Purlin (9x3inch) with 6m clear span

P

PeteMars

Joined
5 May 2024
Messages
16
Reaction score
0
Country
United Kingdom
I'd like to know if anyone else has a roof supported similar size (9x3) pair of purlins at 6m spans, or if a previous owner may have removed some type of structural support.

1000008984.jpg

1000008987.jpg

Question has arisin after having roof retiled from original clay to concrete double Romans. I calc a total load change from 2.5 tonnes to 3.5 tonnes, so just under 50% increase per sqm.

There are no struts to shorten span. roofer didn't think it was any issue as purlins are "big" but a structural engineer on site for another issue said it needs reinforcing by thickening the middle.

95 year old house. Only slightest deflection visible now.

There's also 2 spliced timbers running parallel to the each Purlin, but totally independent and on top of ceiling joists, but fixed into party walls: I just can't visualise what they are doing structurally, unless some ceiling joists are 'hanging' from them, even though the are mostly resting on top of walls below.

1000008985.jpg

1000008988.jpg

All very mysterious.... all insights welcomed.
 

Attachments

  • 1000008984.jpg
    1000008984.jpg
    203.7 KB · Views: 73
Last edited:
It looks pretty straight, there would be evidence of something having been removed if there had been. Knock on your neighbours door and ask if you can look in their loft, if they're terraced houses all the same.
 
Looks pretty normal to me, although someone ( probably the person who fitted the insulation ) has removed all the ceiling hangers! The 3 x 2 timbers on your ceiling are binders.
I don't think the load will have changed much.
 
catlad said:
Looks pretty normal to me, although someone ( probably the person who fitted the insulation ) has removed all the ceiling hangers! The 3 x 2 timbers on your ceiling are binders.
I don't think the load will have changed much.
Click to expand...

Oh really?
What should it look like? I'm struggling to visualise the how the loads should work..!
 
Your old ceilings would have had a row or possibly two depending on the size of ceilings of 2 x1 inch ceiling hangars which prevents the ceilings from sagging. These are normally attached from rafters to ceiling tie or sometimes the binders. These seem to be missing from your photo's.
If you are going to put these back it would be a good idea to prop / jack your ceilings back level first.
 
Very interesting! So i think I understand the purpose of the binders now.
Our binders are actually simply supported in the party wall on one end and the chimney breast on the other end. They run parallel but ~1m out from the bedroom wall below. Might these binders be adequately preventing ceiling sag or you expected to see hangers from rafters to the binders too, or hangers from rafters to point on ceiling joists between binder and external wall? The Binders don't seem to show any deflection.

A diagram or photo showing some examples would be v helpful!
 
The problem with reinstating the hangars (they are literally bits of timber, sometimes old floorboards, hanging the ceiling joists off the purlin) is that they will increase the load on the purlin - which is your concern. If the SE says it needs reinforcing ask him to spec something (probably a steel flitch plate bolted to the purlin) and mention that you propose to reinstate the hangers.

Screenshot_20240509-092424.png
 
Thanks For the explanation . Will do.
Nice pic. Lots of bracing!
 
That purlin would be way undersized using todays regs, even with just taking the standard snow load of 0.75kn/m2 and assuming the purlin supports 1m of roof each side of it i.e. 2m2 , so loading per metre run is 1.5kn, taking c24 timber the actual bending moment in the beam would be 40% over the allowable and deflection (which should be limited to 14mm) would be 35mm. Lets hope global warming stops any snow.
 
PeteMars said:
Thanks For the explanation . Will do.
Nice pic. Lots of bracing!
Click to expand...

That is quite similar, to my roof - similar sized purlins going end to end, and a similar length. The difference is, my purlins are supported mid-span, by a pair of 4x5 timbers, top end with a bird's mouth where it meets the purlin, forming a V, taking the load down onto a solid brick partition wall, via a wall plate. Then horizontally, just below the purlin, was a 3 x 2, simply tacked to the sides of the V. I cut that short, as it made getting through, end to end, difficult, and I made a judgement that it had been fitted to hold everything in place, during the roof construction phase.

Do you have a similar structural wall, onto which you could fit a V support?
 
Thanks For this. Any photo? Yes there is a parallel solid partition wall running +half width of house, at midpoint between each Purlin. Centre of attic floor is currently boarded over for storage so I can't see the top of that wall, but guess that's something that a pair of struts could land on. I'd have to see some examples of bird mouth joints and wall plate joint to attempt this, as I'm no carpenter.

There are also perpendicular partition walls below, each purlin at 2/3 span, but they appear to stop close to below purlin where the bedroom doors are. Seems that possibly a vertical post or small brick column could be erected on these to take some load directly down too. Am guessing a 'bird mouth' is needed on top of post too...

FYI Structural engineer who had a brief look (was here for a diff job) had noted the absence of midspan support and just said reinforcement of centre of purlin was needed, and is (informally) providing a detail for a DIY fix soon (he's v old school) but I like the idea of reducing the load through mid support and I'll prob ask him about these additional supports too. I've Terrible paranoia w.r.t. it being literally roof over our head.
 
PeteMars said:
I'd have to see some examples of bird mouth joints and wall plate joint to attempt this, as I'm no carpenter.
Click to expand...

It's just a V, cut in the end of the supporting timber, with angles set to match the purlin. The birds mouth, means once located on the purlin, it cannot slip off.
 
You must log in or register to reply here.
Back
Top