I am in WI. We need to replace our roof because it is leaking. I know nothing about this!! We have an older house with 2-3 layers of shingles and possibly wood shingles. There are spots that the roof is slightly uneven with dips. Nothing major that we can see. We do have the vents that are in the siding. One on the east & one on the west side.
We have 2 contractors that we have spoken to. One states that no matter what we need to put OSB over the existing roof boards. We have another who states that if the board are bad he will take them out and put OSB in those spots only. Is OSB needed if the boards are in good shape?

If the deck under all of your existing roofing is skip sheeting you need to put plywood. I recommend half inch cdx instead of osb because it is much stronger. If it is solid deck you just need to replace broken or rotten wood


We have another who states that if the board are bad he will take them out and put OSB in those spots only

This contractor I would not use. To spot replace 1x material with OSB. That is cheap constrution. You will have lots of sagging in time. It may look good when he is done. Couple of years it will be sagging.

The contractor that is going over the whole roof with the OSB is giving you a smooth roof.

OSB = No

We have always used OSB over skip sheating only .

Skip sheating usually has about 4-6 inches gaps in between boards and the purpose of the OSB is only as a nailing substrate/filler .

I like to use it because the rough side provides very good grip for walking on your roof .

If i am REPLACING only a few bad sheets i would definitely use CDX or if its visible from underneath ACX with the sanded side down .


If you want to grip the roof, wear the right shoes.


Cons are the best roofing shoe ever. I can run up a 12/12 or walk a 2x4" toeboard on a 21/12. Spiderman has nothing on me, heh.


if it is cedar you are tearing off then you must use new plywood of the existing deck. osb is absolutely fine. you dont need it for strentgh…just smooth substrate. osb is actually better in a moisture situation than cdx. i left 4 sheets in my yard all winter (to test this theory) i live in upstate NY we got 20 feet of snow (at once) and the cdx was almost perfect in the spring. i leave a sheet of cdx outside over night in just the humidity and it curls right into a U shape…let the arguing begin…

i have seen this happen too.

**"i leave a sheet of cdx outside over night in just the humidity and it curls right into a U shape

i have seen this happen too."**

Perhaps the highly flammable glue that holds the wood chips that form OSB together afford it better protection. Want to do a true test? Drop a full lit pack of matches in the middle of a sheet of each and see what happens.

i will have to try that out.

actually i save all my plywood scraps because my wife like to have bonfires…cdx cathces fire much, much faster than osb. i have done the coors light research in this area. 8)

Coors light ?? what the hell kind of roofer are you?

owner/salesman/manager… :smiley:

IMO, I’d be removing every bit of decking product up there, doing a “truss up” (i.e. running a zig zag with string line / chalk line, then marrying or putting in new truss beams alongside the old ones to the bottom edge of the string line) & installing ReflecTech or Reflectix 7/16" (1/2" type) OSB on top of the new beams.

The primary purpose(s) of the decking is twofold:

Support your roofing products
Hold nails.

So long as you get a smooth surface, I don’t have any complaints about the roof deck being OSB.

If you’re going to put on 3 Tab (which I don’t reccomend but might do for cost concerns if that’s what the customer has budgeted), you most certainly will need a fully flat roof deck.

2 Layers over cedar & possibly a 3 layer roof + cedar is an insane amount of weight; I’m expecting you have more than a little bit of sagging in those trusses.

In conclusion, IMO the ONLY thing I’d “go over” is the beams. Based on your description, everything should be torn down to a bare skeleton.

My Imput::

In my findings of 3 months in Dallas, it would appear that a major hail storm back in the mid 90s left a large majority of the homes with an insurance offer that was insane by my opinion but this is what they all got::

They all got…80% of the older homes prior to mid 90s (dallas was all cedar at one point when it was popular)

A comp shingle over cedar. Literally felting over the existing med grade cedar and adding a 30 years shingle over it

Well, now they are finding that they are stuck with this nasty tear off which includes as I Im seeing above

Utimately the best thing to do would be to remove all existing plank boards as well as the roof back to truss and install a quality decking such as at a min 1/2" CDX

Now since the costs of this exceed most peoples budget, your finding in certain areas that the offering of OSB over the existing 1x4 plank boards that are and should be gapped out according to OLD TIME Cedar shake installation standards. The problem these clowns have around here with Cedar beside fire is mainly the slope. These old time Cedar methods that dont include using a 30lb half roll of felt, lapped at 10" shouldve been installed on a 10/12 or greater…nether here or there

So costs would be cheaper by using the OSB over the existing 1x4 plank boards. Any bad ones underneath should be removed, but may not need to be replaced as you only really need them at the plywood ends and 1 or 2 in the center…

Here there is a new OSB with a Radiant barrier that seems to work rather well for reflecting heat. Being in the north I wouldnt think you would need it as much as these homes in the south

Go ahead and use the OSB over the plank boards, but do the entire roof or atleast the entire section

CDX cost vs OSB

OSB over the existing plank boards is even more durable than just 1/2" CDX.

I dont personally like OSB but in this situation, especially in a resell or flipping house situation, Costs mean everything


Meh :stuck_out_tongue: I aint reading all that lol .

OSB has never given any contractor here problems .

Is like everything else install it right or it will fail no matter what kind it is .

But whatever rocks your boat fellow roofers :wink:

Amazing fluctuation on pricing from 3 years ago of being the OSB 7/16 sheet at $21.50 Down to recently quoted $8.50 .

I will be honest and say i will keep using OSB on my re-sheathing jobs .

I must have done over 100 houses easily with it .

Highly flamable ??


Ive seen guys torch almost directly to it .

note :


But i have seen some crazy roofers do it and it doesnt catch fire easily at all.

To my view price/performance OSB > CDX :wink: