# Choosing flashing cement for flat roof



## Ed the Roofer (Jan 27, 2007)

Is there an actual attic space under the flat roof?

The type of roof vents made for flat roofs are shaped like withes hats or tapered cones.

What style are you using?

If the orifice is too close to the roof surface, you may take in water from snow build-up if you live in a wintery region.

Any photos of the roof and the proposed vents?

I never heard of an Intake Vent made for installation on the top of the flat roof. ???

Ed


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## stubits (Dec 30, 2008)

Ed-

Thanks so much for weighing in on this. All really great questions. 

Yes, there is an attic, perhaps 4.5' at its highest and working down to nothing, of course, but plenty of room for me to work. 

My house is a 75 year old brick row house. Most of the houses do not have vented attics, but since installing insulation, we really need it. It seems as though there are not many options for intake vents, so most people around here install some form of intake on the roof.

Also, we live in Washington, DC where we don't get much snow, maybe a couple of inches at a time, but the weather is warm enough that we don't get much build up. 

Here are some not great photos of my porch roof (mostly just small), which is the same material as my main roof. Is there something in particular you're looking for? I can take additional photos.



















I am using the Pop Vent by Active Ventilation Products Inc., for intake. More info on these can be found at http://www.roofvents.com/popventcomm.html. I am using two 14" vents with 6" collars.










And I am using the Aura Ventilator also made by Active Ventilation Products, Inc. More info on this product can be found at http://roofvents.com/roof-vent.html. I also went with two 14" vents with 6" collars. 










Hope all of this helps to give you an idea of what we're dealing with. So, am I on the right track, both with the vents and with the installation method? If not, what do you recommend? And, what do you recommend for roofing cement? Were any of the options I mentioned in my first post any good?

Thanks!


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## Ed the Roofer (Jan 27, 2007)

I am thinking that those vents would work alright for the Intake, but would you consider framing out a curb to lift up any of the vents about 12" off of the roof surface to alleviate any snow build up weather infiltration issues?

Also, at the same time, how about a Solar Powered Attic Fan for the exhaust?

I think that would help out tremendously, especially if you add the right amount of NFVA for the Intakes.

You are on the right track for patching in the metal flanges, but if you do as I suggest and build curbs, you would be better off having a roofer come in for a couple of hours and use a Torch Down APP Modified Bitumen Roof Membrane, along with the required Fiberglass Base sheet as a buffer and also to install Cant Strips along the bottom perimeter of the curbs.

Ed


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## stubits (Dec 30, 2008)

Ed-

Thanks for a really well thought out response. Very much appreciated. 

I spent a lot of time considering the solar powered version of the vents I ended up purchasing. In the end, I decided to oversize passive vents, which purportedly work better than turbine vents. The good news is that I can always order the solar head and switch it out. Guess I should add I've already ordered and received the equipment I indicated.

As for the curb itself, I did a lot of research into this. I called a number of local roofers, all of whom said that they'd install it straight to the roof and without a curb. I ordered the vents with 6" collars, although I could have increased that to 12", but the manufacturer indicated based on my location it wasn't necessary. Here is DC, we get on average 15" of snow over the entire winter, perhaps 2-4" in any given storm and it melts pretty quickly. I am not totally opposed to the curb, but folks around here seem to think it is unnecessary. Any thoughts beyond what you've already said?

If I decide to proceed without the curb, am I on the right track? Cement down the flange, screw it in, more cement and asphalt saturated glass fabric to seal over the edges? Any thoughts on the different cement options?


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## Ed the Roofer (Jan 27, 2007)

Read through these 6 older threads, starting from the oldest to the newest for more detailed explanations on how to use roofing cement and reinforcement fabric to properly seal in a flange.

http://www.diychatroom.com/search.php?searchid=940627

the Chicago roofing term that I learned was called Dope and Minnie, but that varies geographically as you will see from reading through these posts.

Other than that, you are probably okay with your minimal amount of snow with what you want to do.

Ed


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## stubits (Dec 30, 2008)

Ed-

This is excellent. I am about half way through the material you provided, a wonderful tutorial. 

Elsewhere you've suggested that this method should last 2-5 years. Is this accurate for my application? 

Given that I am not certain what my current roof material is (any guesses based on the photos?), what type of cement do you recommend? Unfortunately I cannot contact the installer or the manufacturer because all that took place before we bought. Will any of the three cements I mentioned in my first post work? Which one is the best? Something else entirely?

Thanks!


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## Ed the Roofer (Jan 27, 2007)

The first choice is probably the closest to what a standard plastic roofing cement would be to utilize for this.

At the several year point, the suns UV will start to harden and possibly crack the surface coating of the cement troweled over the reinforcement fabric.

The surface coating is just there to cover the fabric with a very light skin coat. Too much and too thick and it will dry up and crack prematurely.

The maintenance several years down the road would be to just reapply a new surface coat and make sure the exterior perimeter edges are troweled down like a smooth transition to the flat roof membrane material, without a lip left on the edges.



The material could be a modified bitumen with white granules for the cap sheet or a cheap 90# roll roofing material.

Unless I probed one of the seams, I can not tell for sure what type of material it is.

Ed


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## stubits (Dec 30, 2008)

Excellent. Thanks so much! I already bought that first cement, so it is nice to know I don't need to return it.

Would it help at all to apply an aluminum or elastomeric roof paint to the area? Would that extend the life expectancy a bit?

Thanks!


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## Ed the Roofer (Jan 27, 2007)

stubits said:


> Excellent. Thanks so much! I already bought that first cement, so it is nice to know I don't need to return it.
> 
> *Would it help at all to apply an aluminum or elastomeric roof paint* to the area? Would that extend the life expectancy a bit?
> 
> Thanks!


 
Not necessary, but if you do, wait 30-90 days for the solvents to leech out of and gass off from the material.

Ed


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## stubits (Dec 30, 2008)

Ok, good to know. I read it as a suggestion on another forum. Would you mind a few final questions?

First, you think that cement will work well regardless of the type of roof I have, right?

How many screws should I use in each vent? The more the better, or are they there mostly to prevent the wind from yanking it off the roof?

Somewhere else I read a suggestion that you should treat the application like contact cement. That is to say, apply the roofing cement to the roof AND to the bottom of the metal flange and then put them together. Similarly, apply the roofing cement to the top of the flange AND to the minnie (as you call it) and then stick them together. Is this necessary? Helpful? Crazy?

While I am up on the roof I'd like to fix two other problems. First, the lack of an exterior vent for my bathroom fan. It just isn't possible to go through the wall because of the distance. Would a roof vent like this work well? I assume the installation procedure would be about the same. http://www.iaqsource.com/product.php?p=american-aldes_22-041&product=174479

Second, is an issue you already helped me with here http://www.diychatroom.com/f9/flat-roof-leak-47560/. I a relatively small leak related to what must be a break in the flashing around the house's main stack (3" cast iron). I am pretty sure, based on your guidance that there is currently a lead boot. I like your suggestion to replace it with a new aluminum/rubber boot. First would something like this work? http://www.homedepot.com/h_d1/N-5yc...splay?langId=-1&storeId=10051&catalogId=10053 It seems like it is intended for a sloped roof. Will I have trouble? 









Also, do I need to do anything to the old cast iron vent to make this work right, or can I just slide it over? What is the best way to go about removing the old boot, flashing and cement from the roof? I assume I'd like the new flange to be adhered to a relatively clean surface, no?

Really, many, many thanks for all your help with this. Although the summer is almost over, we're really looking forward to reducing the heat up in the attic and in the long term, treating our roof and attic better than it has been treated the past 75 years.

Thanks!


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## Ed the Roofer (Jan 27, 2007)

stubits said:


> 1. First, you think that cement will work well regardless of the type of roof I have, right?
> 
> How many screws should I use in each vent? The more the better, or are they there mostly to prevent the wind from yanking it off the roof?
> 
> ...


1. If you do the roofing cement properly to bond it to the roof, none would be required, but to ensure it is tight, put one at each corner, but do not torque down so much that you squeeze all of the roofing cement out of the sides. If the sides of the flanges are very long or wide, add another fastener to the center if you want.

2. Just apply the roofing cement to ones surface or the other liberally for the portion that is going to get attached to the roof.

3. Yes, it looks like it would be perfect.

4. Don't remember the older thread, but on a Flat Roof, a Lead Boot would be the best option.

5. Cut the base circumferance with a stanley box cutter knife with a straight blade, going easily around the diameter, just scoring the old lead boot at first and a little more of a score the 2nd or 3rd time around. After it has a good line cut in, use the tip of a chisel or flat head screw driver to break through the scored mark and then just continue prying the top lead from the bottom lead base portion just like using a can opener. It will rip cleanly at the scored mark.

The top portion of the lead boot should be trimmed so that only about 2" sticks above the cast iron pipe with the same scoring method just described. Then, after the excess is torn off and removed, take the wooden handle of your hammer and gently tap the 2" lead above the top of the cast iron pipe into the inner circumference slowly, going around and around so that it folds into the interior. Repeat until it is tucked all the way into the pipe.

Ed


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## stubits (Dec 30, 2008)

Ed- Wow, thanks! Everything makes perfect sense. 

Ok, so I shouldn't use the plumbing vent flashing I posted. That would have been too easy, right? Any reason why it isn't right for a flat roof? Where can I get a lead boot from? Never seen them at HD, so I am guessing a roof supply place? Any idea as to cost? 

Alternatively, if the leak in the current lead boot isn't too bad, can I just repair it? Or does it need to be replaced?


How clean does the roof need to be for the cement to work? The top layer of my roof has granules, will they cause trouble?

Thanks!


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## Ed the Roofer (Jan 27, 2007)

Plumbing supply houses and roofing supply houses, such as ABC Supply.

Ed


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## stubits (Dec 30, 2008)

Thanks.

If the leak in the current lead boot isn't too bad, can I just repair it? Or does it need to be replaced?


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## Ed the Roofer (Jan 27, 2007)

stubits said:


> Ed- Wow, thanks! Everything makes perfect sense.
> 
> 1. Ok, so I shouldn't use the plumbing vent flashing I posted. That would have been too easy, right? Any reason why it isn't right for a flat roof? Where can I get a lead boot from? Never seen them at HD, so I am guessing a roof supply place? Any idea as to cost?
> 
> ...


 
I responded too quickly before I read the remainder of your questions.

1. Less than $30.00

2. Yes, it can be repaired with the roofing cement and fabric and roofing cement method previously described, but it would still only be a band-aide. Spend the $30.00 and replace it.

3. Scrape the granules if they are still very prominent in the surfacing or soften it "Carefully" with a small butane/propane hand held blaser or torch kit and just get it soft enough to scrape the top surface granules away.

Probably not necessary, but sometimes it needs to be done.

Ed


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## stubits (Dec 30, 2008)

So, the only real remaining question is what is the likelihood that I screw this up and end up with water pouring into my attic during the next storm?

Seriously though, how long before I can test my work with a garden hose?


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## Ed the Roofer (Jan 27, 2007)

It will be good right away upon completion.

But, just in case, let the roofing cement cure for 48-72 hours if you plan on doing a water test.

It will work though if you follow everything we discussed and it seems as if you have a good handle on it so far.

Ed


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## stubits (Dec 30, 2008)

Ed-

Hope you're out there! Sorry to revive this old post. I still haven't had a chance to get up on the roof, but will do so over the next couple of weeks. Work has just been too busy. The upside is that I've had some time to think it through more completely. The downside, however, is that I've come up with a couple of additional questions.

1) Do I need to be concerned about the outside temperature when I am doing this? Is there a minimum temperature?

2) Do I adhere the lead boot to the roof just as I am installing the various vents? That is to say, the same technique?

3) How do I keep the trowel clean as I am working with it? I will be installing 4 vents, 1 bathroom exhaust and 1 lead boot. I suppose it is going to take me all day, if not longer (how long do you think for a total newbie?) and I don't want the cement to dry on the trowel, especially during longer gaps, like when I am cutting the next set of holes, etc. How do the pros do it?

Thanks so much!


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## federer (Aug 20, 2010)

studbits i am in the dc area as well-how did your project go?


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