lateral stability of deck or porch roof 2

[DoubleStud]

When I do wood framed deck or porch roof that are attached to the main house through a ledger, what kind of check/calculation do I need to do to make sure the roof or deck diaphragm can cantilever from the ledger to take the lateral load? When do you decide you need a diagonal bracing from the columns? I feel like this is something that we never checked at the firm I used to work at. We almost never specified diagonal bracings at these structures.

 

[phamENG]

This is the best article I'm aware of on the subject.

 

[DoubleStud]

phamENG, when do you decide to put diagonal bracing on the columns? Or is it never an issue if the deck is supported by a ledger (non free standing)? Just assume the lateral load is transferred to the house?

 

[gte447f]

DoubleStud, I have often wondered the same thing. I have not had to design a roof structure over a deck yet, but my general assumption is that the vast majority of the ones that exist would have almost no chance of standing up in a code wind event. If I were going to design one, I would start with knee braced framed at the outboard edge and see if they could take half of the lateral load. I would not even consider "just" the diaphragm.

 

[phamENG]

You either cantilever the diaphragm, or you don't. If you do, you have tension and compression chords perpendicular to the wall. You have to develop these into the diaphragm of the house. That's why prescriptive codes require those tension ties within a foot or two of each end of the deck.

Same goes for the roof.

 

[gte447f]

phamENG, good point on the tension ties. I believe the prescriptive deck requirements of the IRC require both approaches to be used simultaneously, meaning knee braces at the outboard edge and tension ties to the building (essentially cantilevering the diaphragm). Belt and suspenders approach, I guess.

 

[DoubleStud]

Lets say you have 30 ft wide, 15 deep deck. Assume the deck is 1 ft deep. Wind is 20 psf. If we ignore the handrails and post, we will have about only 80 lbs of tension and compression. That is easily can be taken by a screw? So when do you decide to use braces and Simpson deck connector into the joist?

 

[phamENG]

Read the article I posted above. There are traction live loads. They were able to create about a 12psf lateral load with 40psf worth of people on the deck if I remember correctly. They were trying, of course, so 5-10psf is probably pretty good. These aren't code required, but prudent to consider.

 

[jerseyshore]

When I have decks with roofs over them I specify heavier duty post caps and post bases. I haven't specified a knee brace for years, no one wants to see them on their decks. I almost always use 6x6 posts and with those heavier connectors I get a little extra rigidity. Around here between poor deck foundations, lots of wind, and salt spray, a high percentage of decks have issues. I try to make sure my decks hold up to the elements, especially the ones with roofs.

 

[XR250]

How are you getting any diaphragm capacity for a deck when most use Trex and hidden fasteners systems that allow the boards to slide relative to each other?
If my customer does not want to see bracing, I do planar, under deck bracing.

 

[phamENG]

XR - yep, if it's composite decking you have to do planar bracing underneath. But there are still lots of decks around here built with lumber.

 

[XR250]

But there are still lots of decks around here built with lumber.

Sure, but there is no guarantee it will stay that way. Also, many peeps around here use hidden fasteners on wood.

"Hey we're changing from screwed-down wood to hidden fastener composite. We have just lost all diaphragm strength. Let's contact an engineer to design us some alternative bracing."
Said no contractor ever

 

[phamENG]

Sure, but there is no guarantee it will stay that way. Also, many peeps around here use hidden fasteners on wood.

This gets into an area that I struggle with. Just how "foolproof" should our designs be? Whether it's the deck fastening or floor sheathing fastening or wall sheathing fastening, if it's called out on the structural drawings then it should be followed, or changes should be run through the engineer. Any contractor can rationalize why this change or that change is "just as good as what the engineer said" without have any idea why we designed it the way we did. Decks may be low hanging fruit, but they are just as likely to leave off the planar bracing ($@^!ing engineer) as they are to exchange one type of decking material of fastener for another.

 

[WinelandV]

I know it's more drafting and paper space, but you could put both options (screwed deck boards and underside bracing) on the drawings for the decks that you seal.

Do you guys seal a lot of decks?

Please note that is a "v" (as in Violin) not a "y".

 

[phamENG]

Not decks specifically, but lots of houses with decks.

I think it would be sufficient to put a note on the drawings - maybe in general notes for wood framing - stating that the depicted decking and fastening are required for structural stability. Any changes in material, fasteners, or fastener arrangement must be approved by the EOR.

I know it's more of a CYA thing, but the line has to be drawn somewhere.

 

[jerseyshore]

I can't remember the last time I saw a deck with planar bracing underneath. I like to specify solid blocking, couple of rows at least.