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Old 05-16-2012, 09:35 AM   #1
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Roof deck removal help


I've been posting like a fiend (in roofing and insulation forums)--trying to decide whether to strip off an old flat roof deck.

My original thought was to remove the old roof, roof deck and then redo insulation from the top (and some misc like pull some wire for new lights, drill one new vent), replace roof deck and re-roof.

Construction is 2x10, 16oc and x-braced--clear span of over 22' in the center section. AFAIK, there are two bearing partition walls--so the center sections rafters run N-S, and the two end sections run at 90 deg to the center. Joists are toenailed into top plates.

Saving the ceilings seems worthwhile because my wife likes the plaster skin coat--and there is a lot of wood trim that would have to be removed to redo them.

The most recent roofer claimed that he would not remove the deck, because doing so would likely disturb the ceilings below anyways because the joists would likely move.

Am I likely to get joist movement like he describes if I work carefully?
(I thought leaving the fascia intact while I work would help)

Any advice from the pros?

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Last edited by paredown; 05-16-2012 at 09:38 AM.
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Old 05-16-2012, 12:08 PM   #2
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Roof deck removal help


you will get some nail pops but that is all. I just finished a job like this last week. Some things to consider.... what is your insulation value? You need to be sure that the dew point of the underside of the sheathing will never be reached or the plywood will rot. this is what happened at this job. Someone used 2" Styrofoam which is too little R-Value. And the venting was inadequate. So venting added moisture in the summer and moist air condensed on the cold plywood in the winter. Every sheet was rotted through from below.

also do not use an recessed lights in this ceiling or you will have the same issue over the lights.

What type of insulation are you using? What venting do you have?

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Old 05-16-2012, 12:49 PM   #3
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Originally Posted by Bob Mariani View Post
you will get some nail pops but that is all. I just finished a job like this last week. Some things to consider.... what is your insulation value? You need to be sure that the dew point of the underside of the sheathing will never be reached or the plywood will rot. this is what happened at this job. Someone used 2" Styrofoam which is too little R-Value. And the venting was inadequate. So venting added moisture in the summer and moist air condensed on the cold plywood in the winter. Every sheet was rotted through from below.

also do not use an recessed lights in this ceiling or you will have the same issue over the lights.

What type of insulation are you using? What venting do you have?
The house was built in 1963, so interior ceilings are that first generation plasterboard (maybe 3'x6'?) covered with skincoat plaster.

As it stands, the house has a continuous soffit vent on both long sides so that each joist bay is vented. It is a cold deck, so the little insulation that is there (about 2" foil faced) is on the ceiling side). AFAIK, there has been no problems with condensation probably helped because the kitchen & bathrooms are in another section of the house.

For cost reasons, I thought I would do what they did in the other sections of the house--block the end of each joist cavity at about 7 1/4" just overtop the wall plate--which would leave about a 2" air gap (but block airflow through the insulation), and filling each bay with an R22 batt (6 3/4" according to Ownens Corning). Not enough I know, and I had the thought originally of spraying a thin layer of spray foam to seal the bottoms of the cavities which would add R value and a vapor barrier.

Also, the only lights in the place are pot lights, so I did think of at least replacing them with airtight/IC units!
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Old 05-16-2012, 01:15 PM   #4
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Roof deck removal help


There is no way to get enough insulation over the tops of these cans. R22 when you know you should have R49.... it's your house. A thin layer of foam will air seal only nothing more. But this is a good thing. Best to be adding foam board above the roof. I just did that on a job last week for the same reasons you have at your house.

fiber glass batts are a cheap way but the worse way to insulate a ceiling. In winter at 25 degrees outside and 70 degrees inside a conductive loop occurs within the batts. This de-rates the effective R-Value to R-2 when you need it the most.
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Old 05-16-2012, 02:25 PM   #5
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Originally Posted by Bob Mariani View Post
There is no way to get enough insulation over the tops of these cans. R22 when you know you should have R49.... it's your house. A thin layer of foam will air seal only nothing more. But this is a good thing. Best to be adding foam board above the roof. I just did that on a job last week for the same reasons you have at your house.

fiber glass batts are a cheap way but the worse way to insulate a ceiling. In winter at 25 degrees outside and 70 degrees inside a conductive loop occurs within the batts. This de-rates the effective R-Value to R-2 when you need it the most.
Thanks for the information--and I'm close enough (NY/Hudson Valley) that the recommended insulation is probably that high for us too If I leave the construction as/built the only way I will get a higher R value is by using all foam (expensive), and I thought some improvement is better than leaving it as-is.

The only way it would make sense to add the roof board though, would be to convert the roof to a warm deck and block the soffit vents. Otherwise the board is on top of a vented roof deck, (which is what I have been having arguments with the roofers about).

The second roofer was going to quote me on 3" ISO (tapered though) which would give ~R22, and I could add the batts as well. My concern with the roof board on top though is that it changes the height of the fascia, and makes the flat roof line look heavier....

GBR (moderator) posted a link to this article on Building Science that shows two ways to get higher R values in a roof section--either meets current code for unvented sections & I've attached the pictures. The article I think assumes dense pack cellulose for the cavity insulation..

(I'd have to figure out how to adapt the diagram a little, since I have large overhangs where the soffits vents are--maybe blocking all the way to the top of the joist cavity at the outside top plate and use the spray foam?)

Alls I know--renovation is not for the faint of heart!
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Old 05-16-2012, 04:26 PM   #6
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Roof deck removal help


A spray foam flat roof only adds 1 1/2". Cost is about $450 sq compared to a built up roof of $400 a sq. In 20 years or less the $400 needs to be replaced. For the life of the building the spray foam roof is guaranteed material and labor to not leak of deteriorate in any way. Payback is 4-7 years depending on your bills. But that does not even consider the costs of replacing the roofing three times in your life being eliminated.

And this type of fix will allow your recessed lights, cut AC energy costs down 30% and does not need to be vented.

I am doing one of these roofs like this next week, I will post pictures on my site and update this post.

Last edited by Bob Mariani; 05-16-2012 at 04:27 PM. Reason: added venting
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Old 05-16-2012, 09:22 PM   #7
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Roof deck removal help


around here there are a lot of conklin roof systems "spray foam" for flat roofs and they work great

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