Diagonal bracing on pitch roof

[Faceache]

I've cut and pitched a vaulted roof designed by others and the building inspector has queried the absence of diagonal bracing.
The rafters are 225 x 50 at 600 centres with a 300 x 100 glulam at the ridge and birdsmouthed at the plate utilising slider shoes.
On top of the rafters are 11mm thick OSB boards, 50 x 25 counter battens in line with the rafters and finally 38 x 25 tiling battens at 100 mm centres all the way up the slope to hold plain clay tiles.
Had I'd been aware of the need for diagonal bracing I would have put it on top of the rafters in the form of metal strap before I put the OSB on but surely what's up there in terms of timber and the amount of nails vertically, horizontally and therefore diagonally would be sufficient to do the job.
I'm not a professional builder so any advice or opinons would be appreciated.

 

[noseall]

What the fk would diagonal bracing do that the OSB is not already doing?

 

[Faceache]

My thoughts exactly. The firm that designed the roof are as bemused as I am. I just wanted a second opinion before I take the building inspector on.

 

[oldbutnotdead]

Or is he querying bracing across the room to prevent roof spread? (rather than between the rafters).

 

[^woody^]

Diagonal braces, any braces are for trusses not rafters.

 

[Faceache]

Or is he querying bracing across the room to prevent roof spread? (rather than between the rafters).

By this do you mean the base of the triangle formed by the rafters would get wider over time? I don't think so as we asked for a vaulted design. I will check with him though.

 

[oldbutnotdead]

Or is he querying bracing across the room to prevent roof spread? (rather than between the rafters).

By this do you mean the base of the triangle formed by the rafters would get wider over time? I don't think so as we asked for a vaulted design. I will check with him though.

Yes that.

 

[bernardgreen]

If the ridge beam is adequately supported at both ends and, if a long span, at other points along it's length and the rafters are securely attached to it then maybe the rafters will not spread at the base of the triangle by any signifcant amount. But do listen to the Buildng Control officer. They have to deal with designs that are not structural sound from "qualified" designers and others.

 

[tony1851]

As previous posts, diagonal bracing is not needed if the roof is diaphragmed.

One thing unusual is the use of glide shoes. These are normally only used for raised tie trusses, to allow for the slight deflection and outward movement
when the tiles go on. But if there is a good supporting beam at the ridge, these should not be necessary...?

 

[RonnyRaygun]

Diagonal bracing is never, ever required for a vaulted roof. Diagonal bracing is used to prevent trusses falling over like dominoes. This will not happen to a cut roof with a ridge beam installed, as the ridge beam will be supported each end and restrained against lateral movement.
Glide shoes will not be required for lateral movement as with a properly designed ridge beam lateral movement of the rafter ends will be vanishingly small.
Likewise, rafters do not require tying at their ends if a ridge beam is used due to the reason mentioned above.

 

[theprinceofdarkness]

I am very unhappy with ridge beams, they can be as strong as you like but the problem remains of how you are going to fix your rafters to them. bearing in mind that if the roof starts to spread that the rafter will pivot at its top edge which will then try to drop. So first off all you have the stress of a long (say 9') rafter trying to lever its way out of the ridge beam, where the fixings will be close to the rafter end, say 3" which gives a leverage of 36:1, and that nail is within 4" of the end of the rafter. If you drop the fixing lower, you still end up with a very high leverage factor and a short distance to the end of the wood. The only way to get a secure fixing is to rest the opposing rafters end to end then nail on a plank across them then rest that plank on the ridge beam. If this seems a step to far, then strapping opposing joists over the ridge beam would work, providing the straps ran at least 12" down each rafter and are held by decently long screws. This method would interfere with nailing on the laths.
Frank

 

[noseall]

I am very unhappy with ridge beams, they can be as strong as you like but the problem remains of how you are going to fix your rafters to them. bearing in mind that if the roof starts to spread .

Ridge beams are designed with near zero deflection therefore zero spread.

 

[theprinceofdarkness]

Yes, that design is a theoretical number, now tell me how you are going to fix the joist in it so it can take their stress without the fixings slipping.?
Having an old barn with a spreading roof, I can see whats going on. Its a typical Victorian construction with a ridge board and where the rafters are moving, their top corners (points?) have dug into the ridge board and actually partially bent it on edge, before it split. While at the bottom of the rafters, there is a distinct gap between them and the ridge board. So I think if the board had been a beam, much the same would have happened, except the board would not have bent.
Frank

 

[noseall]

Yes, that design is a theoretical number, now tell me how you are going to fix the joist in it so it can take their stress without the fixings slipping.?
Having an old barn with a spreading roof, I can see whats going on. Its a typical Victorian construction with a ridge board and where the rafters are moving, their top corners (points?) have dug into the ridge board and actually partially bent it on edge, before it split. While at the bottom of the rafters, there is a distinct gap between them and the ridge board. So I think if the board had been a beam, much the same would have happened, except the board would not have bent.
Frank

Yes, but the design you are talking about is easily countered with a joist with little stress on the fixings. You wouldn't legislate for sudden multiple fixing failure on a modern roof because it can't happen.

Similarly a ridge beam and the gable ends are doing the (anti-spread) work for you. All the fixings are doing is keeping the joints together. The fixings are not subjected to shear stresses unless the beam or the beam supports fail.

 

[RonnyRaygun]

Frank, it's the same principle as below for lean to roofs. Yes, the fixings will need to be strong enough to prevent the rafter slipping down and imposing a lateral thrust on the external wall, but this should not be difficult to deal with if done properly.
Alternatively the rafter can be birds mouthed at the top and sat on a hanger, so that the hanger does all the work, or birds mouthed over the beam itself. This is the detail that I would be more likely to do if possible.
Unlike a ridge board, a ridge beam is designed to carry the weight of the rafters and will not sag so there should be no risk of the rafters pivoting about their top corners.

Lean-to Roofs

• RonnyRaygun
• 10 Nov 2015

Proving Lean-to roofs do not exert a horizontal thrust.