Glad to hear that things are still moving! Despite the problems, at least there is some advance happening.
Another question. Have you heard of or experienced any damage to the drywall occurring on top of the OSB on the middle leaf/attic floor from workers walking on it when installing the HVAC or insulation or other reasons for walking inside there? This was concern for the new builder.
That's why the HVAC goes in BEFORE the ceiling is done...
Nobody walks on that ceiling, ever, so it has no additional live load: only dead load. Perhaps that's why he's so confused about the span handling capabilities of various lumber sizes! He's thinking that it will have to handle the live load of people and furniture on it, as though it were a true attic, which is why he brought up the example of the "old floor" sagging and bending as people walk on it... but nobody walks on the middle-leaf ceiling OR the inner-leaf ceiling.
If he thinks that the HVAC guy needs a surface to stand on for installation and maintenance, then you could put some plywood sheets up there, on top of the drywall, in the area around where the plenums will be, but the area where the AHU, dampers, intake vent and exhaust vent will b,e does not have any middle-leaf ceiling below it: All of that is directly over the lobby and bathroom, where there is no middle-leaf ceiling. I designed it that way specifically so there is plenty of access for HVAC maintenance / repairs / replacement for the parts that need access. Parts of the plenums and ducts are over the middle-leaf, yes, but after the system is installed they should not need touching again, and even if they do need access, it would be simple to slide in some plywood over the drywall temporarily, to protect it, then remove it when the work is done (or leave it permanently, if you prefer!
). However, it's rather unlikely that it would be necessary to access those regularly.
Also, don't forget there is access into the attic space from the large air air vents in the gable end. That one is sized large enough that you could even fit the entire AHU through it, from the outside of the building (using scaffolding, of course), without needing any access from the inside. For example, if the entire unit failed completely, then that would be a relatively easy way to get out and replace it.
Getting back to your GC: He's also not correct about his "rule of thumb" for how much a given lumber size can span: you have to take into account the SPECIES of wood, the dead load, the live load, the deflection factor, whether it is notched or not, whether or not it is exposed to water, and whether it is a floor joist or a ceiling joist. As I implied above, floor joists must carry live loads, ceiling joists do not. Some species of wood can carry much larger loads than others. Some types of lumber are specifically designated as "structural", able to carry a larger load. The others are not. It seems to me that he's not very knowledgeable about wood properties and spans! His "rules of thumb" are not very valid at all.
He's also not reading the plans correctly! either that or the plans he has are not taken from the ones I did for you, because on the design I did originally, those are 2x10 joists, not 2x8! I'm not sure where he got the idea they are 2x8. Both the middle-leaf joists and also the inner-leaf sistered joists are all 2x10. There's no 2x8s in there. The rafters and rim joists are 2x8, yes, but the joists are all 2x10.
In fact, those distances COULD be spanned using 2x8s at 16"OC (barely), but I opted for 2x10 24"OC for several reasons (including an extra safety margin and also improved isolation at low frequencies) .... however, instead of using either of those, 2x8 16" OC, or 2x10 24" OC, instead, he built it as 2x12 16" OC!!!
That's strong enough to park a truck on!... So it's overkill, and expensive.
To be fair, the approved plan didn't state 24"OC.
Ooops! Then the architect screwed up! Because the original design has 2x10s spaced 24" OC...
He said as a general rule of thumb a 2x4 can span 4', a 2x6 spans 6', a 2x8 spans 8' etc. but you can go a little extra.
Ummm....
Carrying what load? And what species of wood? And what deflection?
Those are key points to any discussion of how far a given piece of lumber can safely span. In fact, according to the American Wood Council's span calculator, ANY 2x4 of ANY wood species and ANY structural grade, can easily span 8 feet (not 4 feet, like he said) at 16" OC with a dead load of 10 psf PLUS a live load of 10 psf (assuming deflection of L/360). Most species can span 9 feet like that, and some can even span 10 feet. I have no idea where he got his rule of thumb from that 2x4's can only span 4 feet, and 2x8s can only span 8 feet. That's like saying that if a vehicle has two wheels it can carry two people, if it has 4 wheels it can carry 4 people, and if it has 8 wheels it can carry 8 people.... Easy to remember, sure, but totally wrong, and just plain silly.
According to the same span calculator, for the case of 2x8 joists, ALL species of wood with just one single exception (Norther White Cedar) can span 16 feet with the same factors as above, and in fact in some cases even 2x6s can span that distance. Which shoots down his other rule of thumb, that a 2x8 can only span 8 feet. In fact, according to the same source, most 2x8s can span 18 feet, and a few species can even handle 20 feet. So, considering that your span is 17 feet, it actually would have been possible to use 2x8s... assuming certain wood species. But I didn't do that; I always prefer to go one size bigger for safety, which is why I put 2x10s in the original design. In theory, they could span 22 feet (far more than your 17 feet). The 2x10s at 24" OC in the original design could in fact carry more than twice the load that they will actually be carrying. But the 2x12 monsters that they actually put in, can carry about SIX TIMES times the load... it could carry a live load of about 31,000 pounds, according to the span calculator... (that's live load,
in addition to the dead load! Like I said, you could park a truck on there. That's about enough capacity to carry an entire 18-wheeler big rig...
So yeah, I'd call it "slight overkill" for your middle.leaf ceiling...
It's a great idea to include a margin for safety, absolutely, but I think having enough capacity in your ceiling to hold a big rig, is going just a little overboard.
Anyway, it's done now, and you can't go back. It certainly won't do any harm, structurally, but it sure did cost a lot more than was necessary. The only harm here is a possible slight reduction in isolation for very low frequencies, due to going 16" OC instead of 24" OC. If you are concerned about that, then you could add more mass to the deck on that middle-leaf, by increasing the thickness of the plywood deck to 3/4", and/or switching to OSB, instead of plywood. OSB is a little more dense than plywood (around 610 kg/m3 vs. 560 kg/m3), so you'd get 9% more mass (not much!) just by switching to OSB, and another 18% by increasing the thickness. Between those two, you'd be increasing the mass of that layer by nearly 30%, which is plenty to offset the reduced isolation. Another option would be to use Green Glue between the two layers: that would give you a substantial increase, but it won't be cheap, as there's over 400 square feet of deck to cover. So if you are concerned about isolation, then those would be the options.
I have a question regarding the sound isolation loss and extra treatment I will have to do. Is that because of the 16"OC or the 2x12s or both?
The spacing is the issue. The joists don't really matter at all here (except for the cost!) With wider joist spacing, there is more free area of sheathing in between, so it is able to flex more. It gets complicated, but that's basically the issue.
Final question: The former builder's plan for the exterior wall sheathing was to tack on the OSB using 8d nails with caulk on studs (already done), then green glue, then 2nd layer of OSB using 16d size nails to go through both sheets of OSB into studs. The contractor thought it would be better to not just tack on the first layer, but to fully nail down it using 8d size nails, then 8d size for 2nd layer of OSB. Which would you recommend?
The nails on the second layer need to fully penetrate into the studs, just like the nails form the first layer. Thus, they need to be at least 3/4" longer if the first layer is 3/4", or 5/8" longer if it is 5/8". Normally I use half schedule on the first layer then full schedule on the second layer. That means the the first layer actually gets one and a half schedule, and that's fine! But the second layer does need a full schedule in order to comply with code, and for the same reason, the nails need to penetrate into the studs properly. So whatever your local code requires as the nailing schedule, use half of that on the first layer by itself, then all of it on the second layer. And do try to stagger the nails a bit! In other words, move the nails by a couple of inches on the second layer, to ensure that those nails don't hit nails that are already there on the first layer!
Monday they will begin working on the roof. Things are moving forward. The general contractor is actually going to put his tool belt on and help with the roof due to the number of employees he recently lost and his budget concerns.
Cool!
Don't forget to take your camera, and take plenty of photos to document everything that is happening! And post a few of the best ones here...
- Stuart -