any structure?

 

[joewa]

Yes to Building regs

No have not removed any structure

oh Except the ceiling brace which was not connected to the roof structure and I have put in new joists.

 

[freddiemercurystwin]

So has your Building Control Inspector not asked for any calcs to prove the existing roof structure is OK?

 

[joewa]

I'm using an approved rep who has visited a few times during the build and he's never asked.

I have just left a message with him to discuss this specifically of the back of your post, thanks.

 

[20years]

Shoule I be using joist hangers to fix the additional purling to the brickwork either side or will the combined strength of the new structure be sufficient without them?.

I'd definitely be transferring the weight onto the brickwork somehow, yes. Personally I'd try to get the ends of the timbers set into the walls. Not the easiest of jobs, I know.

 

[joewa]

Building control reply was simple. None of the attic conversions signed off on this building type required additional strengthening. It's fine as it is and can support plasterboard insulation and skim.

However he's also said that using the lower vertical struts to bind the rafters to the joist can only be a good thing.

All looking good then. Final question is a general woodwork one. Should I worry about screwing the lower purlin into the rafters? Will the screws affect the structural strength of the rafters?

 

[20years]

There you go If anything, what you're doing will strengthen the entire structure.

I'd try to spike some big screws through your new purlin into the rafters, yes.

- get your new purlin in place (joist hangers to the brickwork will, no doubt, suffice).

- Use a sharp wood bit to pre-drill your purlin to take the 6" screw or whatever you're using.

- Good luck