In roof framing, how does a ridge beam prevent lateral pushout of home walls? 10

[CivilSigma]

I have been researching site-built roof structures with ridge beams such as the following:

Typically, you see collar ties or ceiling joists to help tie the truss and prevent lateral movement. How would a ridge beam prevent lateral movement in this case?

Thank you.

 

[jayrod12]

Because all of the load applied is vertical, and there's a vertical support at each end of each rafter. There is no lateral thrust to be resisted.

In a rafter and tie roof, the rafters push against each other at the top creating the thrust.

 

[JoelTXCive]

Difference between ridge board and a ridge beam with posts.

See attached Journal of Light Construction Article.

 

[GeorgeTheCivilEngineer]

This question has been asked a lot here! You're trying to imagine where the truss forces have gone when it's actually two beams spanning between the ridge beam and the wall.

Where is the horizontal force coming from? The dead loads act downwards and can be resolved at the two support points vertically. The internal tension and compression forces cause by the angle of vertical forces acting on the rafter balance out. The lateral force from live loads are no different to a truss roof. However - to make this work, the ridge beam must be stiff so deflects very little. It is a 'ridge beam' and definitely not a 'ridge board'.

I've actually built this on my house - I used a steel beam and a plywood sheathing over the rafter so the roof is very stiff and not going anywhere! I also put joist hangers instead of skew nails to make sure there was no slip.

 

[MIStructE_IRE]

If it can’t go down it can’t push out.

 

[CivilSigma]

Thank you all for the explanations!

@MIStruct I first read your reply, and everything else made sense. Very well said, thank you.

 

[dik]

I've often used metal straps at roof joists (rafters) to eliminate/reduce horizontal thrust. The ridge is designed as a beam and not just a 'ridge board'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better? Dik

 

[r13]

Both of IRE and dik's comment makes sense. Actually there is horizontal thrust, but the energy is released through the flexible nature of the wall/vertical support, and the outward deflection is small, as the ridge beam is made stiff.

 

[jayrod12]

Retired, there is no thrust.

Do the statics, if there is no horizontal load, there is no thrust.

Rafter and Tie roof, yes, ridge beam roof, no.

Dik, where do you install the straps in a rafter and ridge board roof to eliminate the thrust? Unless you install them at the bottom, then you aren't reducing the thrust. If you're installing them at the top of the rafters, you're performing the same function as collar ties, keeping the rafters together at the peak under uplift.

 

[MIStructE_IRE]

Jayrod, I think what retired is saying is that the thrust is a secondary effect of the ridge beam deflection, which is correct. As the beam deflects slightly, the top of the walls push out ever so slightly.

I suppose for statics, you could consider the apex a spring and both eaves as horizontal rollers. That would allow you to determine how much the top of the walls would deflect laterally - but that’s just making it way more complicated than it needs to be!

 

[r13]

IRE,

Thanks.

 

[jayrod12]

I'd have to agree with that assessment. However, why do we always need to overcomplicate things? If anything, those types of answers tend to muddy the waters.

If I'm designing a roof with a ridge beam, I'm not concerning myself with any sort of thrust. And I wouldn't tell a junior engineer about it either. Because that's not how that stuff is designed.

 

[MIStructE_IRE]

I’d agree.

 

[CivilSigma]

@Jayrod

I was actually reading the Ontario Building Code for residential construction, and one of the clauses permits you to construct a roof without ties if you have a ridge beam. So that got me thinking about the differences.

The code also requires that the ridge beam is supported on intervals. It makes sense to me now that the beam will prevent the vertical displacement of the roof, and therefore prevent lateral movement of the walls. But, for rafters without a ridge beam support, nothing stops them from displacing downwards, and pushing the walls out.

 

[jayrod12]

But, for rafters without a ridge beam support, nothing stops them from displacing downwards

Correct, and that is why without a ridge beam, you need to have some form of tension tie. In many cases, this is the ceiling joists. But, you must ensure that the ceiling joist connection to the rafters is adequate to resist the thrust, same goes for the connection between ceiling joists where they are spliced over the interior walls.

Without a continuous load path, bad things happen.

I don't know why they indicate the ridge beam must be supported at intervals. If designed appropriately, the ridge beam could span the entire length of the roof. However that is generally uneconomical and therefore you usually need to find somewhere to stick a post down.

 

[oldestguy]

Interesting how this discussion always forgets the beam laying on its side, the roofing plywood.

 

[BAretired]

I'm not keen on the eave detail in the original post. There should be a proper birdsmouth cut in the rafter where it meets the top plate. If the rafter bears on a sloping surface, even when there is a ridge beam, there is a tendency for the top of wall to move inward, resisted only by friction and a couple of toenails. Under heavy snow load, this could cause collapse.

BA

 

[XR250]

Interesting how this discussion always forgets the beam laying on its side, the roofing plywood.

Because the math and detailing is pretty darn messy. That being said I've witnessed plenty of houses constructed without ridge beams and they been fine and I also have seen many that have not. Good luck if you design yours that way!

 

[GeorgeTheCivilEngineer]

Here's my roof... you'll note the proper birdsmouth BA.

The steel was deliberately set slightly low (trying to align with a very old house mean a margin for error). Came in maybe 100mm too low but didn't seem worth taking it out to shift it upwards!
What we didn't do was get a good ridge cut, hence the joists hangers.

This was then sheathed in plywood.

 

[r13]

CivilSigma,

I think you got the ridge beam part right, it needs to be stiff enough to prevent excessive deflection, in turn, avoid excessive outward movement due to thrust. The number of intermediate supports depends on the length of the roof.

But, for rafters without a ridge beam support, nothing stops them from displacing downwards, and pushing the walls out.

Isn't then, tie/ties are required to hold the rafters from falling apart/move too far out!?