Questions on a new roof

[CTburning]

I need a new roof on my 1977 modular built home with a 4 pitch roof. The roof dimensions are roughly 27 x 54 and it is a truss design with "rafters" 24 inches on center. Long story short with all the snow we had last winter and coupled with rain that we got last February, the decking is soft when you walk on it. It doesn't sag but when you put some weight on it (220lbs), you can feel that it has a little give. You can feel where the rafters are because it is rock solid but if you step right in between you can see it move. . I have had several roofers up here and gotten different advice from all of them.

I want the roof overbuilt! I bought the house in 2006 and had planned on replacing the roof in 5 years. It’s the old 3 tab design and probably 15 years old at this point. I have no leaks and no water penetration so far. I have taken pictures of the roof and nail holes and no black rings or evidence of moisture anywhere. My main issue is the decking or the subfloor. The decking needs to be replaced. It is half inch plywood which is code for 16 inch rafters. Just about everyone I asked has given me a different opinion on replacing the decking.

My question is can I lay another layer of decking on top of the old? I have read online that with this design it should have three quarter inch decking or even one inch. I have gotten quotes to lay half inch over the existing plywood (if any are water damaged they will be replaced) and the last company I got a quote from said that putting a second layer down was a bad idea in that it would trap moisture and cause the roof to fail. Other people have said put down half inch or five eighths and I should be good to go. Ironically the company that wants to rip up the old decking and put down three quarters tongue and groove gave me the best price overall. They have all pretty much said that I can wait till next year if I want as none of the shingles are in bad shape yet. Sorry for the long rant but any advice will be greatly appreciated as this is going to totally wipe out my savings which will stress me out a bit.

 

[smokinj]

Short answer imo there should be no moisture to trap! If so done of it will work.....Now with the sagging issue if it sags now just adding a sheet over it not going to help either. The guy how says to replace it all does have the best overall advice though!

 

[northernontario]

As long as the current deck is dry when you put the second layer on it, then it won't rot out any faster than if the roof was solid. There might be a potential if a leak developed and water becomes trapped between the layers..

Increasing the thickness of the sheathing will take out some of the bounce, but the big problem is still the 24"OC truss spacing. Not much you can do to easily fix that without tearing off the whole thing.

 

[davmor]

You could just re-sheet over the old decking just remove anything that is rotten. with such a low pitch I would also consider ice shield on the whole roof not just the edges. I tend to overbuild but it pays off in the long run.

 

[joefrompa]

Do you have access from underneath? I.e. can you access the roof trusses? If so, can you add bracing running perpendicular between them?

Seems like they are supporting the weight of the roof just fine, it's simply the plywood not holding up in between the spans. So why not add some support between the spans?

Just a thought for something on the cheap.

 

[peakbagger]

You really dont gain a lot of stiffness by adding an additional layer unless the layers are fully adhered to each other. Just tying them at each rafter doesnt really do whats needed, its better than nothing but not much. My strong preference for 24 inch on center roofs is 1" pine boards. They take longer to put down but they dont "telegraph' the rafters, I have seen 3/4" plywood do it.

Do be aware that if your building inspector wants to be difficult he may require you to have a professional engineer sign off on adding deadload to the roof.

 

[Later]

You have to remember that when walking on the roof your feet probably put more pressure on the spaces between rafters than even a heavy snow load does.

Our first house, 1974, was a Northern Homes package home. Trusses on 24 inch centers, 3/8 inch plywood sheathing. Upstate NY where we often have 2+ feet of snow on the roof in the winter. We never shoveled the roof. The deck did flex when walked on. The roof never failed. We currently live down the block and that house is still standing and the roof looks as good as ever.. No sag visible.

 

[SolarAndWood]

How about throwing some perlins across the trusses and doing a metal roof?

 

[CTburning]

Thanks everyone for the replies. I can't see how nailing a second layer on top of the first will trap moisture unless moisture is already present. As of now my plan is to add 5/8ths on top of the existing decking but I am holding off until next year. At 17 squares my roof isn't that big of a job and I might hire a roofer to help me complete the job myself. Either way I will be thinking it over this winter and will come up with the best solution.

My plan is to ice dam the entire roof and that is the advice I recieved from the neighboors. The quotes I have recieved so far is about $6k to strip and reshingle the roof and $8k to redeck in 3/4 and reshingle. Of course theres home depot, only $9600 to reshingle the roof, what a bargain. There is quite a financial incentive to do it myself. I will make a post showing the replacement of the roof in the spring. Now in the meantime all I have to do is to build a hearth with tile and stone veneer, replace two ailing triple wall chimneys, install the new stove and ....