Does anyone have experience using tongue and groove decking for roof sheathing? IRC Table R803.1 lists 1 1/2" T&G for rafter spans up to 72". I am looking to space rafters at 72" however I cannot find any information on diaphragm strength of the T&G decking. Just wondering if anyone has used this decking to act as the roof diaphragm. Did anyone add plywood above the decking just for the shear strength?
Yes, I've done a lot of them, the engineers always specify 5/16" plywood diaphragm nailed on top of the decking, but I have no idea what the prescriptive code says.
The IBC references the the NDS Special Design Provisions for Wind and Seismic which has provisions for wood frame (lumber) diaphragms. http://www.awc.org/standards/sdpws.html But, this may be beyond the scope of the IRC.
The diaphragm values for timber decking are so low that except in limited circumstances something more is required. One way it to put plywood or OSB sheathing on top of hte decking. You need to be careful that at the joint where two sheets of sheathing abut that the nailing from each side is in the same piece of sheathing.
It’s been an awful long time since I’ve designed a deck like that. They made beautiful exposed and finished ceiling surfaces on widely spaced GlueLam beams, purlins and arches and the like. I’ve got a bunch of bldgs. on which I did the structure, with this type of roof system.
Of course, the idea was that the T&G improved the vert. load transfer btwn. the individual pieces of decking, to the point that you could use some sort of an improved average plank strength and stiffness vs. what grading rules would allow when considering a single piece of lumber, in effect a repetitive member increase consideration. Then you could also use two span and three span lay-up to improve deflections and capacities a little further. For the most part as I recall, the manuf’er. of the decking provided load and deflection tables for their product. There were also thicker deck products, many laminated, with two or three T&G’s in each piece, sometimes achieved by just shifting the various lams. laterally in the piece to form the T&G. Finally, some of the material was also end matched and could be laid up in a very controlled random length fashion.
Certainly, we considered diaphragm action of these decks in our designs, but it wasn’t very great in comparison to today’s sheet goods, and thus the layer of OSB or plywood on top, for the much better shear diaphragm action. MarkK is right on the money on that account and the nailing of the sheet edges. We also knew that T&G or ship lap wall or roof sheathing was a better diaphragm than just loose boards, how much better was though to get your arms around, there were so many variables. Diagonal T&G or ship lap made a better diaphragm but that was mostly because of the diag. trussing action of the pieces. The best way to see the weakness of this system as a diaphragm is to look back at the photo you posted a week or so ago, to explain horiz. shear in a beam. Your slats moved incrementally w.r.t. each other to cause the horiz. shear failure, or at least slip. In the current case the decking pieces are the slats w.r.t. each other, and their incremental longitudinal movement w.r.t. each other causes to diaphragm to be relatively weak. There are various nailing methods to improve the diaphragm action, but the OSB overlay is easier and stronger for this condition. The sheet goods just distribute these shear field stress much more efficiently and uniformly as long as we pay attention to edge and field nailing, and prevent the sheet from buckling out of plane.
Conarb’s 5/16" plywd. is an early, fairly min., recognition of this situation. I’ll bet he can’t get buy with that thin plywd. today in SanFran. My latest ed. of the AITC still has a chapter on the use of the adz and broad ax for trueing up timbers, so I can’t be much help there. But, I’m sure there must be some current industry publications on the matter; you seem much more up-to-date than I am on some of this current liturature stuff. I would start by superposition and combining the strengths of the two systems, as I understand them, and I would assume this is what the wood industry has done, along with full scale testing to verify a design approach. Are you using your own sawn decking? Then you need to consider grading, allowable strengths, E, etc. for the decking. The OSB diaphragm values are in the NDS, and you combine the two. I’m sure that if you glued and nailed the OSB to the T&G decking you could improve the system performance even further, whether the codes would recognize this might be another matter.
I’m sorry that I didn’t address you, for some reason I thought DRP started this thread. You certainly have my permission to read and digest my post. Since you say ‘rafter spans up to 72"’ and ‘looking to space rafters at 72"’; I assume you really mean deck span of 72" on rafters (?) spaced at 72", and you are not talking about 2x8 or 2x10 rafters (?). Talk to some deck suppliers, they’ll have literature, load and deflection tables, etc., much better than any generic IRC table. And, likely some current literature on diaphragm action. This decking is a fairly specialized product, graded much differently than a prescriptive table could cover; and a diaphragm system more complex than the IRC normally considers, but not disallowed by the IRC.
No problem about not addressing directly to me. Thanks for the incite. And yes you are correct the deck span is 72" (rafters spaced at 72").
We just did a heavy timber patio cover with 2x6 t&g roof decking and the engineer called for 2-3/8" diameter lag screws going into the 4x8 rafters per plank. We were discussing why he called for 3/8" instead of 1/4" as it seemed a little big and he said it wasn't too big and would take care of the lateral shear requirements.
You can download the AITC Standard for T&G Roof Decking from:
Pg. 5 gives the nailing schedule for heavier decking. It only briefly mentions diaphragm action, but doesn't say how much the deck provides. It might be adequate for a smaller house in a non-seismic low wind area (a lot of houses were built like this in the 50s & 60s and are still standing), but a layer of plywood on top would be prudent.
You can get more information on plywood & OSB for diaphragms from:
The main problem with this construction nowadays is getting enough R-value above the deck.
You talked about 2x6 t&g roof decking on 4x8 rafters with 2 - 3/8" lag screws through each 2x6 and into each rafter. To get a feel for what your engineer was trying to achieve, try this experiment. Exact material is not real important because we are just trying to get a relative feel for the situation and the point I’m trying to make. Using a 6 or 8' long 2x6 or 2x8, attach one end to a rafter piece, not too near the end so as to eliminate end splitting of the deck piece. Clamp the rafter piece down and pull on the free end of the deck piece, in the plane of the deck, trying to torque the deck piece w.r.t. rafter piece, i.e. changing the 90° angle btwn. the two pieces to 95°. Keep the various dimensions the same so the only change is the fastener size, and apply the pulling force with a spring scale. Use 2 - 16d common nails, 2 - 20d or 30d common nails, 2 - 1/4" and 2 - 3/8" lag screws, lengths should be more than two times the deck thickness. The lever arm btwn. the two fasteners, on this resisting couple is about 3.5" for the 2x6 and about 5" for the 2x8 and only the fastener size changes. The lever arm to the applied force, at the spring scale, would be about 5.5' or 7.5', and doesn’t change. Certainly, you should see the larger fasteners giving larger resistance to the twisting action.
The way this deck structure works as a shear resisting diaphragm is to sum all of these individual connections (resisting couples), which are preventing the decking from racking w.r.t. the rafters, i.e. keeping the whole diaphragm square, or from parallelograming. You can actually force and feel this parallelograming on poorly nailed decks, both on the whole deck at the free edge or at the individual joints deck to joist. There is some friction btwn. the decking pieces, but it is very difficult to put a reliable number on this, given shrinkage and the like. Gluing these connections would be an improvement, but code approval may be questionable without a specific testing program. Also, rim joists and blocking are important to prevent the rafters from rolling over due to this loading, as the system transmits the load to lower level structure.
Sheet goods are very good at transmitting this kind of loading. Rather than doing this through many discrete resisting couples, the sheet goods do this as a continuous panel, and we talk about the loads or stresses, in plane, as a shear field, or as a shear flow in #/sq.in. The experiment here is to stand a 1/4 or 3/8", 4x8 sheet on edge, with its lower corner against a sole plate; and push, in plane, on the upper opposite corner. The sheet will not parallelogram, but it will buckle, out of plane, under sufficient load. Thus, the need for strict edge nailing schedules, and somewhat lesser nailing in the field, to prevent out-of-plane buckling under the shearing load. The discrete condition here is primarily the summing of the edge nails and their spacing. And, you will see tables which allow greater shear loading on a given panel with tighter nail spacing. Also, stud or rafter spacing comes into play in this buckling problem, so 16" o/c spacing will give higher values than 24" o/c spacing for a given sheet thickness; and this system too can be improved by gluing the joints btwn. the sheet and the rafter, joist or stud. In each case you are improving the system performance by transmitting the forces or stresses as more uniformly distributed shear flows rather than at discrete, relatively weak resisting couples.
The ultimate here is the thin skin on an airplane wing, a shear panel btwn. the ribs and stiffeners. Here we control the skin buckling, but it still buckles several magnitudes of the skin thickness, something we can’t tolerate on most of our structures, and the forces and this slight buckling cause the skin to become a diagonal tension field btwn. the ribs and stiffeners, rather than a rigid shear panel. In effect, the ribs are like the top & bot. chords on a floor truss and the skin (diagonal tension field) is the truss diags. And, now the skin is a carbon fiber composite, only mm thick, and the whole system is bonded to provide the connectivity.