One Ply or Two Ply Modbit?


This is a follow up to a previous question…

Gotten some estimates on having my flat roof redone and have the option of going with either a single ply or two ply torchdown modbit system.

My question is: Is it worth the extra expense to go with a 2 ply?

To my way of thinking I don’t necessarily see the advantage of adding a second ply.

It seems that adding more material only increases the chance of premature failure somewhere in the system.

Is there some advantage to a 2 ply that I’m missing?


you should always have at least 2 plies,

  1. base sheet
    2)weldable bitumen
    over here its rarer to put a double ply of rubberized bitumen,unless you are building a deck on top of it,or using it to protect the roofing from pavers and such,but properly applied the 2nd ply of bitumen can add 10 years to the product warranty single plys 1)EPDM-Functional more commercial look than residential,unless you use white which is newer 2)TPOs–generally can have issues(from what I`ve seen)of material failure,and are harder to repair/weld over time
    3)PVC-IB/Sarnafil—very nice clean materials that allow for clean welds,and can come with up to a lifetime warranty,and deal with water ponding problems without problems

If you see the roof,probably the most matched look would be a mineral surfaced rubberized bitumen system containing
1)base sheet
2)1 smooth layer APP weldable
and 3) a top layer of mineral surfaced rubberized bitumen

regardless of what roof you may pick,More often than not the issues arise from the quality of installation,and finishing details,So what Im saying is Make sure you spend whats necessary to get the job done properly,rather than looking for the cheapest alternative,remember you get what you pay for


Thanks for the info, I probably should have been more specific.

Ok, the single ply quote is for:

(1)base sheet
(2)top layer of mineral surfaced rubberized bitumen

The two ply quote is for:

(1)base sheet
(2)1 smooth layer
(3) a top layer of mineral surfaced bitumen

There is about a $1500 difference between the 1 ply vs 2ply.

Adding the 2nd ply doesn’t double the warranty, but adds 5 years.

Following this line of thinking… Seeing as how there is a greater chance of failure due to an installation issue during the warranty period vs a premature product failure.

Wouldn’t it stand to reason that adding a second layer would increase the probility of having an issue somewhere down the line? Regardless of Cost


I would go for 2-ply just for the fact it is over living area.


If torched properly there would be no problem,just make sure it is fully torched not just seam torched


Quite the opposite actually. the seams will be staggered between the layers of modbit and be melted together. That way if the top layer ever does have a seam pop or something the underlying layer will be water tight still. Plus it doubles the puncture resistance.


Don’t get me wrong… I don’t mind spending the money for a better roofing system.

But am I adding a 2nd layer essentially “in case” there is a problem with the 1st?

Are there any other pro’s to adding a 2nd layer?


Yes a 2nd or a top sheet will add longevity to your roof plus if it: a traffic area will hold up longer.A base is not a real water barrier and a good app system with a cap sbs will give you yrs of roofing life.with out seeing pics it hard to sell you on the up grade but if it where me i would go with it if it was in the budget.



Wouldn’t it stand to reason that adding a second layer would increase the probility of having an issue somewhere down the line? Regardless of Cost[/quote]

The complete opposite is true.

Additional layers of protection = less quality ?
Is this the twilight zone?

Most of my career, i have installed one ply.
One layer of smooth or one layer of granulated.
After much experience, i now install both if the homeowner can afford it.
If its flat- than i demand it. No option.

Unless your roof is extremely small. 1500 would be way too cheap to install a second layer.
I would choose a different roofer.

If your roof is flat and your roofer gave you the option of one ply- choose a different roofer.

Choose a roofer that cares about your roof.
Not cares about making it cheap enough for you to choose them.


No question about it. base,modi smooth,granulated torch. This will be bullet proof and let everyone sleep well.I too,insist on this with flat roofs.Goodluck.


Again, Thank you for all the replies and sorry for the long posts!!

To answer a few questions:

The flat roof area receives no foot traffic to speak of, other then myself occasionally getting up there to clear the gutters.

This is in Southern California (which I probably should have stated) So the weather is very moderate. No snow, ice or hail. Generally we have a few months of rain in a good year.

As far as longevity, Here is what I’m trying to reason with…

I understand that adding a 2nd (or middle ply) will increase the lifespan of the roofing system and serve to act as a secondary barrier in the event the top layer fails, but here is where I question the cost/benefit factor.

For discussions sake:

If I get 10 years out of a single ply (base w/mineral surfaced top) and roughly 15 years out of going to a 2 ply (base, smooth, mineral surfaced top)

Wouldn’t it stand to reason though, that after 10 years if the top layer has reached the end of it’s serviceable lifespan and is starting to fail. I’d need to look into replacement, since the overall system is compromised?

Or is the longevity of the top layer extended by adding a middle layer?

If not, aren’t I more or less just buying some extra time. Delaying the inevitable replacement, but adding costs at both ends?

Basically $1500 more up front and XX more at tear off for an additional layer.

Again, I’m not against spending the money.

But if realistically after 10 years I’m going to need to look into replacement, regardless of whether I go with a 1 ply or 2 ply system I don’t see the sense.

Can anyone give me the pro’s and con’s of both options?


Wouldn’t it stand to reason that adding a second layer would increase the probility of having an issue somewhere down the line? Regardless of Cost[/quote]

Sorry, Im not trying to offend anyone… That is a serious concern.

Reason 1: Given the human factor and knowing that adding a second layer results in twice as many seams, theoretically this doubles the chances of a failure somewhere along the line.

Reason 2: The laws of physics come into play. In that the more you heat the material (which obviously has to be done whenever adding a subsequent layer) the more it’s cell structure starts to breaks down resulting in a certain level of degradation.

Reason 3: I’ve read posts on the forum here about how it can be more difficult to trace leaks in a 2 ply system since the water can run between layers before leaking out.

As I said, I’m not against paying for a better roof. And I’m not trying to offend anyones sensibilities.

I’m just not one to take it at face value that if 1 layer is good, then 2 MUST be better.


A perfectly installed torch down one ply on a flat(no drainage) roof fails in a short time.

10 years? only in your dreams…

I charge 60 dollars a sq to tear off each additional shingle roof tear off.

I would charge zero per sq to tear off a second additional layer of modified. Both layers would come up at the same time because they would be melted together as one.


So how does adding a 2nd ply of modified make for a better roof?

If a single ply is due to leak from mechanical failure in a short period of time on a flat roof. I’m assuming due to pooling water and the resulting degradation at the seams.

Don’t I still have the same issues with a 2 ply?

Standing water entering in at the seams, resulting in material degradation and ultimately mechanical failure?

Is there a better option?

The contractors I’ve spoken with so far have said my two options are:

Hot mop tar and gravel or torchdown modbit.


A 2-ply is a way better roof if done properly,no question.


But why is it better? This is an honest question as I’d really like to understand.

From a consumer stand point this is what I’m hearing.

If you install a 1 ply modbit your roof isn’t going to hold up.


If you install a 2 ply modbit it’s going to be a great roof.

Now keep in mind, Your suggesting that I use 2 ply’s of the same exact material, that you just told me wasn’t good enough in 1 ply.


If done properly the first layer is waterproof and same for the second layer.If the first one fails you have a back-up.A single-ply is not near as forgiving.



Now keep in mind, Your suggesting that I use 2 ply’s of the same exact material, that you just told me wasn’t good enough in 1 ply.[/quote]

Incorrect logic.

One ply is Smooth** or** Granulated torch-down plus your underlayment.

Two ply is Smooth and Granulated torch-down plus your underlayment.


2ply torchdown roofs are not a good idea for me.
if you have a problem with 1st layer water will get inbetween the 2 and make it hard to find and/or fix.

2ply torchdown roofs are only for people who cant make 1 layer last.



One layer does last a decent amount of time and is perfectly acceptable–if there is no ponding water.
Just my opinion though.