Double sheetrock roof decking?

I saw the strangest thing today and I can't believe it is true. Apparently, I own a property that has a flat "hot tar" roof over multiple layers of "tar paper"(?) -- and the roof decking under under the tar paper is just two layers of sheetrock. There is no wood decking -- just a double layer of sheetrock nailed to roof rafters on 16-inch centers and the roofing material on top of that. Has anyone ever heard of or seen that?
Here are the details:
I own an end-of row two-apartment building. I am not sure when it was built, but some records indicate that it may have been built in 1930, another indicates possibly 1935. And, a third anecdotal report is that maybe it was built around 1945 because all of the walls and ceilings are constructed with studs and ceiling joists on 16-inch centers with a double layer of 1/2-inch sheetrock. There are no plaster walls, and someone said that around 1945 they had just started using sheetrock and, because it was new, they did double layers of sheetrock. I am only explaining this is case it gives any clues as to the type of roof construction that was used back then.
I bought the property a few years ago and the second floor tenant that I inherited just moved out a few days ago, so I am re-doing the second floor apartment. The ceiling of the second floor apartment was a drop ceiling and I took just that out. There were no signs of any roof leaks on the drop ceiling, but after I took down the drop ceiling I could see that there used to be water leaks by looking at the old sheetrock ceiling. And one small part of the old sheetrock ceiling was damp and warped. So, I started taking down the sheetrock ceiling to find the leak and to repair the ceiling. The sheetrock ceiling is a double layer of sheetrock that is nailed to the bottom of the ceiling joists. Above the ceiling joists are the roof rafters, and nailed to the top of the roof rafters is a double layer of sheetrock. That double layer of sheetrock on top of the rafters is the actual roof decking. Some of that sheetrock had been water damaged in the past and was pulled down is spots. So I can see the underside of the actual tarp paper roof base and the hot tar roof on top of that. I can even see roof nails coming through the roofing material and through the double layer of sheetrock.
So, again, the is absolutely no wood roof decking under the hot tar and tar paper -- is is just those asphalt/tar products on top of a roof deck that is only made of two layers of 1/2-inch sheetrock nailed on top of 2x6 roof rafters on 16-inch centers. This is how it is for the entire roof (I checked other areas to confirm this).
Has anyone else ever seen this or heard of this?
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WOW! What part of the USA? is this? Never heard of it anywhere that I have ever lived, NJ, Fla, and IL
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wrote:

WOW! What part of the USA? is this? Never heard of it anywhere that I have ever lived, NJ, Fla, and IL
+++++++++++
This is in New Jersey, across the river from Philadelphia, PA.
Like you, I never heard of anything like this before. I can't see how people could even safely walk on the roof without at least cracking the double layer of sheetrock that is used as the roof decking.
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RogerT wrote:

What is the pitch or slope of this sheet-rock roof?
Are there regular shingles applied as the final top-layer of this roof, or just some sort of tar-paper sheeting?
Do you know anything about the repair history of the roof? Has it been re-tarred (or what-ever you do with a hot tar roof) ?
Is there any printing or marks, logos visible on the sheet rock?
Has anyone stood on this roof? Can it support a person's weight?
Were there any building inspectors around back in the 1930's or 40's ?
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Home Guy wrote:

The pitch is very slight. A quick measure looks like it drops about 5 or 6 inches from the front of the property to the back -- which is a distance of about 33 feet.
I took some photos this morning and I'll see if I can get them posted here.
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RogerT wrote:

Hopefully, these 5 picture links will work:
http://tinypic.com/r/2w6zztv/7
http://tinypic.com/r/22hsnm/7
http://tinypic.com/r/262ulon/7
http://tinypic.com/r/2hrk11c/7
http://tinypic.com/r/oa29up/7
The first two are exterior views.
The third one is from underneath on the inside along the flat part of the roof near the back of the property.
The fourth and fifth photos are from underneath near the very front wall of the property through a hole cut out in the ceiling. It is a view up into the section that has a built up shingle on the front as shown in the second photo. In these two photos, you can see the wood "decking" that makes up the vertical part of the front rim/roof where it is shingled, and you can see the double layer sheetrock that makes up the back slope of the front rim/roof -- and it also shows a little of the flat roof from underneath. The "angle" where two different planes of sheetrock meet is the angle where the slope-off from the rim/roof meets the flat roof.
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RogerT wrote:

Sorry, I'm not too good at this picture upload stuff.
I should have written:
The first two are exterior views.
The third photo is from underneath near the very front wall of the property through a hole cut out in the ceiling. It is a view up into the section that has a built up shingle on the front as shown in the second photo. In this photo you can see the wood "decking" that makes up the vertical part of the front rim/roof where it is shingled, and you can see the double layer sheetrock that makes up the back slope of the front rim/roof -- and it also shows a little of the flat roof from underneath. The "angle" where two different planes of sheetrock meet is the angle where the slope-off from the rim/roof meets the flat roof.
The 4th and 5th photos are the flat roof from underneath, near the back of the house.
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what an interesting way to see if your insurance is all that it should be
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RogerT wrote:

See here:
http://www.inspectionnews.net/home_inspection/roofing-system-home-inspection-commercial-inspection/13801-sheet-rock-decking.html
One of the comments:
My recollection is that it was used for roof sheathing during or after the war (WW II) when plywood was in short supply for being used for planes and boats, then new homes for the GIs returning home, and it proved to be fatally flawed for that use.
Also, this:
http://www.wconline.com/Articles/Column/090578779d768010VgnVCM100000f932a8c0
During the 40s, gypsum wall sheathing and gypsum roof sheathing products were used along with gypsum board in domestic and overseas military construction.
This is apparently in current use:
GYPSUM BOARD ROOF UNDERLAYMENT SYSTEMS http://www.gypsum.org/pdf/GA-276-05.pdf
Effectively preventing fire from penetrating through the roof eliminates updrafts, both limiting the potential for fire spreading on the roof surface and reducing the generation of flying brands. Adding gypsum board to roof systems also inhibits the spread of fire within an attic while protecting against burning brands from fires originating in another unit or from burning vegetation.
If most of your sheetrock is in good condition, you might want to just make some patch repairs (replace with new sheetrock) and then throw a layer of 3/8" or 1/2" wood decking over it (or press-board or what-ever it's called where you are) and then apply final layer of tar or shingles.
But it might end up being really heavy, and snow loading might be a problem.
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Home Guy wrote:

http://www.inspectionnews.net/home_inspection/roofing-system-home-inspection-commercial-inspection/13801-sheet-rock-decking.html
Thanks for finding that! It's a very interesting link and it looks like I am not the only person to have seen this. The OP on that link is a home inspector who saw the same thing.

I am wondering the same thing. I have to fix the sheetrock ceiling in the apartment. But, before doing that I am hoping to get a roofer out there to look at it from underneath on the inside so he can see what I see. I doubt that many people, including roofers, have seen this, but maybe they have.
Then I have to decide what to do. I am considering a few possible options.
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On 9/4/2011 8:51 AM, Home Guy wrote:

http://www.inspectionnews.net/home_inspection/roofing-system-home-inspection-commercial-inspection/13801-sheet-rock-decking.html
Wow- learn something everyday on here. I knew a lot of corners got cut in construction in WWII era (like plastic faucets), but never heard of drywall roof decking. I gotta wonder if it was some special extra-high-compression stuff, and maybe fiber laced. And since asbestos was considered a wonder material back then, that is likely what the fiber would be.
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On 9/4/2011 2:07 PM, aemeijers wrote:

The other one I have heard of is steel electrical wiring. Fortunately the average home back then probably had one circuit on knob and tube and a half dozen lights and outlets so one would think any steel wiring is gone.
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On 9/4/2011 2:30 PM, George wrote:

K&T was mostly long-gone for new installs by that era. Greenfield cable or early cloth-covered romex was SOP.
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wrote:

Don't count on it.
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harry wrote:

Looks like I figured out and received some good news on this situation this morning. We took down the rest of the ceiling in the two back rooms, so we can now see everything that is going on with the roof from underneath. It is also raining fairly heavily today, and we can see what's going on with the roof from the top using the vantage point (another nearby property) that I used to take the earlier exterior view photo. At this point in time, the roof is not leaking despite the constant rain.
From underneath, we tried pushing up on the areas where we can see the actual roofing due to the double layer of sheetrock having broken off. The roof is rock solid, rigid, and strong when pushing up from underneath. I think what must have happened is that somewhere along the line someone added a new wood roof deck over top of the original roof and then did a second roof over that. The roof is way too strong and flat for that not to be the case. And, that would explain the rows of nails that we see coming through the original roof -- it must be the nails that were used for the new roof decking that went on top of the original roof.
The roofing contractor that I expected yesterday showed up today and he also said the roof is not leaking and that there must be wood decking on top of the original roof. He owns a large commercial roofing company that has been in business a long time and he said he never saw sheetrock decking like this before -- but that there is clearly wood decking above that even though it is not visible. He is going to come back in a day or two after the rain ends and go up there and take a closer look.
We (not the roofing contractor) saw a small 1-inch by 2-inch hole about 4 or 5 feet in from the back of the property along the line where the sloping ridge along the side meets the flat part of the roof -- maybe up an inch or so on the slope. That's where we did find the one and only slightly wet ceiling tile when we removed the dropped ceiling from the back room. It appears that probably that hole doesn't get much water in it because it is above the flat roof level, but during the recent hurricane style rains water probably did get in through that area.
So, the bottom line is that the roof is strong, it is in overall good shape, but it will need a little roof cement and/or fabric patch and it should be good to go.
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Hope the supporting joists for the roof can take all the weight that has built up with the multiple layers of roofing.
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On 9/6/2011 5:05 PM, hr(bob) snipped-for-privacy@att.net wrote:

And that OP never gets a sudden 18" wet snowfall wherever he is.
Not a fan of overlaying roof or other structures. And the fire dept HATES the dead spaces they produce. (Parts of my office have 3 layers of drop ceilings, with spaghetti wiring and ducting running through all the layers.)
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aemeijers wrote:

I am in New Jersey, across the river from Philadelphia, PA. We did have a 28-inch snow storm in February of 2010 and some other heavy snow falls that year. The good news is that the roof survived those and all other storms for the past 4 years that I have owned the property.

I agree that dead spaces are an issue in the event there is a fire, and especially when there is one roof, then an open dead space, and then another roof above that. In my case, there is no dead space, just multiple layers of roof directly on top of each other.
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hr(bob) snipped-for-privacy@att.net wrote:

I hope so too, especially since I already own the property and have owned it for the past 4 years. So far, no evidence of any weight issues with the roof. I now have the entire ceiling down in all of the rooms so I can now see all of the rafters and the goofy sheetrock original decking for the entire roof from underneath. It looks okay with no signs of stress.
If I ever do need a new roof, it will be probably be a complete tear-off down to the rafters, or possibly just a tear-off down to the mysterious so-far-unseen wood decking.
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