# Low Pitch Roof: Hot Mop vs Torch Down?



## ddawg16 (Aug 15, 2011)

In the process of doing my 2-story addition, I had to modify my existing roof to butt up against the addition....










The slop pitch in the area is 1'9":12"....

Per my drawings....the roof in this area is to be:



> GAF BUILT-UP ROOFING SYSTEM. ICBO REPORT NO.
> ESR-1274. HOTMOP TOP LAYER OF FIBERGLASS CAP
> SHEET EMBEDDED INTO HOT ASPHALT APPLIED AT 30
> LBS. PER 100 SQ. FT. OVER 2 - LAYERS OF FIBERGLASS
> ...


Per my inspector....he says torch down is ok as well....

I already have one quote...a roofer I have used before....(so have neighbors and in-laws)....he says hot mop is better...

I have another guy coming out Saturday....during my first phone call with him...he mention torch down as being better...

So....which one is it? Hot mop or Torch Down?


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## Grumpy (Dec 12, 2003)

Such a small job not worth getting out the kettle. Honestly very few people mop anymore, and on such a small job I doubt anyone will want to. A 4 ply granulated surface modified bitumen will last as long as the shingles, and you can get colors similiar to that of the shingles. If I were bidding this job, I would be proposing the 4 ply modified. 

Better? Hmmmm If it were a shopping center, I would say PVC is better. But seriously a 5 ply hot mop BUR will be better than a modified bitumen. But at the same point in time the modified, if installed correctly and installed 3 or 4 plys will defiently last as long as the shingles, and that's really all you want.


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## shazapple (Jun 30, 2011)

"Hot mop" and "torch down" are not roofing systems, they are how the layers of the roofing system are attached. It's like calling shingles a "nailed roof". 

What you describe in your original post is essentially a 3 layer roof with a rolled roofing top. The bottom layer is nailed and the other layers are mopped with hot asphalt. Not many people install these anymore, and the cap sheet mentioned is crap. Grumpy is correct; for something that size I would not bother with a roof that needs to be hot mopped. 

"Torch down" refers to Modified bituminous (Modbit) roofing. Typically it would be installed by nailing down the base sheet, an then taking a propane torch to melt the cap sheet and base sheet together. It looks similar to rolled roofing, but is a much superior and longer lasting product. Instead of nailing and torching this could be hot mopped as well, but again, not very popular for small roofs. Some contractors are not comfortable with using the torch down method, but they could use glue or 'self adhering' modbit membranes instead. 

There also EPDM, PVC, or TPO roofs (which are typically glued or screwed down). All good roofs as long as the installer is good. I suggest you get more quotes other than the one you posted.


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## ddawg16 (Aug 15, 2011)

Good info guys.....shaz...thanks for the nitty gritty details....

With that said....it's my assumption that the same asphault shingles on the rest of the roof will also be on this section of roof.

Hence....the nails through the layers will not be an issue?

Lastly....what does one do 30 years from now when it's time to replace the shingles? do you rip up the torched down layers and start over?


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## 1985gt (Jan 8, 2011)

Original question, Hot mop is better, but see the advise above.

If you get 30 years out of the shingles I would be surprised. But if the shingles need to be done before the flat roof it's possible and fairly easy to do if hte contractor knows what they are doing. Same with the other way around. It's really a non issue for the right contractor.


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## Grumpy (Dec 12, 2003)

ddawg16 said:


> Hence....the nails through the layers will not be an issue?
> 
> Lastly....what does one do 30 years from now when it's time to replace the shingles? do you rip up the torched down layers and start over?


Make sure you have at least a 2' over lap where the shingles meet the low slope. In other words the low slope membrane will wrap 2' up and under the shingles. I also like to back off my shingles a few courses so they aren't all the way down to the lowest portion of the low slope to steep slope transition. Furthermore if tieing in shingles with modified, both asphalt based products, I like to sptrip in with ice shield. Then I know there can never be any back up. Doing it right will cost extra though.

When it comes time to replace the roof depending on code and condition you might be able to go over. However I usually suggest tearing off, specially a critical area like a pocket. See my reply about about "that's really all you want." What I was eluding to is the low slope should match the steep slope in terms of longevity. Don't go cheap or super premium on the low slope since I recommend replacing both at the same time in your situation.


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## ddawg16 (Aug 15, 2011)

Grumpy said:


> *Make sure you have at least a 2' over lap where the shingles meet the low slope.* In other words the low slope membrane will wrap 2' up and under the shingles. I also like to back off my shingles a few courses so they aren't all the way down to the lowest portion of the low slope to steep slope transition. Furthermore if tieing in shingles with modified, both asphalt based products, I like to sptrip in with ice shield. Then I know there can never be any back up. Doing it right will cost extra though.


Good point...and makes sense.

FYI....being in California, I think an ice damn is not something I need....if it gets into the 30's where I am...the news would be going 24 hours talking about it....

Regarding the transition from the 5:12 slope to the low slope...I just assumed that valley flashing would be installed. I have it on all my other valleys....in fact, any roofer that says I don't need it...I would not use....

I've seen too many roofs in my neighborhood without it and the valleys look like crap.


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## ddawg16 (Aug 15, 2011)

While waiting for more comments......I've been doing more Google searches.....interesting how the hits change just by changing the words just a little....

Anyway....it would seem that torch down has a slight advantage over hot mop....with the main reason being that it's a bit more forgiving if done wrong (up to a point)..

Hence, the next question....

What about EPDM?


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## 1985gt (Jan 8, 2011)

ddawg16 said:


> While waiting for more comments......I've been doing more Google searches.....interesting how the hits change just by changing the words just a little....
> 
> Anyway....it would seem that torch down has a slight advantage over hot mop....with the main reason being that it's a bit more forgiving if done wrong (up to a point)..
> 
> ...



Hot mopping has the advantage of most likely not burning the whole place down. I'm not sure where you got that info but IMO it is 100% wrong.


EPDM would be a great choice for this application and it would be what we would go with if we were bidding or installing. The devil is in the details.


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## Windows on Wash (Aug 30, 2011)

1985gt said:


> *Hot mopping has the advantage of most likely not burning the whole place down. I'm not sure where you got that info but IMO it is 100% wrong.*


*

:laughing:
* 


1985gt said:


> EPDM would be a great choice for this application and it would be what we would go with if we were bidding or installing. The devil is in the details.


I would look at membrane as well.


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## JWilliams (Sep 22, 2010)

Grumpy- In my neck of the woods you'd be surprised on who still hot mops. and no one here who torchs puts any plys down when they torch. its just base sheet then cap unless requested which in turn costs more.


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## 1985gt (Jan 8, 2011)

Around here very few mop any more, us and two other contractors actually, and even less torch, in fact I think only one company will torch anymore. We don't too much liability in it, and it's even more of a lost art then mopping.


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## RoofJack (Nov 7, 2012)

I would take a hot roof any day over a torch down. I have done thousands of squares of both!


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## ddawg16 (Aug 15, 2011)

Well.....I thought I would post an update......

My house did not burn down.......










You can see where they used 1x6 on the edge....then ran the torch down up the side....this weekend I'll nail z-flashing to the top of that 1x6










They ran the torch down almost 2' up under the existing shingles










I need to dig up a 'before' picture of thie area.....I think they did a good job of redoing the shingles....

Note...some of the shingles may not look good...they are actually ones that I pulled off to expose the roof...so I could do all the work. Their just a little dirty.....one good rain should clean them up....we used existing shingles on this section so as to match....my color is no longer available so the new section of roof will be new shingles....real close in look...I doubt anyone will notice the difference....


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## shazapple (Jun 30, 2011)

Looks like they did a pretty decent job. Application methods vary by region for sure. Around here we do over a hundred thousand square feet of hot mop and torch down each year.


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## ddawg16 (Aug 15, 2011)

Final pics....


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## JWilliams (Sep 22, 2010)

what is up with your ridge cap?


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## ddawg16 (Aug 15, 2011)

JWilliams850 said:


> what is up with your ridge cap?


What's wrong with them?


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## JWilliams (Sep 22, 2010)

why are they sitting up and not laying flat? must be some new way to vent. it looks like there is something underneath them. i dont know ive been roofing since i was 17 and ive never seen anything like that before.


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## Windows on Wash (Aug 30, 2011)

JWilliams850 said:


> why are they sitting up and not laying flat? must be some new way to vent. it looks like there is something underneath them. i dont know ive been roofing since i was 17 and ive never seen anything like that before.



Cobra vent.


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## JWilliams (Sep 22, 2010)

ive used cobra vent and the ridge caps never sat up like that. oh well


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## Windows on Wash (Aug 30, 2011)

Caps almost look metal in those pictures with that hard crease in them.


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## OldNBroken (Jun 11, 2008)

JWilliams850 said:


> what is up with your ridge cap?


What do you mean? It's arch hip and ridge. Looks fine to me.
You've never used Z-ridge or Decoridge or any folded ridgecap?


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## ddawg16 (Aug 15, 2011)

OldNBroken said:


> What do you mean? It's arch hip and ridge. Looks fine to me.
> You've never used Z-ridge or Decoridge or any folded ridgecap?


Yep....folded ridge cap.....

I've looked it over real well....I can't find anything wrong.....I think he did a great job.......

Best $3500 I've spent in quite a while......it sure is nice haveing a roof......

So if you will excuse me....I have windows to install....


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## OldNBroken (Jun 11, 2008)

What brand of roofing is it ddawg?

From the details in the pic, your roofer was very conscientious


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## JWilliams (Sep 22, 2010)

i do alot more metal than anything and down in my area im never seen hip or ridge like that. pardon my ignorance.


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## ddawg16 (Aug 15, 2011)

OldNBroken said:


> What brand of roofing is it ddawg?
> 
> From the details in the pic, your roofer was very conscientious


Certainteed


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## Grumpy (Dec 12, 2003)

I was thinking it was the CertainTeed Mountain Ridge. It's thicker than the traditional shadow ridge. A much more distinctive look... A hard sell though since most guys are just cutting up 3-tabs. 

I don't like his pipe boots, I hate those, but at least he caulked the top. Most guys in my area that use those, don't caulk the top and if not they WILL leak.


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## Grumpy (Dec 12, 2003)

OldNBroken said:


> You've never used Z-ridge or Decoridge or any folded ridgecap?


Nope. never even herd of them.


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## JWilliams (Sep 22, 2010)

All the people in my parts use three tabs and cut them for the hip and ridge


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