Hello All, New to this site… so in advance, I apolgize if this has already been discussed… Lots of great information/discussions to read - It’s good to be able to discuss/listen to various issues with other ‘professionals’ who obviously take their trade very seriously. I’ve been roofing about 16+ years in the North East (Boston) area. Lately I have noticed a huge increase with mold on the underside of roof sheathing. No matter what the root cause may be; poor ventalation/condensation, bath room vents vented into attic and not to the exterior (many, many times) or just that the structure or lumber was left exposed for too long during construction - It always seems to be on a plywood roof deck and never on the old plank boards. Any thoughts? Why is this?
Plywood holds/draws moisture more than plank boards.
Plywood is veneers glued together. This does not let moisture pass thru. The moisture once it passes thru the first veneer it starts to destroy the glue. The glue is formulated to last with exposure to moisture. Thus it does not let moisture pass thru for a ling time.
Planks will let moisture pass right thru the lumber itself with no resistance. The moisture is then on the outside of the wood. Felt lets the moisture pass thru. Shingles shed water. You will not have a problem with planks. It naturaly vents itself. The gaps inbetween the planks also lets out moisture.
So these types of houses will not need the ventilation that plywood houses need. This is not address when the ventilation manufactures teach their stuff.
I have seen it on 1 by roof decks so it is not limited to just plywood. The roof had 2 vents in it. After adding vents and cleaning the mold off it is mold free now.
First of all, are you sure it is mold? Not to doubt you, but without seeing what you are talking about, I thought I’d mention another option. If what you are seeing is a blackening of the plywood sheathing, it is also possible that you are coming across old FRT plywood that has “bloomed.” Check the stamps on the plywood, and take mental note as to whether you are coming across the same plywood, or if it is really mold.
the word “mold” doesn’t always mean bad as most think it does. This is a huge topic in roofing. mold = or doesn’t = black mold. The topic mold and the topic black mold are HUGH topics of discussion. There are thousands of types of “molds” - some good - some bad - if you have questions of what type yorus is - have it tested for sure.
The only times I can ever remember seeing mold on planked roof decks is when there was an existent unattended leak. I have never seen it from mere humidity within the house. Now plywood… if you have plywood on your house, chances are that the house has been built at least close to code, in regards to insulating, ventilating etc. or is probably built wrong even for its time.
The worst case scenario in terms of venting nightmares has to be when there IS insulation and no baffles (I never knew what that word meant, I call them cora-vents), and the house has a plywood deck. The plywoods nature to repel water, even just over a very short time, results in the plywood breaking down. I’ve seen five year old plywood SHOT, because of no ventilation. So in essence, I guess I’m saying that I agree that modern ventilation is required for modern building styles, and maybe depending on location…
…but these victorians I’ve been stripping up here have been standing a long, long time, and the concept of venting is fairly new relavent to them.
Cerberus; very interesting. I will take note of this as it appears in the future. What is ‘FRT Plywood’? timothy_71 I absolutely agree - not all is bad and always will refer customers to a company who handles these (mold) matters. S&G; the older homes are much, much better off than some of these new ‘code built’ homes.
If you were building your own home today… would you insist on using plank boards for the roof sheathing? or Does ‘proper ventilation’ actually work to shed excess condensation?
honestly… I don’t think I could afford to demand the planking. Thats why its no longer in practice. It’s WAY more work to install the planking. Newer standards are fine when theyre done right, but whats right for a 1980 ranch probably isn’t necessary for a 1900 colonial or whatever… thats just my opinion.
[quote=“roof-you”]Cerberus; very interesting. I will take note of this as it appears in the future. What is ‘FRT Plywood’? timothy_71 I absolutely agree - not all is bad and always will refer customers to a company who handles these (mold) matters. S&G; the older homes are much, much better off than some of these new ‘code built’ homes.
If you were building your own home today… would you insist on using plank boards for the roof sheathing? or Does ‘proper ventilation’ actually work to shed excess condensation?[/quote]
FRT stands for Fire Resistant Treatment. The FRT plywood was involved in law suits back in the 80’s if my memory serves me correctly. It probably isn’t FRT, since FRT was primarily used on townhouses and such, but I would keep an eye on the APA stamp. Now, assuming it isn’t FRT, then you should look at the APA stamp to ensure whomever installed the plywood used CDX or an approved exterior grade plywood for roofs. Substandard plywoods used on roof decks will tend to be more hygroscopic in nature and prone to mold and moisture damage.
Fire resistant plywood/ The whole sheet will turn brown and it is also delaminated if it turns this color. It will not be safe to walk on. Even before you remove the shingles.
If we know that it is present. We will not bid the job. I fell through shingles and plywood and damaged a ceiling. That was the last one.
Heat and moisture set the plywood to delamination. So if there is humidity and heat the chemical reation is set in motion. The plywood will not last the shingles life. The plywood is shot in 20 years. To the point of dangerous to walk on.
**Does not resemble mold at all. **
I agree with that!
Lefty, heres a funny one from a few years ago:
I was measuring a roof while the customers husband was on the roof with me talking while i measured the job. Well i wlked over a bad part of the roof not knowing it and wham right threw the roof i went into the bed of the customer with his wife in bed. Never heard a scream so loud in my life. Apparently there was a bad leak there he did not tell me about. I slammed my knee into a rafter on the way down. I got the job right then and there. I think from fear i would sue. Not sure but boy did i raise the cost up a bunch. I thought i gave the lady a heart attack.
I can keep my wits about me when this is happening. It is still scary. You never know where you are going to land.
The one I talked about. The home owner was a contractor. He made me fix his ceiling. Tried to get me to redo his driveway. We never had a truck on it. He said we parked trucks on his driveway.
some of them always want something for nothing