Dimensional on 2.5 pitch (not a new topic)

Hurricane Wilma tore off the tabs of about 50 % of the 3 tab shingles on my 2.5 slope roof here in Miami. The roof is estimated to only be between 10 and 15 years old. I had been seeing washed off granules in the gutter and the grass around the house for years. I am appalled by the flimsiness of the 3-tab shingle and the premature failure of its tab adhesive.

I intended to replace the roof with Owens Corning Weatherguard HP 40 year, which I chose for its UL 2218, Class 4 impact rating wind resistance up to 130mph and its claim to use a very aggressive adhesive.

Now I am receiving mixed messages:

Pros on this forum reject all dimensional shingles for slopes less than 3 / 12.
Of the roofers who got on my roof to give me a quote 1 outright refused to use dimensionals, others would install it but claim it offers no benefit over the 3 tab.
Yet another claims not to have had any problem with installing dimensionals.

The manufacturer specifies 19 “ overlap on the felt underlayment on slopes between 2 and 4 and current Dade county code (supposedly the strictest in the land) allows it to be used if so installed. The old South Florida Building Code, of 1999 says: “dimensional shingles are limited to 3 ½: 12 slope (min.)”. When calling OC they claim – surprise surprise - never to have heard of the problem.

Is what I’m getting here a blend of hearsay, folklore, “that’s the way we always do itâ€

I have seen about 7 lower than 4/12 that have leaked because of that reason.

What were the mechanics of the leakage in those cases?
What is it about the design of the dimensional versus the three tab that allows the leak? How do we know that the leak is not just simply faulty installation or deterioration due to age as happened with my three tab?

You had just better follow that Florida building code if youre in Florida.

Dims will hold water on a low pitch roof. Due to the way that tey are made they are a 2 piece shingle. when the water sits on it any wind will back it to the 2 layers that are glued together and has a possiblity of leaking. It is not advisable. Some will use ice and water but that will not last. Do it right the first time.

Cooley make a single ply that looks like a dimensional shingle roof. This is perfect for your low slope roof, while still appearing to be shingled.

The main reason (in my opinion) for the deterioration of your shingles was the low pitch of the roof. The low pitch allows water that would normally run off the roof, to sit on your shingles longer, and along with the sun, causes them to break down. This will be the case if they are 3tabs or dimensionals.

Yep, and that’s why dennis makes the big money. 8)


The water lays longer on the shingles. When the sun comes out and the roof is still wet. The water acts as a magnifing glass. Thus the shingles wear out faster.

The laminates let water go cross ways under the one layer of the shingle. The lower the pitch the worse the problem.

if ya still need a roof in florida you can call me
you can email me at roofsbygweedo@ij.net.


thanks for y’alls input.

ditto gtp1003,
demensionals trapp water were they butt together on the sides.
the lower the slope the worse it gets.
3 tabs have water keyways that form a small gap were shingles but
together that helps water not get trapped.

there are no shingles that are made for low slopes, its just
that some styles work better than others when applied to a low slope.


ice+water shield the entire roof 1st,3-tabs are better because they`re not fatter on the bottom side like the dimensionals-either one will not last the full mfgr.s warranty

The reason I’d go with a Dimensional is due to high winds; they are stiffer & hold up better vs. 3 Tabs.

The real problem is in the leakage & faster deterioration as discussed.

In a perfect world, you’d have a roof that’s @ least a 4:12, but that’s probably not an option. You could always install a stiffer 3 Tab type product, i.e. lower water resistance like a 3T but stuff like a 30 year.

Here’s an idea…ROOF IT RIGHT…do not use shingles on that slope. Use a low slope product. It really is that simple.

Yep, anything 3/12 and under need a low slope app, not asphalt shingles.
Even 4/12 is questionable IMO. It’s a pain to do torch down on a 3-4/12 so I’d check into either GAF Liberty or EPDM. Of course it depends on your local code, Fla has some crazy reqs.

sorry AARON,my reply covers mfgr specs for low pitch shingle applications,which would last much longer than most flat/roll applications,the point of three tab being better in that application RANCHHAND is strictly for good drainagE.Shingles are good on a 2 1/2" on 12" pitch,less than 2"pitch,i would agree w/ AARON