How do I add a slope to a flat concrete roof?

I am planning on re-doing my flat concrete roof before I get leaks—it is pooling. I would like to add a slope so pooling will not be an issue going forward. Total sq feet is under 1500. The length at it’s furthest is 40 feet by 23 feet—all flat.

How would you do it?


You can get insulation that is custom tapered for your roof, it’s expensive.

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What system is on it now? How much ponding?

Here’s pictures…I recently bought it. As far as I know this roof has been on it since at least 2005, just has been coated every other year. The dark marks are where it pools, it’s like Algae now. Without a pitch, it just evaporates over a couple days. There is several air bubbles, none with cracks yet.

Need to find out what the existing coating is to determine compatibility

Thanks…do you have a way of me looking at it and knowing?

A roof coating contractor might know what it is, other than that you have to do test patches. Might get help from a coating mfg tech rep. That’s their gig 24/7. 800-766-3411 is united coatings / GAF and they sell water and solvent based systems

Hard to tell how many SF you have, but floor leveler will fill the ponds. Is the coating intact?

It’s peeling up in places, and bubbling…i would like to rip it off and re-do it…but i was wondering the feasibility of adding tapered insulation to create a slope…i don’t know what it would cost…but i reached out to a distributor of HunterPanels and will try to get pricing…not sure if i can do it on my own…

You’ll need to secure the filler and protect it from UV/moisture degradation. Get a quote for a polyurethane foam roof. Might be comparable due to the securement issues you’ll discover.

Thank you! Appreciate your advice

After considering my options…I think I will pressure was/clean the roof, and then apply Henry’s 887 Tropicool 100% silicone to it…does this sound reasonable?

Silicone is like herpes… Once you have it, you have it and cant get rid of it.


Recoating will be a waste of 100% of your money if you don’t identify the existing coating and prepare it properly. Also if moisture is present below the roof but above the deck, your coating will not stick. It will bubble and peel.

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If you are considering just adding another coating instead tearing it off, then I would recommend GacoFlex S2000. I know its not near as good as a tear off and install of new roof system. However, I have used it in certain situations, and it has worked great. Just another option to look at if new roof isnt in the budget.

That looks like either a spray foam roof system or an old smooth build up. The spray foam is usually coated with silicone. Unless they are recoated regularly they soak up water like a sponge. They are usually installed over old roofs and rarely installed directly over deck. Which means outside of a coating the building code will require a tear off to deck.

To add slope you use tapered insulation and have to lay it out correctly. The insulation is 4x4 pieces and you can get it in 1/8", 1/4", or 1/2" slope. It is either mechanically fastened to the roof deck or adhered. This insulation can be used with pretty much every flat roof system.

Your other options are some kind of coating but as others have said you are asking for trouble if you have water in the roof.

Adding slope is more expensive initially but you will get a longer lasting roof out of it. What you do depends on what you bought the property for and how long you intend to stay there. A tear off and new roof should get 20-25 years before you have to think about it again if done right. You could have problems our of a coating in 5 years especially since you are going over previously coated roofing with water and air trapped in it. Eventually you will have to tear it down to the deck, coatings don’t last forever. The longer you wait the more expensive it will be as inflation for construction is higher than national currently.