Structural vs. non-structural ridge

[Timberland]

Got a question that's been driving me crazy for some time now.
I've seen the terminology floated around here and at work from time to time and I have my ideas but i'm not too sure if I am correct or not and hope you guys can send me in the right direction.

What exactly is a structural ridge in typical applications as opposed to a non-structural ridge? I am thinking that it's a ridge that is under the rafters and not flush to them? How do you determine if/when you need a structural ridge?

Tim

[Joe Bartok]

Re: Structural vs. non-structural ridge

Timberland, the article on Page #37 has a good non-technical description of the differences between the ridges. (Page #37 using the buttons, Page #35 on the actual document. The article is titled "Pity the Poor Rafter Pair").

[Dancing Dan]

Re: Structural vs. non-structural ridge

A structural ridge is, naturally enough, one that carries a structural load; i.e. that helps support the roof load. If you think of a Cape, each triangular "slice" of the roof would be a pair of rafters meeting at the ridge, and the floor joists forming the bottom of the triangle. The joists hold the rafters together so the walls don't spread, and all the ridge does is give you an easier way to build the roof - I work on a lot of older houses, and often if it's mid-19th or 18th century, there wasn't a ridge at all. A lot of timberframes don't have a ridge either. And the gable walls don't support any roof load.

Alternately, in a cathedral ceiling, there's no bottom chord to keep the roof load from pushing the walls out. You need to essentially "hang" the rafters from a structural ridge, so half the roof load is supported by the ridge, and the load gets traced down to foundation (typically) via posts in the gable wall.

Hope this helps.

[Timberland]

Re: Structural vs. non-structural ridge

Thank you Dan and Joe.

I think i got it now!

[Timberland]

Re: Structural vs. non-structural ridge

So for a structural ridge, I see two methods of attachment for rafters:
1. Flush w/ ridge (same as non-structural connection)
2. Rafters lay on top of the ridge. With this method, do you guys use special hangers to connect the rafter or do you cut a birdsmouth at the top of the rafter?

Dan,

I see what your saying about the older homes not having a ridge at all....i own a 100 yr old farmhouse and have done some exploring, but i'm not quite sure on why a structural ridge is needed, can't the new rafters butt up against a new non-structural ridge the same way a dormer would be built on a new house or is it just about not trusting the old lumber?

Tim

[Bill Lacey]

Re: Structural vs. non-structural ridge

If you're using a structural ridge that you want hidden from view, then use an adjustable slope hanger (such as the LSU series from Simpson) to attach the rafter to the ridge beam. If you're looking for an exposed ridge beam, a birdsmouth with an appropriate anchor would work.

[Joe Bartok]

Re: Structural vs. non-structural ridge

We use flat bar recessed and lagged into the upper surfaces as in the attached image. I don't have pictures of my own of rafters but I believe I have seen them with a bird's-mouth cut to accommodate the ridge.
Any roofs I've done the figuring for used purlins to supplement the ridge beam. The ridge beams are housed in the trusses. This roof was finished with SIPs; some are framed over with 2×14 rafters.

EDIT: Check this out. Is this a Ridge Beam or what?

[Timberland]

Re: Structural vs. non-structural ridge

So the way i see it is that it's a lot of extra work to do a structural ridge (obviously) but really only done when you want an open cathedral ceiling while stick building to eliminate the need for ceiling joists and collar ties or just working on older homes.?

Thanks guys for taking the time to help me out. I love residential construction especially framing, it's just too bad i never took it up but i try to learn when I can.

[Dancing Dan]

Re: Structural vs. non-structural ridge

I don't think you're getting it yet, Timberland - basically, if a roof is symmetrical to the ridge, and has a bottom chord (think gable roof), then the forces balance out and are held in tension, so you don't need a ridge. But if you have a simple gable roof, and you cut out 20' and put in a shed dormer, the forces on either side of the ridge are being applied unequally, the rafters aren't holding each other in, etc - you may not necessarily need a structural ridge, but you (or your SE) certainly need to determine whether or not that's the case. Plenty of dormers get built without them, and plenty of those probably should have them.

[Bill Lacey]

Re: Structural vs. non-structural ridge

See if these make sense.

[Tim Uhler]

Re: Structural vs. non-structural ridge

So the way i see it is that it's a lot of extra work to do a structural ridge (obviously) but really only done when you want an open cathedral ceiling while stick building to eliminate the need for ceiling joists and collar ties or just working on older homes.?

Thanks guys for taking the time to help me out. I love residential construction especially framing, it's just too bad i never took it up but i try to learn when I can.

We nearly always use a structural ridge and the engineer has not yet spec'd hangers for the commons or the hips. We don't deal with snow loading and you might, but here I think the loading is 25 or 30 live and 15 dead.

I walked through a framing job a few months ago and the engineer for that job let the framers use joist hangers, but the framers make a kerf in the plumbcut of the rafters for the seat of the joist hanger and then they nailed the hanger into the rafter and the ridge.

One thing you should be aware of, is that structural ridge does not necessarily mean big beam. We use 1 3/4" x 14" LVL regularly and on the job we are on now, there is a section of 1 3/4" x 16" LVL that is about 22' long or so. When we use beams, we toenail into the side and hold the ridge 1" low for ventilation (these are typically cathedral ceilings). We do set the rafters on top sometimes like in picture

[Sweep8]

Re: Structural vs. non-structural ridge

Be cautious about using a single large LVL as a beam. Manufacturers require that 16" and larger LVL's only be used in multiple member beams. There may be circumstances where this is not necessary (such as full height bracing with steel rafter/joist hangers) but only an engineer would be able to determine that.

There also may be good reasons to use a single 16" LVL as a 22' ridge board but structurally it would not be able to carry much more than the dead load of a 28 ft wide gable roof house.

It is easy to size an LVL from a chart but because they can carry larger loads than dimensioned lumber, the connection, bearing and lateral bracing details are much more critical and should be designed by an engineer.

[Timberland]

Re: Structural vs. non-structural ridge

Bill,

For some reason i can't open any PDF's posted on this forum. Maybe I don't have the latest Adobe reader....I don't know.

Tim,

Our roof loads up here are 50 LL and 15 DL. Thanks for posting the pic. Pictures always help me visualize stuff a lot easier.

This may be a dumb question but it's within this topic: Are valley and hip rafters always structural then?

Thanks again everyone for the info.

[Joe Bartok]

Re: Structural vs. non-structural ridge

Here's a really loo....onnng Hip and Valley Rafters thread in this forum discussing structural and non-structural hips.
As you read that thread, keep in mind the saying: "When you're up to your ass in alligators, it's hard to remember that your original objective was to drain the swamp".

[Bill Lacey]

Re: Structural vs. non-structural ridge

If you want I can email the pdfs to you or I can create jpg files. I can't get the files small enough (pixel wise) to attach them as jpgs on the forum.