How much buildup is generally tolerable on a flat roof?


Hi all, we are thinking about putting a layer of silicone coating (GACO) on the existing roof. The existing material is pretty built up though. There is probably over an inch of tar. Under all of that, there is metal. No insulation.

The house is 1925 construction and very solid (I hope), with serious hardwood beams.




Is the roof currently leaking?


Hi MPA, It’s not leaking as far as I can tell. It has spray foam on the rafters, so it’s very difficult to know.



If it’s a good solid roof, not leaking bad, Not blistered, and nice and tight when you walk on It, silicone can be good solution to extend the life. Looks like you have 2+ built ups over the original tin.

I would have your local supplier get you in touch with the gaco rep, give them a call/email some pictures. Silicone reps at least in my area seem to be very avalible and willing to help.

Because it is exposed asphalt they are most likely going to want you to prime it first, then hit any cracks, flashing junctures, and where you cut that hole with trowel grade mastic and fabric first, followed by rolling on two coats of silicone.

Done right it is a great product just have to follow the right steps for your particular roof, if not you are just wasting money, and it’s not cheap. A little research and a few calls helps, not something you just pick up at depot and throw on in an hour and magically fixes everything.


Thanks MPA! That’s super helpful. I think I was treating it a little bit carelessly, so it’s good to get a check. As long as the weight is OK, I can put a primer layer. What happens if done improperly? Does it peel up?


Primer from what I have been told by a few different silicone manafactures is mainly to seal the asphalt and keep it from leaching into the silicone, opposed to a bonding primer. Alot of it is common sence roof needs to be clean and dry ECT. Just did a job in a previously coated roof with a different brand and for warranty they sold us cleaner…i was actually amazed how much dirt came off by just spraying it on.

Just do some research before you slap it on. It’s not that it will peel up you want to follow the right steps for your roof, so you get the best, longest lasting job for your $$$. I assume it is your personal roof you are looking to use it on?


Yes, it is my personal roof. Thank you so much for all of your advice and help! I was sort of under the impression that silicone is an impermeable and inert substance that can resist the fires of hell, but when you think of it as being mixed with oil and dirt on the roller, that all makes perfect sense.

Oh, the old roof is 1 1/2 inches of built up tar, so I’m guessing it is about 5 previous roofs, one on top of the other. I guess I don’t have to worry about leaking under those conditions. :slight_smile:


I would still talk with gaco first for there recommendations on product and coverage they recommend for build-up. We did alot of research for one big complicated job that requires warranty and for some reason alot of the manafactures don’t put all there spec out on there sites, when you call them up there were all more than willing to provide info and recommendation.


You better check the codes in your region about putting another roof or coating over so many layers/thickness of roofing material.


Well, the silicon is light. We are probably long over code. I will check with an architect.


What type of roofing tar is on top of that rock?
Is it well embeded into that rock?
That rock might need to be spuded/ removed.


Hi Roof_Lover. Sorry, I’m not familiar with roofing terms. What is rock? Do you mean the tar or the tin underneath?


You have a “built up” roof
Or a Rock roof
A tar and gravel roof
With a layer of some type of tar on top.
It could be emulsion poured on top.

Also, it is very strange for a gravel roof to be on top of a metal roof.
I cant imagine running a hot mop and fiberglass underlayment over a metal roof.
I would look in another spot.
I am thinking that you just happened to open that spot over a metal patch.

I really think you need this all removed.
I dont think GACO will work with this.


In the city of Philadelphia we commonly find tin under multiple layors of hot and modified. Many of the row houses originally had tin and then just kept building, sometimes 5-6 inches.


It is tin in multiple spots under hot and modified, no gravel anywhere. What you are seeing in the picture looks like gravel, but it is just broken up chunks of bitumen. Thanks for all of your help roof_lover and MPA!


I once seen a homeowner associates sued a roofing contractor because they went over a built up roof that was 2" thick stating the roofing contractor went over 3 layers. When I spoke to the contractor they stated they offered the home owners aassociate to tear off the roof but preferred to just have a go over.