Flat Roof Drainage and Leaks: It May Be Plumbing and NOT the Roof

General Roofing Systems Canada provides Flat Roof Repairs throughout Canada. Repairs are completed in the following cities: Calgary, Red Deer, Edmonton, Fort McMurray, Lloydminster, Saskatoon, Regina, Medicine Hat, Lethbridge, Canmore, Banff, Cranbrook, Kelowna, Vancouver, Whistler, Winnipeg, Toronto and points between. British Columbia. Alberta. Saskatchewan. Manitoba. Ontario.

Below you will find a simple account of a service call inquiry. Oftentimes, there are multiple contracting parties involved in a roof repair. This includes plumbers, the roofers themselves, other construction services in the area, and obviously the stakeholders themselves.

In this case, a simple misunderstanding concerning the purpose of water testing culminated in repeated requests for service calls outside of the ongoing work we had been performing for the client. Finally, our client’s property manager provided us with the implements to deal with the problem. As suspected, it was the plumbing – not the roofing system itself – which was causing major leaks.

We have included a comprehensive explanation from our national project manager as well as a follow-up correspondence by a third-party plumbing contractor concerning this matter.

Water Source Needed to Test Drainage System

As you know, for about a year and a half, GRS has been requesting a water source at the building to be able to test your drains. On each occasion, we have been told that a water source is being to be made available. As well, our crews have been told that it is likely the water source has been covered up in construction, but that we may be able to use a neighbour’s water source until a proper solution could be found. Up until today, no water source has been offered.

The Reasons Drains Need to be Tested

a. The source of the leaks were previously at the drain system. We requested the drain assembly (the connection to a new drain and U-Flow, and the connection to the drain) be redone with a plumber – we would provide the roofing services and the plumber would provide the connection services. This was not done at that time. My understanding is that the plumber only cleared the drain system. So, we requested a water source so that we could test the drains/plumbing ourselves to eliminate them as the main issue, or conclude that they are the suspected cause of your leaks.

b. The plumbing system and assemblies are old. It is common for drainage during times of heavy rain (which is when your building is having issues) to back up and leak at the drain bowl assembly connection to the drain pipes. As the water rises and then meets the bowl assembly, if there is not a U-Flow expander and/or proper drain bowl, the water will then get between the pipe and the connection to the bowl. What occurs is the water that is backing up will then get into the roof assembly and begin to run along various areas and manifest themselves through the ceiling. At the time of your roof replacement, there was no work order for the plumbing and drainage to be modernized, and normally this is not an issue. This this does become one, it is typically solved with a quick water test and simple remediation work (such as bowl replacement, U-Flow backup expander installations, and/or repairs to the drain pipes).

Discussion with the Site Manager

Water testing only works when it is not raining and is intended to isolate the drainage system from the roof membrane that we replaced. This is the reason for the need of an external/outside water source. We test the drainage system with a hose, which allows us to isolate the drainage system on its own. If that test does not show that it is the drainage system, then we go on to retest the membrane. We also test various other penetrations, such as rooftop units and/or vents, pipe boots, etc. Finally, we move onto testing various other components of the building envelope, such as the connection of the roof to the wall and the flashing areas.

Your site manager also asked that an emergency be done (such as pumping water off the roof), to which I replied that we could, if requested, dispatch an emergency crew for that, but we would have to charge extra because we have requested on numerous occasions for the water test to be done to the drainage system, and that has not been done. The site manager did not seem to want the emergency service under those terms.

I should also note that on the other occasions that we did respond in this fashion, that we did not charge for the emergency service. We did this despite finding that it was a drainage problem by the plumbers at that time. Furthermore, I will also note that on each occasion we did recoat the roof and did not charge for that either.

The only reason we are now saying that we have no choice but to charge a service fee is because we have requested number times to have water access. Part of our servicing parameters included that should the water test not to be done, that we would charge a fee for the emergency service.

You also asked us about why water was pooling on the roof. I explained to the site manager that at the time of the roof replacement, an engineered sloped insulation package and rebuild of the building exterior (parapet wall construction, sheet metal, siding, etc.). We were only approved to go ahead with the roof membrane replacement only. He asked why that was so, and I said that it is an option and that not all (in fact, less than 50%) of the buildings we service have a sloped insulation package installed. I directed him to all the neighbouring buildings as examples of those that do not have such sloped systems.

Discussion with the Property Manager

I explained to the general property manager that we needed to test the various components of plumbing, membrane, units, vents, plumbing boots, and possibly the building envelope at the roof to wall connections. Finally, we have been promised an external water source to isolate the issue based on our tests.

Plumbing Contractor’s Follow-up with the Property Manager

August 21, 2015:

I was sent out to have a look at water leaking in the building. I found that the roof was full of water and it is what was pouring into the suites. The one drain was working, but very slowly. I called in the drain cleaners and went through it with them.

The snake was getting caught up on something, so I had them use the camera and we found there is something stuck in the piping. We traced it to the piping in the ceiling of a first floor room. We will need to return when the water is all gone and open the ceiling, remove the piping, and then get the cause of the blocking out. As it stands, the system is draining as much as it can with whatever is causing the blockage.

Also, the building has some piping up to the roof drains. These are not ran well and are back-graded for parts of the system. I have left it disconnected for the drain that has the problem.

General Roofing Systems Canada (GRS)
24 Hour Emergency Roof Repair: Call +1.877.497.3528 toll-free.


Calgary, Red Deer, Edmonton, Fort McMurray, Lloydminster, Saskatoon, Regina, Medicine Hat, Lethbridge, Canmore, Banff, Cranbrook, Kelowna, Vancouver, Whistler, Winnipeg, Toronto and points between. British Columbia. Alberta. Saskatchewan. Manitoba. Ontario.

Rooftop Patios, Decks, and Terraces

General Roofing Systems Canada provides roof repairs throughout Canada. Roof Repairs are completed in the following cities; Calgary, Red Deer, Edmonton, Fort McMurray, Lloydminster, Saskatoon, Regina, Medicine Hat, Lethbridge, Canmore, Banff, Cranbrook, Kelowna, Vancouver, Whistler, Winnipeg, Toronto and points between. British Columbia. Alberta. Saskatchewan. Manitoba. Ontario.

Flat Roof Patios – Roof-Top Patios, Decks, and Terraces Roof Top Patios, Decks, Terraces

The challenge with an outdoor patio or deck on a flat roof is that the waterproofing material or membrane used on the substrate/decking has to be done perfectly. The drainage for the low slope, and the decking material put on top has to be precise

(Some patios we service are on a flat roof itself or on a terrace balcony, while others could be on a garage.)

There are various types of rooftop decks and the different waterproofing membranes that can be used. There are flat roof membranes such as 2 ply SBS Modified Bitumen, 4 ply Built up Roofing, EPDM Rubber Roofing, PVC which is a

poly vinyl chloride, TPO which is thermal plastic. There are also products that can be applied to seal a substrate such as a concrete substrate with membranes like hot applied rubber asphalt systems and cold applied liquid rubbers (elastomeric coatings etc.)

The next issue that you would face on a flat roof would be the drainage issue. All repairs would need to be done with extreme precision in order to allow the proper drainage and slop.

After you have chosen the type of rooftop deck, the appropriate waterproofing membrane for on the substrate, and the drainage is correct. You then have the issue of what you place the membrane on itself. You will need to determine if any decking material on the membrane is required. We have seen customers use a variety of surfaces on rooftop patios, with everything from ceramic tile, paving stones, treated lumber, composite tiles, to Spanish tiles and granite.

Our recommendation to those looking at repairing, installing, or replacing a rooftop patio is to speak to a waterproofing expert first. The Home Depot or Lowes store decking material associate may be able to provide some suggestions on what tile to use, yet they may not understand the proper flat /low slope roof waterproofing and drainage practices. Roof top waterproofing for a deck system is different than normal flat and everyday roofing. With rooftop decks and patios the two worlds meet. The staff at General Roofing Systems Canada can assist you in designing a rooftop patio system that should last 20 or more years. In order to achieve these results, it is vital that owners do not try to take the least costly way out. When Owners begin to cut corners in an attempt to save costs that is where the errors occur and end up being more costly later on due to future repairs.

Below we have a report that came in to the GRS office from a roof repair technician. It shows a rooftop patio that is not very old and was installed incorrectly. Due to the improper installation, the owner is now having to pay for costs that could have been avoided. As a result of this the deck material, patio’s flat roof membrane and the sheathing/substrate will have to be removed and replaced. There is even the possibility of remediation for mold and trusses being replaced.

We hope that this article will help home owners to be more aware when purchasing decking materials and to contact a waterproofing specialist first. This tip will help you to enjoy your patio/deck for years to come without the need for repair.

Waterproofing: https://www.grscanadainc.com/Waterproofing.html

Flat Roof / Low Slope Membrane Information: https://www.grscanadainc.com/Flat_Roofing.html

Initial Client Inquiry:

I have a 300 sq. ft. rooftop deck that currently is leaking.  I was wondering if I can get someone to have a look and estimate what is required to resolve the problems we are having.

Assessment, July 27 2011:

The roof top deck is having leak problems. The rooftop deck is 95% tiled. Under the patio tile is a  deck-type waterproofing membrane and then plywood substrate. The worst area of the rooftop deck is in front of the sliding doors of master bedroom. It is not tiled. The roof top deck is covered in a paintable rubber deck membrane (low cost liquid coating). Where the tile and deck meet is a metal flashing that is letting water in. The wood decking is rotted. Not sure if you can put grout on PVC decking material, I suspect when the rooftop deck was originally tiled this is probably what caused the problems. The homeowners don’t want the ceramic tile on anymore. To fix this rooftop terrace/deck it needs a whole new rooftop deck replacement. The deck needs a tear right to the substrate and inspection of the substrate wood. This terrace is unable to be repaired and will required a full re-do


Flat Roof Deck
Damage and possible Mould due to Leaking patio
Flat Roof Houses
Closer view of the mould forming due to the water damage caused by the leaking patio
Flat Roof Mold
Here you can see that the underlay and membrane have completed rotted through
Residential Flat Roof Deck
The tear in the roof top membrane which resulted in the leak
Roof Terrace
Water damaged caused to the tilling


Roof Patio
More water damage on the tilling
Roof Top Terrace
Here you can see the tile separating and lifting from the membrane


Rooftop Deck
Tiles separating: View 2
Roof Top Patio Deck Terrace
Separation along the door frame and the roof patio deck
Flat Roof Terraces
Roof deck patio prior to PVC membrane installation
Rooftop Patio
Completed PVC membrane installation

we will follow this post up if we are the contractors hired to do the repair work on the leaking patio

Until next time,

The Team at GRS

General Roofing Systems Canada (GRS)
24 Hour Emergency Roof Repair: Call +1.877.497.3528 toll-free.


Calgary, Red Deer, Edmonton, Fort McMurray, Lloydminster, Saskatoon, Regina, Medicine Hat, Lethbridge, Canmore, Banff, Cranbrook, Kelowna, Vancouver, Whistler, Winnipeg, Toronto and points between. British Columbia. Alberta. Saskatchewan. Manitoba. Ontario.