Ridge placement

[4thorns]

Hi All,


I’m trying to determine if the ridge pole is carrying the load of the rafters regardless of where it is placed.

Situation 1: I put a structural (load bearing) ridge between the rafters and they are secured properly(ties etc.). - I assume the ridge is carrying the load.

Situation 2: I put the exact same structural (load bearing) ridge under the rafters. - - I assume the ridge is carrying the load.

In each case my thought is to determine the load in exactly the same which would mean that the ridge required to carry the load is the same for both. The only difference in the two scenarios is the placement of the ridge. To me, if the rafters are secured properly, in either case the ridge is doing all the work so it doesn’t matter where the rafters are attached. Does this sound right?

Thanks for your time,

Doug

 

[msquared48]

Technically, it makes no difference as to whether or not the rafters frame to the side of the ridge member or rest on the top of the ridge. The ridge still takes the same load.

There are exceptions to this rule involving cantilevered rafters, but I will not go there.

Mike McCann
MMC Engineering

 

[Ron]

The ridge beam should not carry lateral load, but reacts the vertical load of the rafters.

Maybe I'm missing something, but I don't see the difference between your two scenarios. Please explain.

 

[BAretired]

Sounds like the ridge beam carries its tributary width of roof in either scenario. So where is the problem?

BA

 

[dhengr]

4thorns:
Situation 1 - The ridge board can be a 1x6, 1x8, maybe a 2x; one of the rafters is end nailed through the ridge board, the other rafter is toenailed into the ridge board and other rafter; this nailing does not provide for much of a vert. reaction to the ridge board. In fact the ridge board primarily spaces the rafters at 16" o/c, and assures that they react against each other through the ridge board. The primary reaction on a rafter at the ridge plumb cut is horiz., btwn. the rafters. This requires detailing at the rafter heel, at the ext. brg. walls, to take an equal and opposite horiz. thrust. This might be a birds-mouth seat cut, special thrust hardware, a ceiling joist (tie) to take the thrust, etc. There is also a vert. reaction at the brg. walls which, on a symmetrical roof is one half of the load on the entire roof. This is fairly standard stick framing for a roof and the ridge board is not intended to take a vert. load over a significant span length. It will account for minor unbalanced loading, but end nailing and toenailing are just not a very good structural connection. The all important condition here is the tie at the rafter heel and a strap (tie) at the ridge is not particularly effective here, because if the tie at the heel isn’t working, the tendency is for the heel to move out horiz. and the plumb cut at the ridge to settle vert. pulling away from your tie over the ridge.
Situation 2 - The ridge beam under the rafters (no longer a ridge board) attracts primarily vert. loading from the rafters. To assure that there is only a vert. reaction at the ridge and at the heel (seat) cut, these cuts should be min. sized bird-mouth seat cuts. Now a strap or tie over the ridge holds the rafters together and aligned because they can’t settle.
Free body diagrams of the rafters show the following: Sit. 1 - horiz. reaction at the ridge plumb cut, and an equal and opposite thrust at the rafter heel, plus a vert. reaction to the brg. wall, with the rafter acting as an inclined beam; Sit. 2 - has vert. reactions, each with one quarter of the roof width contributing, and no appreciable horiz. reactions. An unsymmetrical roof framing scheme or signif. unbalanced loading are a whole different problem.
In each case it is not how you “determine the load in exactly the same which....”, but rather, that you determine the loads to correctly model the way the structure actually acts.

 

[4thorns]

Sorry for the delay in response.

Thanks for your input guys.

Quotes:

"Technically, it makes no difference as to whether or not the rafters frame to the side of the ridge member or rest on the top of the ridge. The ridge still takes the same load"

"The ridge beam should not carry lateral load, but reacts the vertical load of the rafters.
"Maybe I'm missing something, but I don't see the difference between your two scenarios."

"Sounds like the ridge beam carries its tributary width of roof in either scenario. So where is the problem?"

These three posts pretty much are in my line of thinking. In each situation the ridge is the only support for the rafters..No collar ties etc.

I understand your post dhengr but to me it refers to a situation where collar ties or another means of support exists at the heel of the rafters. In this case the ridge is structural and the only thing supporting the rafters. I realize that attaching the rafters to the side of the ridge is not as strong as on top of the ridge, but if they are secured properly I don't see the difference between the ridge between or below the rafters.

Doug

 

[msquared48]

The only issue would be the need for venting in a situation where sheetrock is below the rafters. It is easier to vent with the ridge beam dropped, otherwise you are looking at a ridge vent.

Mike McCann
MMC Engineering

 

[msquared48]

Also (obviously not the "only" issue"), simplistically speaking, if the center of the roof diaphragm was not at the ridge where the wind/seismic shear is zero, then there would be a shear transfer issue, and dropping the ridge beam would be the better choice.

Mike McCann
MMC Engineering

 

[Ron]

No collar ties can be a disaster! Would you like some photos of a roof failure with not collar ties?!

The lateral thrust at the top plate can be significant. That's why you need collar ties. The ridge beam is a relatively simple issue as compared to the lateral thrust.

 

[msquared48]

Ron: Sorry if I misunderstand you here, but...

If the rafters are notched for flat bearing at a structural ridge beam and wall double top plate, the only lateral force I can see would be due to the vertical deflection of the ridge beam, causing the walls to move ourward slightly. Normally, this should not be a problem.

However, if the ridge beam is non-structural, as in a 1X8 or the like described above, then I agree that you'd better have some form of collar tie or lateral thrust mechanism to hold things together.

Sometimes lateral expansion is not a good thing - waist lines included.

Mike McCann
MMC Engineering

 

[BAretired]

Ron,

If the ridge beam is load bearing, you do not need collar ties. The rafter span from ridge beam to wall and there is no problem.

BA

 

[Ron]

Mike and BA...for most ridge beams in conventional framing, there is no end bearing for the beam, therefore some lateral force will result at the top plate. This is more critical for balanced load cases (wind parallel to ridge (inward on wall), uniform snow load (outward on wall), or loading the roof with shingles/tiles/slate for new or re-roofing (outward on wall).

I agree that if the ridge beam is sufficiently competent and has bearing, lateral is small.

 

[hokie66]

If the ridge beam has no bearing, I don't see how you could call it a beam.

 

[4thorns]

Thanks to all for your posts.

In every design our ridge beams are supported at both ends and has sufficient bearing to carry the loads. I've learned a couple other things with this thread and thank you all for your time.

Doug

 

[paddingtongreen]

I'm with hokie66, is it a ridge plate or a ridge beam? The beam provides vertical support, the plate doesn't.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[BAretired]

The OP stated the ridge beam was load bearing.

BA

 

[msquared48]

Ron: Sorry, but I am really confused by your statement here...

"for most ridge beams in conventional framing, there is no end bearing for the beam"

I guess I would have to ask what you consider to be a ridge beam as I ALWAYS, no exceptions, nill, no way, ever have one without end bearing. I assume you mean bearing for vertical loads. Are you talking end bearing to resist loads parallel to the ridge beam?

No offense intended here, I'm just confused. Thanks.



Mike McCann
MMC Engineering

 

[geoffwatson]

If the ridge member is designed as a "beam" with load bearing supports the rafters would have to have their fixings at the "beam" designed for the vertical load or the rafters can be birdsmouthed over the beam to ensure vertical load transfer. If the ridge member is merely acting as a means of transferring rafter thrust then it can be of any thickness or not present at all if the rafters are accurately aligned as in mediaeval oak framed roofs. A ridge member does also stabilise the rafter heads together with local battens.