Roof replacement advice needed

[jimmyj]

Hi gang.

I need your advice about the work I am about to do at our home. I am going to get the local code requirements tomorrow but in the meantime I'm in a quandary.

The house is not quite 30 years old and the existing roof is OSB sheets 16" OC. The OSB is frighteningly thin, I think it's only 3/8" which apparently was allowed even though it's shocking to me. When I walk on the roof I can feel the flex and I guarantee if I stomped really hard I could break through. The other issue I have is the roof is shingled, but when I check the pitch it's way too flat for shingles. The pitch is less than 3 / 12, I'd estimate about 2.6 / 12. The current shingles are failing badly, I feel because of this, and about 3 of the sheets of ply show rot when I inspect them from the attic. The rot appears to be where the 3/8" OSB has sagged and water is simply going around and under the shingles (although I cannot confirm that to be 100% accurate). The baffles that vent from the soffit are made of plywood and 2x2s, but at least there are baffles! The venting is by way of a ridge vent and all soffits are vented. Roof size is two halves, each 53 feet long and about 14 feet wide so total roof is about 1500 sqft.

So, here is what I was thinking.

Option 1. Remove all shingles, replace all bad existing OSB with identical thickness new OSB. Then, lay an entire new layer of 1/2" plywood overtop to give me a thicker combined roof deck (I already have this ply on hand leftover from another project). Then, put complete coverage of ice and water shield over the whole roof (the torch down kind) and lastly put a sheet-steel roof over the whole thing (with all proper drip edge, new fascia flashings, etc etc.). Stick with ridge venting but install some sort of gaskets to prevent wind pushed water from flowing up hill and down into the ridge. (Goodness knows why we have not had water flow up the shingles and in the ridge to date, I know the ridge vent has a bit of a lip inside).

Option 2. Remove all shingles and all existing OSB (seems a shame considering 95% of it is fine in terms of no rot). Install all new plywood and then the same final covering with the ice and water shield and sheet steel.

Option 3. Replace the entire works with new trusses with a steeper pitch and the steel roof. This would create all sorts of extra work as the gable ends would no longer match, plus other issues with the intersecting peak of the house etc. Also I would have to involve an engineer and it would be very costly.

I would rather do option 1, I estimate it would add about 1500 lbs of ply to the roof, then on top of that the ice and water shield and steel. Overall I would have to calculate the total weight to make sure it's ok but considering how much snow and ice we've had on this house I can't imagine it would be too heavy. The current trusses are engineered but only 2x4, again on 16" centers.

So wise TBN'ers, what do you think? Have you ever double thicked roof decking on a house? I know they do for wood board houses but not so much ply over OSB.

Any comments would be appreciated.
Jim

 

[ray66v]

Your roof seems dangerously flat, for Canada? Does it meet code?

While, OSB is spongy on a roof. Plywood roof sheathing delaminates, and has not fared any better in the end, on any reroofs I have done. So, I still use OSB on roofs.

You can use heavier sheathing. And, or, increase the pitch of the roof. That's not a bad thing.

Whatever you do, use a good shingle underlayment.

Ice and water shield the whole thing, with Grace brand, if you can get it, and can afford to. Grace Ice & Water Shield 36 in. x 75 ft. (225 sq. ft.) Roll Roofing Underlayment in Black-52 - The Home Depot

OR, at least use a good grade of synthetic roof paper, on the bulk of the roof, with big overlaps.

That will help insure the sheathing never gets wet from the top.

You may have issues that are allowing it to get wet from underneath. i.e. improper venting of bath fans, and or lack of ventilation. Which can cause issues with the sheathing getting soft and spongy.

You need an experienced assessment of all that.

 

[Trials]

Option 4: strap it and install metal roofing over the whole mess

 

[jeff9366]

I like TRIALS idea, above.

Consider applying foam insulation under the roof. Pretty cheap material, large benefit.

 

[jimmyj]

Re does it meet code? I honestly don't think so. It's stupid flat in my opinion. I agree regarding top quality underlayment, we would only use something very very good even if it cost a bit more. The Grace brand is just like the stuff I looked at although a different brand name.

My plan for the last 10 years was to just strap it and put steel over the whole darn thing. Our attic insulation is R60 or something ridiculous like that so I don't need any more I don't think unless it improves the life of the steel. Then last year a big nasty black spot appeared on the underside of some of the ply as I mentioned in the OP. So regardless I have to strip and replace I think. I wish I put steel over the whole works 5 years ago.

Thanks for the inputs so far. I guess I should have put Option 4 as "just fix what's rotten and steel over the whole mess" but I feel in my heart those terrible curled split crappy shingles should be taken off. So maybe option 4 is really like my Option 1. I just can't bring myself to cover over and leave the 2 layers of ruined shingles.

 

[topbuilder]

Option 4: strap it and install metal roofing over the whole mess

My vote as well. I would think the current pitch would be fine for a metal roof. We can go 1/12 . I do not know how snow load affects this though.

 

[jimmyj]

I'm not worried about snow load, we've had feet and feet of snow on the roof, even wet and it's been fine.

But obviously you suggest I replace the bad OSB before. So what would it work like, strip only the shingles in the bad spot and then re-ply? I don't understand how I'd replace only the bad spot but leave the shingles etc on the rest.

 

[topbuilder]

Strip the shingles. Fix osb. It will all lay flat. You don't need the dead weight.

 

[Trials]

Keep in mind that you will be paying for the disposal of any old shingles, they won't accept that stuff at your local dump. Normally you remove the shingles where the strapping goes, the strapping nails to your roof trusses and you can use shims to make it set level where required. The original roof sheeting needs to be free of moisture rot and bugs, but serves no real purpose once it is covered by the steel. (yes indeed it will contribute to dead weight) Steel can be used on a lower roof slope then shingles. You will need to buy the appropriate materials to seal around the outside edges where the new roof material sets a little higher then the original fascia. Highly recommend you buy the roofing that over-laps at the seams and does not leave exposed fasteners to the weather. It will cost you more for the materials in the short run, but it will also be the last roof you ever need to put on it. … modern asphalt shingles suk!

 

[jimmyj]

Ok lads! I feel it coming together. Strip, fix, ice and water shield, all new flashings, fascia and eaves troughs and steel er up!! Now I'm excited.

(I know, to be excited about doing a roof is akin to madness, but I really want the old bad crud gone and nice new in place. I love making things better.)