I am adding a great room to my house (roughly 20’X20′). The ceiling has unalam beams of radius 24’11” measured to the top of the beam. I have nailed 6″X3/4″ pine T&G perpendicular to the unalams for the ceiling . I need 7″ of closed cell spray foam for R-49 (live in NYS) so I have to run 2X8s parallel to the T&G (I will use right angle brackets lagged into the unalams through the T&G and screwed into the 2X8s) to make a cavity for the foam. Then I need to sheath over the 2X8s in preparation for a membrane roof. My question pertains to the sheathing. Architects call for 5/8″ sheathing but do not specify type. We’ve been using Zuber ZIP 1/2″ for other flat roofs in the renovations. Will there be any problem bending the ZIP to the radius of curvature of the top of the 2X8s (24’11″+3/4″+7 1/4″). The 2X8s are 24″ on-center (but measured perpendicular to their tops: the spacing of the bottoms on the 3/4 T&G is slightly closer). Which thickess should I use (some say 1/2 is fine and easier to bend)? What fasteners should I use to avoid having them pull through the ZIP.?Would plywood be a better choice?
Thank you, Tim.,
Replies
tim, i would suggest sticking to the design. you have plans to go by, why stray from that? if you have plans that specify thickness of plywood, then there is also going to be a fastening schedule. if there isnt a fastening schedule i would maybe look at 3'' deck screws with a small washer for the areas you are concerned about pulling out because of the radius.
Decking
Mark, that was what I was leaning towards, thank you very much for your reply! Tim
The convex geometry will naturally give the zip panel more rigidity. You should be fine with the 1/2" zip, but I would check with the architect first. I would get the 10' long sheets* for less joints/smoother curves. Recommended fastener length is generally 3X material being fastened. I would recommend this fastener:
http://www.mcfeelys.com/8-x-2-1-2-promaster-wood-screws-nocorode-plus-round-washer-head-combo-drive-qty-1000-9jdm3.html
They're round washer heads, but will distribute the widest range of clamping pressure per fastner. The head should pull just about flush with zip panel face.
*7/16" zip wall is available in 4x10', but not sure about the full 1/2" thickness.
Curved Roof Fasteners
Deadnuts, thank you for the advice. Per your advice, I did go back to the architects and they gave me the following choices: 16" on center, 1/2 inch ZIP, 24" on center, 5/8" inch ZIP. I did check with Zuber and they don't make the 1/2 or 5/8 in 10' (unfortunately, that would have been great for this application). The architects recommended screws with washers but I really like your fasteners (killed two birds with one stone) so I ordered them. Wasn't quite sure what you meant by " The head should pull just about flush with zip panel face" ... do you mean to try to sink the washer face below the panel surface so the top of the heads are flush? Thanks, Tim
We're sheathing a (non curved) roof with 5/8" zip right now. It's fairly stiff material. While I'm sure you can get it to fasten, my fear is that you will not get a smooth radius with this 8' material at 24" O.C. It may take on a segmented curve effect with more pronounced bridging at the panel joints. Hard to tell w/o trying it with a test panel. It alsot depends on how accurate you want to be with the curve. I imagine you want to be fairly particular with the honesty of the curve.
We used the 10' x 7/16" zip for our walls and I feel this this stuff would readily follow your curve. I'm wondering if the architect would accept 12" O.C. furring with the 7/16" zip. It may be worth the extra furring and clips to do this.
My comment about screw head pulling flush is about driving the screw deep enough to bury the raised round head. It is not optimal to break the surface of the zip by torquing it down this much. HOwever, how much you torque the screws depends on how many you use and what kind of underlayment you roofer is using. Residential EPDM underlyament is usually 1/2" and soft enough to absorb fastener bridging issues on top, as well as underneath. YOu may want to discuss the fastening schedule and detail with your architect and roofer before deciding how deep you need to set the washer head screw fasteners for zip panels.
ZIP Board
Deadnuts, thanks again for your response. The arclength over the tops of joists is around 25' so we will need between 3 and 4 ZIP panels to get up and over and about 5 to get across. I suspect you are correct - it is going to look a bit segmented but with the staggering the segments will also be staggered. The 5/8 (after several discussions with the architects, they want us to use this thickness on 24" on center) comes with regular edges and tongue and groove (I think only on the long edge) ... using the T&G might help. Do you think I could make T&G work or should I stick with flat edges? Thanks, Tim
Let me get this right...
on most post you tell the world to go hire and engineer, hire and architect, or ask what do the approved plans (drawn up by the architects) say...
now you are recommending to this man that he continue to go against the architects reviewed opinion about the plywood.
12" OC Joists
Deadnuts, my objections would be (1) too many thermal conduction pathways through the joists out the roof (2) little chance of installing in-ceiling light cans in desired locations (3) a lot more cost (joists and hardware). The roof is twenty five feet up and in the back of the house with woods, so almost no vantage points or neighbors.
All are reasonable objections. Thanks for sharing them.
Counter points to these you may want to consider:
1. Thermal bridging on a 2x8 edge is fairly minimal. Your getting it anyway with 24" O.C. but granted, 12" O.C. is techinically twice as much. There is a fairly easy way to reduce thermal bridging in either case: rip 1/2" polystyrene strips on table saw and contract spray these to edge of joists before setting in place. Permatex makes a heavy duty adhesive made primarily for car headliners. Almost instant bond without reacting with polystyrene and takes heat well w/o breaking down. The 2.5" screws for 7/16" thickness should still be plenting good even with 1/2" pad because your using more fastners.
2. I would argue (and reputable building scientists like MJartin Holladay) that recessed cans within your thermal envelope is a bad idea to begin with. Since your using 2x8's, an IC rated light can is going to take up most of that depth and you could have dew point issue in those areas of your ceiling. If you're worried about thermal bridging in #1, then I would argue that you really ought to be worrried more about it here.
I recommend going surface lighting. There are plenty of good options out there.
3. Addressed in #1, Twice as much joisting and hardware. No way around that. However, given the size of your structure and custom nature of your project, it doesn't seem like a game changer to me.
4. No real argument on this point. If nobody is going to see it, then a segmented arch is probably is not worth sweating any extra equity on. That said, my guess is that it may cost any extra $1500 in time and material to do this, so it would still be worth considering if I were doing it for myself. Again, I would have it approved by the architect, but I don't see a structural objection (at least mentioned to date).