1/2 ply on 24" on center?


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Old 04-01-04, 04:28 PM
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1/2 ply on 24" on center?

I have a new roof planned this summer. My plywood now is 1/2" on trusses at 24" on center. No H clips & most joints do not line up even. You cant see it from the street. But it looks bad from the attic.

Im getting a 50 year Malarkey shingle , so should I replace all the plywood with 5/8" OSB before? At $23.00 a piece Im wondering if I can skip the OSB.

Thanks

Mike
 
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Old 04-01-04, 04:46 PM
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We modern folks are spoiled by plywood and osb.

If you have ever studied a roof built on 1x4s, any decent application of 4x8 panels is heads and shoulders above.

I believe 24" oc is fine for 1/2". If the APA stamping is visible you should see a notation indicating o.c. for roofing, o.c. for flooring.

If you insist on additional sheathing I wouldn't strip off the current plywood. You could cover it with 1/4 osb and 1" sheetrock screws. Stagger the joints. Drywall screws would pull everything up tight.

Personally, I doubt you have a problem.
 
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Old 04-02-04, 05:08 AM
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steve you really shouldnt be giving roofing advice from what i've seen
 
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Old 04-02-04, 08:06 AM
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GiantScale, I'll share my own experiences as a roofing contractor. You're the owner, only you can decide what is appropriate.

Although the architects and builders tell us that OSB is just fine for sheathing (and cheaper than plywood by almost half), we stay away from it for roofing applications. OSB works well as long as it is not wet. If it gets wet from condensation or water infiltration, then the result is breakdown of the board over time. However, unless your ventilation is excellent, and the roof surface is truly water-proof (which composition shingles aren't), there is a good chance for moisture (condensation or seepage) for going onto the decking. In our experience, solid wood decking (1x4, 1x6) tends to resists moisture the best, followed by plywood (the thicker grades have more plies and deteriorate less under moisture than the thinner ones), and finally OSB which resists moisture the least. We've seen OSB boards lose their structural integrity in as little as 5 years when persistently wet.

1/2 inch OSB is less rigid than 1/2 plywood, and we do not use anything less than 5/8 plywood when the rafter spacing is 24 inches. If you are in an area with significant snowfall, you need to take into account that snow weights about 40 lbs per cubic foot. The Malarkey 50-year shingles weigh 2.75 lbs per square foot. Effectively, if you have 1 ft. of snow on your roof, you will have about 95lbs. of weight being supported by the sheathing between the rafters spaced 24 inches apart. If the roof sheathing gets wet and deteriorates, what's your margin of safety?

You mention that your existing panels have no H-clips. In our area that would be a big NO-NO for both building code and practical reasons. If I had your situation locally, I would need to remove the plywood and reinstall, just to meet the code.

So if it was my home, I'd do the following:
1) Strip the deck to the rafters and resheath with 5/8 tongue and groove plywood properly spaced. Personally, I'd prefer 16" spacing between rafters, but you may not be able to change that.
2) Ensure that you have proper, unobstructed ventilation from intake at soffits to exhaust at ridge or similar vents. I use the general rule of 1 ft. of ventilation for every 150 sq.ft. of attic space. Note that the minimum air channel should be 4" in height to allow proper ventilation by passive convection.
3) Install Ice and water shield membrane along eaves on all roof sections where water infiltration is possible due to ice damming or poor flashing detailing. That would include any valleys.
4) Install 30 lb. felt, or a synthetic felt like Triflex, over the entire roof. Malarkey recommends their "Right Start UDL" underlayment if you are using their shingles.
5) Install the appropriate metal flashing at roof joints such as valleys, endwalls, eaves, and chimneys.
6) Install the Malarkey shingles, following their published procedures.
7) Ensure that your living space is properly sealed from the roof/attic with vapour barrier and the appropriate amount of R-value insulation for your area. The first will prevent moist air infiltration into the attic (causing condensation), and the latter will prevent heat loss which can melt the snow on your roof and cause ice dams.

Good Luck.
 
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Old 04-02-04, 08:30 AM
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Just wondering if the stamped steel clips couldn't be retrofitted by bending the flanges, applying from below and having a second person on top hammer the flanges down? The reason I suggest this is the context of D.I.Y., a contractor wouldn't do it, but a homeowner might see it as an attractive remedy.

Person inside the attic would need a 5 pb hammer to act as anvil.


Is the building an inhabited and furnished dwelling? Without the decking, how much damage and liability would be caused by a brief rain shower?

I am confused regarding weight of snow. An inch of rain creates a foot of snow. That would be 1/12 of a cubic foot (7.5 gallons), or 1/12 of weight of 7.5 gallons. A gallon of water weighs 8.33 pounds. Unless there is 9 inches of water (as snow) on the roof, you won't have 40 pounds of snow. A roof may need to support 40 lbs/ft of load, but most of that is expected to be wind load.
 
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Old 04-02-04, 09:15 AM
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Steve,

Retrofitting the H-clips isn't just about getting them in. Not all areas of the attic are accessible to allow your proposed technique. Secondly, bending the metal back and forth weakens it. Thirdly, if the H-clips were omitted by the original builder, I suspect that other "sins" were also committed. Finally, if the original decking is already deteriorated, it already is home to fungi and mold. Why would you want to have a new roof with a "rot-starter" built in?

The weight of snow obviously depends on its air content, and we've seen both "light" snow (uncompressed), and "heavy" snow (compressed and water-saturated). A cubic foot of water weighs 64 lbs. We use 40 lbs. as it represents the type of snow which has experienced both compression and thawing/freezing, increasing its ice content and lowering its air content. I am not saying that a roof needs to support 1 foot of snow. The building code in each location determines the amount of snow loading the builder has to accomodate in his design. I'm pointing out that the snow load is not insignificant. The practical result (in our area) is to see panels bowed in between the rafters. We can often see these from the curb-side even before we conduct the roof inspection. What we usually find is either OSB which has been weakened by moisture, or 3/8 inch plywood (which is no longer allowed in our area). It is the persistent weight of snow that causes the deformation of these panels, as can be seen when you compare the same roof, but in an area where snow does not build up.

I don't understand your reference about damage to the dwelling without decking. Anyone contemplating redoing the sheathing has to take into account the weather, and plan the work so that only the appropriate amount of sheathing is removed and replaced each day. Any good roofer, or competent do-it-yourselfer will take this into account.
 
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Old 04-02-04, 07:05 PM
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Thanks Paul, Steve & Mojo

I just had a Amish contractor on the roof this afternoon. Im in Iowa. He said he would only replace the 1/2 ply which was in bad shape. I liked that. But after hearing from Paul Im having second thoughts again so I called about 5/8 cdx. Its cheaper than 5/8 osb ($22.65) at the moment. $21.65 & $1623.75 for the 75 sheets 5/8 plywood cdx needed.

The only thing I disagree with you about is the tongue and groove because this does not allow for expansion like a H clip would. Or do you allow 1/8 space with the tongue and groove as well?

Now the gamble. Will plywood costs go down later in the season (august)? Or should I prepay now to lock in the current price?

& do you think the Malarkey legacy shingle is the longest lasting shingle out there?

Thanks
 
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Old 04-02-04, 08:06 PM
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Mike, we put 1/8 inch between the sheets even with T&G. We like the T&G because it holds the sheets around it along the entire perimeter, whereas the H-clip is every couple of feet or so. I've seen one case of the H-clips popping out. However that was more due to bad spacing of the sheathing.

I don't have personal experience with the Malarkey shingle, but I haven't heard anything bad about it. In my opinion, 80% of the quality of the roof is determined by the installer, in the care that the installer takes with the various details. On most roofs where you find leaks, the problems are usually poor or inadequate flashing installation. Good roofers spend the time to do these right, bad ones skip the "hidden" details. To give you a concrete example, we've seen 35-year shingles fail in as little as 7 years due to poor installation. What's the point of paying for premium materials when they are not installed correctly?

The shingles that will last longer than the composition shingles are the metal ones, but you're talking of more money. Most of these systems must be installed by qualified roofers, but some can be also installed by homeowners. To people buying metal shingles, cost is not as important as some other properties. Again, it depends on what your needs are and how much you are willing to pay.
 
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Old 04-03-04, 03:31 PM
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Giant Scale:
I have answered alot of your questions before, and I know just where you are coming from. You have gotten very good advice from the other answers. Now if I were you, this is what I would do. Tear your roof down to your sub roofing. Next put on 1/2" 4 ply cdx, and screw it over your existing sheathing. This would be the least expensive. Your roof was incorrectly put on originally, and it would take a bunch of money to redo the whole thing correctly. Your existing 1/2" + your new 1/2" will make 1", which is sturdier then 5/8" anyway. At this point forget your clips. As far as buying your plywood now, it will not go down until next fall. I just tied in 100 sheets of 5/8" osb at $21.95 a sheet from Menards, and they will warehouse it for me. It will be $25 + in a month I think. I will be in Cedar Rapids in May. Good Luck
 
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Old 04-03-04, 05:35 PM
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Thanks again Jack. I appreciate your patience.

My problem is that 1/4 of my sheathing does need to come off since its trashed from a pair of un-vented bathrooms below on the back half of the home. It looks like rolling sand dunes! I just dont see how I can make a transition from two layers of 1/2" to a single layer of new 5/8"

Can I replace the back half of the home with 5/8" and add the 1/2" second layer on the front?

Mike
 
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Old 04-04-04, 09:15 AM
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Can I replace the back half of the home with 5/8" and add the 1/2" second layer on the front?
Why not ---- "do it" You could add a little liquid nail in front if you want Home depot here has 5/8 CDX $21.59 right now


ED
 
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Old 04-04-04, 05:10 PM
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Giant Scale:
Yes, you can. Not a problem.
 
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Old 04-05-04, 06:53 PM
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WOW.

I just spent 1800 on ply.. Gulp
 
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Old 04-10-04, 11:02 AM
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Got 5/8 cdx for $19.00

I talked to a roofing contractor who will only tear & replace plywood if I sign a agreement. That if my ceiling drywall gets damaged from my trusses moving when the ply is off (only two sheets at a time) , its not his fault.

Maybe I should leave the 1/2" on & go over..

Mike
 
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Old 04-10-04, 07:48 PM
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Plywood will take a warp when wet, but that was in the past, correct?

What is your contractor suggesting as to the warped sheets? Does he consider them rotten or beyond repair?

If the warping is at the edges, those edges could be blocked from below, using deck screws to pull it all together. The blocking doesn't really need to be tied to trusses, it just needs to straighten out the offending area. Neighboring warps can share the block, or each edge can have it's own blocking.

Is your contractor thinking the decking can be saved, or it needs to be removed?

Plywood is quite a forgiving material, unless it's rotten.
 
 

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