Supporting loft purlins with struts...angle issues

[TonyBuk]

Hello All - have been scratching head on this one. Need to get strut supports to sit on load bearing wall (where central props currently are just off-centre). To angle them so one end sits bird-beaked to purlin and other end sits on central load bearing wall seems too acute an angle to offer any real support to roof. There is only one central load bearing wall below the loft that’s all.

Any advice on how to fix this one greatly appreciated many thanks

 

[^woody^]

What's wrong with the existing arrangement?

 

[TonyBuk]

The strut supporting the purlin bears on to the nominal ceiling joist and surveyor insisting it bears on to central load bearing wall (where the upright props are sat).

Can’t work out how to ensure strut sits on same load bearing wall

 

[^woody^]

Can’t work out how to ensure strut sits on same load bearing wall

But as said previously what is wrong with it now? ie why does it need to be moved (ignoring the surveyor for a moment)

Have you asked the numpty who insisted it bears on the wall how to do it? Like "How do I get a strut that should be at 90° to a the purlin to reach the centre of the loft?" Do they do different geometry in surveyor land?

If he mentions spanning a beam across, and forming a truss, then he wins a bronze clipboard.

 

[John D v2.0]

Generally the structural engineer would specify precisely what's needed, and any surveyor would take it as read that it's alright given the calculations. Presumably you've bought a house that's been modified without building regs signoff and the surveyor is making an opinion on the existing structure? I think a structural engineer might save you the hassle.
Also, struts don't have to be perpendicular, as long as there is a perpendicular component, but again the se would advise.

 

[TonyBuk]

I see you share my pain. The house has been fine for 120 years.

The exact request was install a system of new struts to transfer the weight to the load bearing centre.

I think this photo might show that it’s a real acute angle from purlin to centre wall where props sit.

The front roof even more acute still.

 

[^woody^]

The exact request was install a system of new struts to transfer the weight to the load bearing centre.

Classic. Telling you what to do, without telling you what to do, and accepting no responsibility for how you do it.

If this was a building surveyor, unless he's a bit special, then he wont have a clue about structural design.

If this was a structural engineer, like it should be, he'll have told you either what to do or not to bother.

 

[TonyBuk]

Thanks will speak to SE tomorrow to unravel mystery

 

[catlad]

That set up has been altered , and not by anyone with structural knowledge.

 

[John D v2.0]

That wood is definitely not all 120 years old. And I'll agree there's something strange, the roof doesn't seem to know if it's a triangulated roof or one where the rafters are all supported vertically.

 

[TonyBuk]

Thanks yes the struts just replaced struts in there previously there. No birds beak nothing just sat against the purlin (and not bearing on any wall below just the rafters).

Bit of a cowboy job all round.

I suppose however the question remains the same: how do you support the purlin so struts bear on to central load bearing wall.....waiting to see what structural engineer says!!!

 

[Ian H]

I wouldn’t fancy having them at an angle just incase one side of the roof weighs more than the other.

 

[tony1851]

The more acute the angle, the less effective the support will be, so what the surveyor suggests would mean replacing a sub-optimal support with an even more sub-optimal support (you know that intuitively).

But there is another way of looking at this. Because the strut is at an angle, the compressive force in the strut can be ‘split’ into a vertical component V and a horizontal component H.

Consider these separately. Assuming the connection between the base of the strut and the ceiling joist is reasonably secure, force H will be resisted by the floor as a whole. The chipboard deck will join all the ceiling joists together making it one big rigid horizontal plate – you could regard this as a large horizontal beam. It will effectively support H, so the strut will not be moving horizontally.

For resisting force V, this will depend on the strength of the ceiling joist. These are often small, eg 75 x 30, but as the connection seems to be near the end of the joist where it bears on the supporting wall, it might not be a problem. This can be checked quantitively to make sure that the joist is not being over-stressed, but the SE would need to do some number crunching.

FWIW, my hunch is that it’s OK – not by the book admittedly - but still OK on the basis that the roof is still there. Maybe try get your SE to write a brief report, perhaps with a few numbers thrown in, to make everyone happy?

(The ‘correct’ bomb-proof alternative would be to span a steel beam directly below the purlin and prop off that = £££s )

 

[cdbe]

That looks virtually identical to my old house, except I had no propping. At that time it was common to replace the lightweight slate roofs with heavy concrete tiles and the "selling point" of many of the local roofers was that they would install additional support for this in the loft - I imagine this is what you'd get.

 

[^woody^]

The more acute the angle, the less effective the support will be, so what the surveyor suggests would mean replacing a sub-optimal support with an even more sub-optimal support (you know that intuitively).

But there is another way of looking at this. Because the strut is at an angle, the compressive force in the strut can be ‘split’ into a vertical component V and a horizontal component H.

Consider these separately. Assuming the connection between the base of the strut and the ceiling joist is reasonably secure, force H will be resisted by the floor as a whole. The chipboard deck will join all the ceiling joists together making it one big rigid horizontal plate – you could regard this as a large horizontal beam. It will effectively support H, so the strut will not be moving horizontally.

For resisting force V, this will depend on the strength of the ceiling joist. These are often small, eg 75 x 30, but as the connection seems to be near the end of the joist where it bears on the supporting wall, it might not be a problem. This can be checked quantitively to make sure that the joist is not being over-stressed, but the SE would need to do some number crunching.

FWIW, my hunch is that it’s OK – not by the book admittedly - but still OK on the basis that the roof is still there. Maybe try get your SE to write a brief report, perhaps with a few numbers thrown in, to make everyone happy?

(The ‘correct’ bomb-proof alternative would be to span a steel beam directly below the purlin and prop off that = £££s )

That reminds me, how is the bridge coming on Isambard?