Is gravel stop needed for a torch down roof?
I am building a contemporary home with a flat roof that will use torch down membrane. It has a 2-inch tall curb and the roof is sloped inward to drains located in the center of the roof. We are using metal roofing in a vertical orientation around the top two feet of the building to act as decorative fascia for a cornice.
This is necessitating the fabrication of a custom piece of trim that would historically been called the gravel stop.
The question is, “What does this custom piece of trim have to look like?” And more specifically:
1. Is it necessary (or advisable) to have gravel stop for a torch down design?
2. Is it necessary (or advisable) to have a drip edge when the roof has a curb and the fascia is metal? When I say “drip edge,” I mean a half-inch kick-out at the bottom of the metal trim that directs the water away from the side of the house.
I’ll try to attach a drawing to make the questions more clear.
My confusion is that people still seem to be using metal trim that has gravel stop and a drip edge, even though torch-down roofs seem to have become the norm. I don’t want to delete it from the design of my home and then find out that it served a purpose of which I was not aware.
Oh, one last thing. The metal trim is Galvalume with a Kynar finish in black. I assume that the torch down membrane will stick to it and the heat will not ruin the black coating, but it is worth asking you if that is a potentical concern.
Thanks for taking the time to share any words of wisdom! I really appreciate the effort that each of you go to help out folks like me that are struggling with some basic questions.
There are basically 3 types of roof edges: Gravel Stop, Canted Edge, Parapet
1) Yes, some sort of metal edge flashing is required. A gravel stop is used all the time, although the two others I mentioned above are used a lot as well. It serves as protection for the membrane, looks a lot better, and allows an overlap with the wall siding for better water tightness.
2) Yes, the drip edge adds rigidity, as well as keeps water from dripping off the edge and down onto your siding (to reduce staining, wear, possible moisture intrusion...)
I prefer the Canted edge, which looks similar to this detail
because it does not allow rain to blow off the roof and cause many of the above mentioned problems. Also you don't have to worry about compatibility with the metal and torch down membrane as it is not directly linked. It also allows for a easy change in direction of the membrane, because a sharp change (like you showed in your second drawing) can be problematic, leading to cracking and difficulty with installation.
As long as the applicator uses the proper primer for the metal it should stick fine, however this is a weak point in the roof as it ages (another reason why I like the canted edge).
What I mean by "gravel stop."
Thank you very much for your reply, explanation, and the link. Very useful!
If I can ask for a little bit more clarification....
What I meant by "gravel stop" was the angled up area built into the sheet metal at the top. I know that the entire piece of metal is called "gravel stop," but I'm trying to understand the need for that little peak. Can it just be flat and rely on the curb to keep water from going over the edge?
I don't think that it makes much difference in terms of keeping rain from blowing off the roof or onto the siding. The curb itself is made of a 2x4 laid on its side. If that 1 1/2 inch tall curb doesn't stop the water from blowing off, then I would think that another 3/4" in the metal wouldn't stop it. Am I underestimating the importance of another 3/4 inch?
Of course there is 3 1/2 inches of exposure from the 2x4 where rain will land and either roll off the side of the house or back towards the rest of the roof, but I wouldn't think that would be of much consequence....unless I am missing something.
The drip edge adding rigidity is an interesting consideration. I hadn't thought of that. But in terms of keeping water from running down the side of the house, once again, it is only the water that would collect on the top of the 2x4 curb, I would think. If the water gets high enough to go over the curb, I would think that would mean that something has gone wrong with the drains.
Hope I am not trying to put too fine of a point on this. I appreciate your help!
Hmm, I suppose I never questioned the top bit. I'm going to have to ask around about that! Other than wind blown rain, I'd guess it is more for aesthetics. The end of the membrane isn't exactly pretty, and when you torch it down there will be a little bit of 'bleedout'. Ideally there would be a bead of caulking at the end of the membrane as well.
Keep in mind that your contractor may be getting a pre-manufactured gravel stop, so it might be cheaper going that route than custom bending a piece without the stop.
EDIT: I gave a call to our local roofing guru and he agrees that it is for rigidity and aesthetics. When you torch to it the metal is going to twist and warp without the 'stop' ('Oil canning' is what they call it. You can see it in big pieces of metal that are in the sun). The size of the stop is also somewhat dependant on the machine that is making it (ie most cannot make it any less than 1/2")
Thanks for your follow-up and going the extra mile by calling your local guru.
I'll give you the bottom line, first. I redesigned the trim to have the pointy part that forms the gravel stop AND the drip edge, too. In my old age I'm realizing that it sometimes makes sense to do it the way everyone else is doing it, simply because there might be several hidden reasons that no one can articulate right away. My motto is becoming, "Don't invent anything new." At least that is my motto on this project at this time!
The reason that I am having this trim custom fabricated is that I'm using a metal fascia at the top edge of the house and I want the trim to match. Also, there are several trim pieces that are designed to hold the fascia in place, so I might as well have them fabricate this one, too.
Regarding the oil canning and the peak detail and drip edge providing rigidity, I think that is a very interesting theory. One that is convincing enough that I gave up on believing that I have any clue as to what I am doing!
However, I was a mechanical engineer in a former life, and I must say that I am having trouble understanding how these features will provide rigidity and help reduce oil canning. Actually, I think that little 45 degree peak detail, makes the metal a little "flopier" -- at least along the axis of the bend. Hmmm, maybe that is why the oil canning will be less severe -- perhaps it acts as a stress relief.
The idea that the drip edge makes it more rigid -- yeah, I can see where that might be the case because it is along the edge of the metal as opposed to be close to the bend at the corner.
Anyway, I am spending far more time trying to understand the mechanics than I should. I'm just going to trust those of you with the voices of wisdom and experience.
Thanks again for your help. This discussion has gotten me unstuck!
gravel stop was meant to stop gravel from washing of your roof from the old tar and gravel roofs nothing else it is not meant for torched roofs
ever seen a flat drain on a flat roof for tar and gravel it has a lip around it
to stop gravel from washing down the drain same thing
you should use a 24 gauge metal for torch on are thicker other wise the heat from torch well mess up the metal
rooferguy, shazapple and you also have good points. Yours is that you are overthinking it. :thumbup:
Rooferguy gave you the original purpose of gravelstop and shazapple was right in that it is more asthetically pleasing from the ground than a flat drip edge.
Perfectly fine to use it.
BTW, oilcanning is rarely a problem on pieces w/ <6" face. This is why you see stiffening ribs in large metal fascias and such. It is not going to be an issue on your detail. Also agree with roofguy, minimum 24 ga metal.
The general term for any edge piece like this is drip edge. That hemmed bend at the bottom of the vertical you are calling drip edge is called a kick.
If you have any ponding water on your flat that rock stop will just add to the problem. Rock stop is a bad idea for torch down.
why not just make a coping to cover it all, imo it will have more longevity than that detail currently shows.
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