New roof sheathing over old?

[JKW05]

50 year-old house with 3/8" plywood sheathing on trusses spaced at 24". Roof shingles are to be replaced. Attic ventilation comprises of a poorly installed ridge vent with no soffit vents. Walking on the roof feels very "spongy". There does not appear to be any roof leaks but, from poking at the sheathing from below, there appears to be "dry" rot.
My first thought was to replace the sheathing with 5/8" plywood. Any opinions on if 3/8" sheathing could just be applied over the existing sheathing, eliminating the removal? The ventilation issues will be addressed with proper soffit and ridge vents.

 

[frv]

Doesn't seem like a good idea.

The new sheathing needs to function as a diaphragm and you're going to be attaching it through rotting material into the supports. Have you checked that the trusses themselves aren't presenting signs of deterioration? I don't know of any provision that prohibits it, but it seems like a poor decision.

 

[TomDOT]

If it were my house, I would strip and replace any questionable sheathing - at a minimum.

As far as thickness, what does your local code say? Personally, I like something thicker. I would expect any 3/8" plywood to feel "spongy" when walked upon.

In my climate I would also use "solarboard" for any significant replacement, to reduce attic temperature. Might actually be worth just redoing all the sheathing for the energy savings from the solarboard - depending on how much is rotted. My local Home Depot keeps it in stock. Solarboard has a low-emissivity (foil) facing which goes on the attic side.

http://www.norbord.com/na/roof-sheathing/solarbord/solarbord-installation

Also, are you using EnergyStar shingles? This will reduce roof and attic temperature. If you pick the right vendor, the cost is about the same as normal shingles (some roofers will try to rip you off and charge an extra couple thousand for the EnergyStar shingles)

You may be able to get tax credits for the solarboard and/or EnergyStar shingles. You will have to investigate current rules yourself - I know I got back a $1,500 federal tax credit for using EnergyStar shingles a few years ago, and I picked a roofer who charged the same for the EnergyStar or normal shingles. The rules change all the darn time, though. Looks like the current credit is $500 (10% of material cost) only, whereas mine was 30% of the overall cost, capped at $1,500. There may be local incentives as well.

http://energystar.supportportal.com...Article/19337/Is-there-a-tax-credit-for-roofs

 

[OHIOMatt]

My guess is, that you do not have "dry" rot. You have wood that was exposed to elevated moisture levels, rotted, and then dried. A 50 year structure has had to have had the roof replaced before. I would remove the sheathing, verify the truss chord condition, and then replace.

 

[msquared48]

Strip and replace any bad 3/8" panels, then overlay with a minimum of 1/2" CDX ply with 12 d nails at 6" OC edges and 12" OC field.

Mike McCann
MMC Engineering

 

[SLTA]

My first concern is added weight from the extra layer of plywood. 50 year old house - are they definitely trusses, or rafters with collar ties? Run some quick numbers for the load first. If you do have dry rot, or fungal activity just waiting to start up again, then giving it new food doesn't help, either. I'm in the remove and replace camp.

 

[Ron]

Agree with others...replace the sheathing but do not overlay. Minimum 15/32" thickness with 24" spacing is typical. Even that is spongy, though and you'll see waves from expansion, even with proper gap at the joints. I would go thicker...5/8".

 

[msquared48]

7/16 ply is only about an additional 1.4 psf dead load. If the roof can carry any amount of snow, this should not pose a problem.

Mike McCann
MMC Engineering

 

[JKW05]

Thanks for your responses. Didn't feel real good about covering it up, but just wanted some other opinions.