Built-up roof question (mopped to plywood)


#1

Hi Everyone,

I am in charge of putting together options for re-roofing our 130,000 sq. ft. warehouse in Concord, Ca. The existing system is a multi-ply BUR with capsheet installed over the original BUR which was mopped to the plywood deck.

My question is: should we remove the top BUR, leaving the original in place and overlaying the original with a new BUR, single ply, foam? I realize that tearing off the original system will basically destroy the plywood which would have our costs esculate.

All opinions are appreciated.

Mike


#2

Couple of questions:

  1. What is the condition of the existing roof systems? Are they blistered?

  2. Did the original roof system utilize stone-aggregate or slag surfacing, and was the newer roof installed overtop of that, or was there insulation installed between the two roof systems?

  3. Steel framing with plywood roof deck? And are you sure the BUR was mopped directly to the plywood and not to a base sheet that was fastened to the plywood?

Until you can answer those questions, I can’t tell you what to do for sure, but you may want to look into installing another layer of granule-surfaced modified bitumen membrane overtop the existing if it is in good enough condition to accept the new membrane.

Do you have any overview photos, and maybe a close-up or two of the membrane?


#3
  1. What is the condition of the existing roof systems? Are they blistered? Poor condition. Blistering, small splits

  2. Did the original roof system utilize stone-aggregate or slag surfacing, and was the newer roof installed overtop of that, or was there insulation installed between the two roof systems? Mineral surface capsheet. The new system had been installed over a base sheet which was nailed over the original

  3. Steel framing with plywood roof deck? And are you sure the BUR was mopped directly to the plywood and not to a base sheet that was fastened to the plywood? No steel framing. Wood only. I took test cuts which revealed that it was mopped directly onto the deck (original installed in 1968!)

Until you can answer those questions, I can’t tell you what to do for sure, but you may want to look into installing another layer of granule-surfaced modified bitumen membrane overtop the existing if it is in good enough condition to accept the new membrane.

Do you have any overview photos, and maybe a close-up or two of the membrane?


#4

pics on the way…


#5

i26.photobucket.com/albums/c113/thecheebs/6.jpg

i26.photobucket.com/albums/c113/thecheebs/1.jpg


#6

With a job that size, I would recommend you hire a consultant to survey the roof and provide a scope-of-work. You won’t need a set of specs, as a scope-of-work should suffice.

Now, if you don’t want to go that route, I would first confirm the second roof was installed over a base sheet that was fastened to roof deck through the original roof. If that is the case, and the second roof can easily be removed, that would be the way to go. Although, that all depends on the condition of the original roof, and whether it was smooth-surfaced or gravel-surfaced.

I can’t guarantee you anything, but if you like I can contact a roofing material manufacturer in Fresno and see what they might can do for you. If they are in the region they might could help you out. They are a modified bitumen roof membrane manufacturer, so if you are looking to go that route they might could help.


#7

Yeah, I agree with everything Cerberus said, except for hiring the consultant :smiley:

If you have a comepetent consultant, he could do you good.

Nuke scans are sometimes a useful tool in determining if you should tear off or lay over. Nuclear scans can pick up on any bit of moisture present, and therefore tell you if tear off, or partial tear off is needed, providing youre not surpassing the load bearing capacity of the building.

I have found that on buildings with large roof-to-floor ratios, a monolithic foam roof roof and coating can pay for itself in the way of thermal insulation values. Also, if you go this route, you may be eligible for up to 60 cents per foot tax credits for adding insulation to reduce the energy consumption of your building. Please see your tax advisor and the DOE for more details.

SPF roofing is lightweight and low-maintenance, and you can get warranties renewed when you re-coat it, if it has been non-neglected.


#8

Thanks to all for your recommendations.

Another option I heard of was tearing off the top layer of roofing and going over the original with a spray applied emulsion, reinforcing with fabric and coating with an elastomeric product. Someone else mentioned a durolast single-ply system. There seem to be so many options out there.

I guess my main concern is that it will be alright to go over the original without having to do a complete tear-off which would be costly


#9

I would advise against the emulsion and coating over the original roof. I also would advise against using a single-ply membrane like Durolast, as if and when it leaks the water can accumulate between the two roof systems.

As for the nuclear back-scatter scan, I wouldn’t recommend it for your building because you don’t have any insulation and the deck is wood. Besides, the scan is done in a grid and any entrapped moisture between grid points will be missed.

Once again, with the size of your job I recommend you get professional help either through a consultant, a reputable manufacturer, or competent contractor. Either way, you need someone who can survey the roof, take several cores, and decide what is best for your building. The problem with using a contractor for this purpose, is that they generally will send out an estimator who wants to sell you a new roof and put money in his pocket. Not all of them are that way, but how will you know which one you have? The problem with using a manufacturer is they will look at the job as only using their membrane. The consultant SHOULD not push any particular type of system on you, he should analyze the roof and determine what system is best, and he will charge you the same amount of money no matter what his findings. Now, the problem there is knowing whether you have a competent consultant or not. Many a contractor has hung the “consultant” shingle outside their door, and instantly think they are consultants. Likewise, I’ve met too many consultants that are book-read consultants and don’t have any practical hands-on knowledge about roofing. I’ve met engineers and former material manufacturer’s salesmen that have become consultants, but it takes many years for them to understand what they are doing. So, if you go the consultant route as I recommend, make sure you have the consultant give you a statement of qualifications, and quiz them about their background.


#10

good stuff from the guys above.
id just like to see ya get all that crap off there and start over.
get yourself a well, well, weeellllll, recomended roofer.
good luck.

gweedo.


#11

Insulation or not, the nuke will pick up wet wood decking, and the scan is thorough. I had one for a 1300 square roof, and it picked up any bit of moisture present…very nice. Cheap way of finding out what you have to plan for.


#12

I’ve performed a lot of nuclear back-scatter scans, and I can tell you I prefer infrared surveys over the nukes. Both together are even better.


#13

Why do you like infrared better?

I didnt even know you were registered with the nuke-overseeing agencies.


#14

That would be the Nuclear Regulator Commission if memory serves me correctly. I used to be nuke certified with one of the consulting firms I previously worked for, and performed nuke surveys on everything from prison roofs for DOC in Virginia, to colleges, hospitals, etc…

The reason I like the infrared scan better is because you get to view 100 percent of the roof surface, and someone who knows what they are doing can disregard areas that glow and give false readings. Take the photograph below for example:

http://i96.photobucket.com/albums/l187/Cerberus1960/IR_02b.jpg

The roof surface has cooled down, and the underlying wet roof board insulation is glowing white (the squared off area in center of photo), but the pile of gravel also retains heat and is glowing too. Now, I knew the gravel was stockpiled and would retain heat longer than the rest of the gravel-surfaced BUR, so I disregarded those areas. I was, however, able to detect the wet insulation beneath the gravel, so I marked it and we indeed found water damaged insulation.

Now, take a look at the photo below:

http://i96.photobucket.com/albums/l187/Cerberus1960/IR_03color.jpg

The heat anomaly was an unusual shape and probably not wet. I still marked the area out of curiosity, but I was later proven right in that the area was not wet. Here is a photo of the same area in the daylight:

http://i96.photobucket.com/albums/l187/Cerberus1960/DSC03.jpg

Now, when you do a nuclear backscatter scan of a roof, you mark a grid on the roof and take readings in specific locations. I can’t remember if the grid is usually 5 or 10 feet, but I believe it is every 5 feet. So you are taking reading in specific spots that may or may not be representative of the surrounding roof area. Alright, here is where my memory gets fuzzy, but when you push the bottom on the Troxler gauge you send out neutrons (heck it could be protons, or something along those lines) and they bounce back up toward the gauge off of hydrogen molecules. So, the more water (H2O) you have in a roof, the more neutrons (or whatever) bounce back up to the gauge where a reading is taken. The count of the gauge is representative of the neutrons or whatever that bounced back up (back scatter) and were counted. Now here is the problem, there is also hydrogen in asphalt, so in places where you have heavy pours of asphalt you are going to get a high reading. You have to plot that number on your histography, and unlike the infrared you can’t just ignore it. Also, if you have a 3’ x 3’ area of water collected beneath the roof membrane, and it falls in between your grid, you will not get a high reading and will be ignorant that you have water in that location.

Alright, I know I’ve probably given you bad information with regard to how exactly the Troxler gauge works, but the principle that I discribed is generally correct. If you look at the Troxler site, they say the top four inches are where they get the readings, and it is from gamma rays. I know the gauges I used didn’t rely on gamma rays, so things may have changed. Still, I prefer infrared over nuclear back scatter for finding moisture in a roof system. I do like the nuke gauges, however, for testing soil compaction.

FWIW, the reason you didn’t know I WAS nuke certified, is because you never asked and I never talked about it until now. If you have anymore questions about nuke and infrared, I’d be more than happy to answer them to the best of my knowledge.


#15

Thats cool stuff, man.

How would the IR work an a foam roof, with water beneath an original BUR from a far-off leak, for example? My mind is thinking it wouldnt, so then do you go to nuke?

Also, how do the IR scans in those non-heating, non-coolong days when temps are comfortable, and there is not a great temperature differential? This has always seemed the trickiest for me, but I still have not used IR. I go with the plug method, but sure would like to have an IR thermograph with me to avoid cutting into perfectly good roofing if I didnt have to.

What about costs/vs. quality? What am I looking for in a rugged, contractor grade IR?


#16

I can’t say that I’ve ever tried to do an IR scan on foam, but it should work fine if the foam isn’t too thick. It is the same principle, wet foam should lose its R-value and where it is wet should glow when exterior temps drop.

Ideally, you want a 20 degree temp swing from day to night, so the IR will be most effective. Some people also prefer nuke because you do it during the day, whereas IR has to be done at night.

The prices of IR cameras have gone down drastically. The older models used to use liquid nitrogen as a cooling agent, and back then they cost around $60K. Nowadays, I think you can get a decent IR camera for around $15K, but I’d have to check pricing, because I haven’t checked in a while.


#17

15K? LOL not this year…

Anyhoo…As you know, a foam roof directly to the roof deck prohibits lateral water movement, since it is fully bonded to the deck…When you go over a BUR or other membrane that is not fully bonded, you can get a leak in the roof all the way down through the BUR, you will not get this added benefit of full adhesion. This is where my question lies…with water sitting under the insulated foam roof, you may not get a great heat differential showing on the IR because the water is under the BUR and the foam…especially hard if it has traveled a long way under the built up to get there. This could leave the foam dry. Am I whack, or could this be a restriction in reality?


#18

[quote=“michaelkelleher”]Hi Everyone,

I am in charge of putting together options for re-roofing our 130,000 sq. ft. warehouse in Concord, Ca. The existing system is a multi-ply BUR with capsheet installed over the original BUR which was mopped to the plywood deck.

My question is: should we remove the top BUR, leaving the original in place and overlaying the original with a new BUR, single ply, foam? I realize that tearing off the original system will basically destroy the plywood which would have our costs esculate.

All opinions are appreciated.

Mike[/quote]

Depends on how solid the original mopping was. Normally they would sprinkle mop base and this would come up pretty easy in cold weather. If it is solid mopped to deck you may want to go sigle ply as this is very light material and the extra costs would offset the demo of first two systems


#19

You could go with a seamless SPF roof and we KNOW you would recoup the costs in rising energy cost savings in a few years.

Makes the most long term roofing budgetary sense, also.


#20

Is there a minimum size for a SPF flat roof?