Plywood necessary on top of old tongue in groove?

Generally speaking you do not need to overlay, especially if you have T&G. It is usually fir or yellow pine. With older homes that used non T&G "Roofers" you might need to. Non t&G that is spaced over 1/4 inch, or are out of plane, I would overlay. It is usually a good idea to hand nail where your substrate does not present a uniform resistance to pneumatic nailers. If you hit a void with them, the nail can blow right through, leaving a big hole. Generally Speaking, Once the shingles seal together, you can depend on the well nailed shingles to keep the whole array secure in high winds. You might want to increase the number of nails per shingle at the rakes, eaves and ridge to assure good holding power. I would insist on ASTM D226 Type 1, or even Type 2 Felt under your shingles for more support also.

I just looked at Wrongdaves roof. I would have re-sheathed it if it were mine, but I wear a belt and suspenders. By the way, if you really care about warranties, most shingle manufacturers require Plywood or OSB under their shingles to qualify. I know for sure that they use the lack of it as a reason to reject a roof under warranty.

I just checked Certainteed. If you have 3/4 inch T&G that does not qualify for warranty. 1 inch minimum for separate boards. Of course if you have full 1 inch boards they will most likely be gapped too far apart to qualify. Other MFGRs may diff er, but I doubt it. :whistling2:

You are not wrong at all, Dave. I have been fighting the warranty war since I have been involved with roofing. I keep telling people that warranties are written by the manufacturers lawyers to keep the MFGR out of trouble, but when an owner knows nothing from nothing about what he/she is doing, and somebody like me tells him/her that a new roof is going to cost between say 6-8 dollars a square foot, (we are talking commercial here) and joe schmo the roofer man rolls up in their pickup with a torch hanging in their back window, and tells them he will install a 20 year warranted roof for 2.50 a square foot, I have to explain why it cant be done, and in many cases, what you now have to say falls on deaf ears. I have a theory about warranties:

Remember when you received any type of certificate of accomplishment, that it had that really neat scalloped gold trim on it? Thats the same thing they do with warranties. Now why do you suppose they do that?

Right! That certificate made you think you were something special, and you probably were. You see where I am going with this. Nuf Said.

The best way to a long lasting roof is:

1. A good specification with job specific details.

2. Good materials with a proven history of performance.

3. A good knowledgeable contractor

4. Monitoring of the installation

If you have those things you dont even need a warranty, because you will never need it.

I wanted to applaud wrongdave's posts and work that he linked to. It is great that you took the time to research and do all of that. You see a bunch of lists on "How to choose a contractor" and the most important items needed on the list aren't there. I would add to every list as number 1 - "To intimately understand what you are buying." and from jagan's post as number 2 - "Monitor the installation."

Wrongdave also had a great write-up on the ice and water. Ice and Water wasn't needed till the invention and use of plywood. Ice and Water should be looked at that way, there to protect it, because plywood will not take the abuse or last as long as board decking does. I certainly don't look at Ice and Water as a way to stop leaks. If that were the case, there is a problem with the roof system I am specifying and installing. Underlayment, felts, should be installed over the ice and water, starting from the eave line so no roofing (metal, shingles) make direct contact with the ice and water.

My opinion is, I am for the outright ban on "granulated" ice and water for the reason wrongdave described. I have been on jobs where architects refused it's use. I haven't used if for at least 7 years. GAF's Stormguard is affordable and comparable to the cost of other granulated ice and water, and is rated high temp.

I also place little value on the shingle warranties. It should be pointed out that Certainteed calls for "nominal" 1" thick wood.

I also agree with Joe and Andy - just wanted to explain. For Architectural (laminated) shingles, for the best installation you want to hit the true nail line.

From Certainteed's Instructions:

I am talking about the shaded area for steep slopes, not the expanded area they now give on shingles. These should say the nail area for all slopes, or at least everything over a 4:12. This way a nail is penetrating both parts of the shingle. All roofers have repaired and you see them all over town where the bottom part of the shingles slides apart and/ or falls out when the nail line isn't hit. If you have gaps in your decking boards, there is no way to guarantee that the "true" nail line won't end up over a gap.

With tongue and groove decking you don't have these gaps, therefore no reason to cover with plywood. Assuming it looks like what I am thinking, the tongue and groove will likely last a century longer than the plywood ever will.

Plywood did not, and does not cause ice dams. Ice dams are caused by snow melting because of heat loss from the warm interior flowing down to the cold overhanging eaves and turning to ice. The ice locks itself to the surface of the shingles and causes a dam which continues to build up as melt water makes its way down the roof.

Why is there heat loss right at the exterior wall? Trusses made with 2 x 4's. This causes a pinch condition where the truss sits on the top plate of the wall. A condition where the builder cannot get enough bat insulation in to stop heat loss. There is an answer, but builders are too cheap to incorporate it. The answer is to use 3 inch isocyanurate foam 4 feet in from the exterior wall, and foam it in place around the edges with a froth pack.

Back when houses were stick framed, and usually used 2 x 8's for ceiling joists, and at least 2 x 6's for rafters, Ice damming did not occur. There was more space for air flow under the deck, and more room to properly insulate.

Like everything else, tract builders try to squeeze every last penny out of their building techniques. If you look at the building codes, 3/8 inch plywood is actually allowed as roof sheathing in some cases, and I have heard of 1/4 inch being used!!! Does that mean you should use it? Not in my book. 19/32 4 ply Fir minimum, if you want a good substrate to nail to, that won't picture frame under heat load.