Oshawa Commercial & Industrial Flat Roofing
Flat Roofing Oshawa: Commercial Flat Roofing and What Happens When the Rain Can't Leave
✓ Stevenson · Thornton Corners · Farewell · Northwood · ✓ Drainage and overflow assessment · ✓ Large industrial floor plates · ✓ Commercial & industrial only since 1977
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4.8 Star
Rating
We run water down every drain, find your overflows or confirm you haven't got any, and give you the answer in writing.
Since 1977 · 50 years of flat roofs | Rated ★ 4.8 by Google reviewers | Licensed & fully insured, WSIB covered | Free spring/fall inspection
A Manufacturing City That Kept Its Floor Plates
Oshawa carries roughly 1,100 gross hectares of designated employment land, organised by the City into four employment areas: South, taking in the Stevenson employment lands and Thornton Corners; Southeast, with the Farewell and harbour area and Colonel Sam Business Park; North, which includes Northwood Business Park and the Columbus lands; and Northeast.
The city ran about 87,000 jobs in 2023, and the City's own growth work projects roughly 38,600 more by 2056, taking it toward 113,000 — with something like 44 per cent of that growth expected to land on employment lands rather than in offices downtown.
General Motors is still the anchor. The Oshawa Assembly plant opened in 1953, covers 5.1 million square feet, and GM has put around $1.5 billion into it since 2020, including a further investment announced in early 2026. It is the maker's only North American plant building both light- and heavy-duty Silverado pickups on one line.
Around it, the economy has broadened — four post-secondary campuses, Lakeridge Health, Ontario Power Generation, food and logistics operations.
What has not changed is the building type. Big single-storey floor plates with a lot of roof, and all of that rain has to get off somewhere.
Who's running the hose
50 yrs · Commercial flat roofing, nothing else, since 1977
4.8 · Our Google rating
Our crews · Crown roofers only, nobody hired out
Aug 2024 · Our Oshawa job, posted in the gallery
A drain test is only worth something if the person doing it knows what a choked sump looks like from above. Ours are the same roofers who'd do the repair. Read what other owners have written below.
Your Drains Were Sized for a Storm. Your Overflow Might Not Exist
Every low-slope roof is a shallow tank with holes in it, and the holes are engineered. Drain capacity is calculated against a design rainfall intensity for your location — a fifteen-minute rate, taken from the province's climatic data — applied across the area plus a share of any adjoining wall that drains onto it. That is the sum, and drainage design on a large commercial roof is arithmetic before it is craftsmanship. It also assumes the drains are clear.
They frequently are not, and there is a reason it is not bad luck. Rainfall runs to the low point. Debris is carried along with it, so it also ends up at the low point. The primary drains are at the low point, because that is where drains go. So the system is arranged such that everything washing across it arrives precisely at the opening it can block.
On its own, a blocked drain is an inconvenience. What makes it serious is what is meant to happen next.
Ontario's Building Code requires a secondary route off the roof in defined circumstances: where a parapet is more than 150 millimetres high, or higher than the adjacent wall flashing, emergency overflows or scuppers have to be provided, along with a minimum of two roof drains. Overflow scuppers are sized generously — designed to carry at least double the fifteen-minute rainfall intensity — because they are the failsafe, not the plan.
The problem is that on older buildings this is often missing, blocked, or set at the wrong height. And a parapet roof with no working overflow is not a building with a drainage inconvenience. It is a container.
That is where it stops being a leak and starts being a structural matter. The Code treats accumulated rain as a load, and it applies whether or not it has drainage, because the question is what the shape and deflection of the deck allow to collect. National Research Council work described the mechanism decades ago in terms nobody has improved on: water in heavy rain caused progressive deflection and accumulation until the loading collapsed the roofs.
It is a feedback loop, and that is what makes it dangerous. Rain collects in a low spot. The weight deflects the deck slightly. The deflection deepens the low spot, which holds more of it, which deflects it further. On a long-span warehouse roof with a limber deck, that loop can run faster than anyone gets up there with a drain snake.
None of which requires a dramatic storm. It requires a blocked drain, a parapet, and no working way for the rain to leave. On a building measured in acres, what the deck underneath is rated for stops being an abstract number very quickly.
Four Questions, and Most Roofs Fail at Least One
A drainage assessment on a large roof here comes down to four questions, and we would rather answer them in September than in April.
Are the primary drains clear and actually draining? Not clear at the grate — draining. A drain can look perfectly open and still be restricted below the sump by years of compacted debris, and the only way to know is to run a hose into it and watch.
Is there a secondary route off this building, and would it work? This is the one that turns up missing. We look for overflow drains or scuppers, check they are set at the right height relative to the primary drains, and confirm they are not blocked, painted over, or flashed shut by someone who thought they were a hole in the wall.
Where is the run-off actually going? Roofs deflect over time and the low point migrates. Drains placed correctly in 1974 can end up somewhere the run-off no longer reaches. Scanning the wet areas after dark tells us where rainwater has been sitting long enough to get into the assembly, which is usually a better map of the real drainage pattern than the drawings are.
And what has the ponding already done? Wet insulation, a deteriorating sheet, corroded deck under a chronic pond. That is where a drainage problem becomes a roofing bill.
The fix for most of this is cheap, which is the frustrating part. Clearing and checking the drains twice a year would prevent a meaningful share of the emergency calls we take — and it takes an afternoon.
The Crown Rain-Exit Assessment
The four questions above are what we answer on the roof. Here's the whole visit, start to finish.
1. Put water down every drain
Each primary drain gets a hose test, because an open grate can still sit over a choked sump.
2. Find the second way off
Overflow drains or scuppers located, their height checked against the primaries, and confirmed clear rather than painted over or flashed shut.
3. Scan and core where rain has been sitting
An infrared pass after dark, with cores where it points, so we know whether ponding has already reached the insulation.
4. Write it up, drainage first
A written report answering all four questions, then a fix in the right order: drains and overflows before membrane, repair before replacement where repair does the job, and an engineer called in before we price if the deck worries us.
Drainage First, Membrane Second
When a building with a drainage problem gets re-covered, the sequence that matters is the one most quotes get backwards.
Drainage is designed first. That means tapered insulation to build positive fall toward the drains that exist, or relocating and adding drains where the fall cannot reasonably be built, and it means sorting out the secondary route — installing overflow scuppers or drains where they are absent, and setting them at the correct height so they engage before the load does. A new membrane over an unchanged drainage arrangement is the same problem with a new cover over it, and it will fail the same way on the same schedule.
Then the system. On these large floor plates, PVC single-ply is usually the sensible answer, with material warranties up to twenty years. Where the deck is concrete and the building takes heavy service traffic, SBS modified bitumen's two welded plies are the better call, and we install those. Hot-asphalt built-up still suits some of the older plants here, and carries a fifteen-year workmanship guarantee from us. TPO and EPDM are membranes we maintain rather than membranes we lay.
Tapered insulation and a proper tear-off go together, because you cannot build fall into a roof you are recovering — and on a building where rainwater has been sitting, recovering also means sealing wet insulation and a possibly corroded deck permanently out of sight.
There is one thing we will push back on, and it is worth saying plainly. If a building has chronic ponding and a deck we cannot satisfy ourselves about, we will ask for a structural engineer before we quote rather than after. That occasionally costs us the job to somebody who will simply re-cover it. We would still rather ask.
Commercial Flat Roofing Services in Oshawa, and What We Don't Do
Crown is a commercial flat roofing business and nothing else. The work is survey and infrared scanning, repair, planned maintenance, tear-off and full replacement, and new build alongside a general builder. Warehouses, plants, schools and clinics across Durham.
A word on what falls outside that, because it saves everybody a phone call. Houses are not our trade — no home roofing, no shingles, no siding, no eavestrough — and if that is what your building needs, say so early and we will point you at somebody who does it properly. Coatings sold as a way of avoiding a drainage fix are not on the menu either.
Every Crown project is staffed by Crown roofers. Nothing is hired out, which on a plant with a sign-in book and restricted areas is usually the first question a facilities manager asks, and it means whoever inspects the assembly in October is whoever worked on it in June.
Narrow has its costs. Where a building needs four trades, we are one of four numbers in the file. What it buys you is half a century spent on properties of exactly this kind, and a firm with no reason to argue for a bigger job than the one actually in front of it.
Contact us and we will tell you which job that is before anybody writes a price.
Flat Roof Repair, TPO Roofing and What We Won't Put On
Repair is the right answer far more often than a replacement, and on a building here it very frequently turns out to be drainage work wearing a different name.
Where the job is genuinely a repair — a split lap, a curb somebody reflashed badly, a pitch pocket that has shrunk away from the pipe — the method does not vary: open it back to sound insulation, rebuild the detail the way it should have been built, then photograph the result so the file shows what was done. Where the same bay has been patched three winters running, we will say that a roof replacement is the honest answer rather than book a fourth visit.
On what we will and will not put down: TPO roofing and rubber sheets get looked after on buildings that already carry them, but neither goes on a Crown job as a new assembly. Expert flat roof installation on this stock means PVC or SBS, laid by people who do it every week rather than twice a year.
Two flat roofs of the same age on the same street can need completely different things. The only way to know which one you own is to have somebody go up and look.
Before you call: "it's probably just a drain"
Is a blocked drain worth a visit?
Sometimes it's leaves on a grate and you'll have cleared it before we'd even get there. What we'd want to know is whether your overflow would have coped if it hadn't cleared. That's the difference between housekeeping and a load problem.
Won't you just try to sell us a roof?
You won't be billed for the spring or fall visit, and the written report comes with it. If a drain fix or a repair does the job, that's what gets quoted, line by line, and if the deck needs an engineer we'll tell you that ahead of any quote.
Our Oshawa Roofing Job, and Half a Century of This
Crown has done flat roofing and nothing else since 1977. Brian Sharp founded it; Joe Ferrier is President; the crews are ours. Fifty years on buildings like the ones in the Stevenson and Farewell lands.
Our Oshawa job closed in August 2024 and is in the gallery with the rest of our completed work, sorted by city and month, photographed up there by the crew.
Here is the practical suggestion for anyone managing a large building here. Before winter, get somebody up to run a hose into each drain and watch what happens, and to find your overflows. That is a short visit and it does not have to be us.
But if nobody can tell you where the overflow scuppers on your building are — or whether there are any — that is the thing to resolve first, ahead of any conversation about membranes, and we are happy to come and answer it.
Reviews From Oshawa and Durham Customers
Our business in Aurora had several leaks in the roof to our shop. Crown Roofing was amazing to deal with. Their worker Arvin was such a great guy and was really thorough in explaining where all the leaks were. He took photos to show us where the leaks were coming from. They also found a forth leak and fixed it in good faith without billing us.
Amanda Helps · Google review
I waited for all seasons before giving this review as I wanted to give a transparent and accurate review. I received multiple quotes and decided to use them to replace the roof on our building.
Susan Mahabir · Google review
Rated 4.8 on Google.
Overflows, Permits, Blocked Drains and Ponding
How do I know whether our building has overflow drainage?
Go up and look, or have someone look for you — it is not something to assume. You are looking for either overflow drains set alongside the primary ones at a slightly higher level, or scuppers through the parapet wall. Two things commonly go wrong. The first is that there simply aren't any, which is not unusual on older buildings. The second is subtler: they exist but have been blocked, painted over, or sealed during a past re-roof by someone who did not know what they were for. Ontario's Building Code requires a secondary route in defined circumstances — broadly, where you have a parapet above a certain height — so on most parapet buildings there should be something. If nobody can tell you where yours are, that is worth resolving before winter.
Do we need a permit to re-cover a commercial building in Oshawa?
It depends on scope, and the published exemptions do not clearly cover commercial low-slope work. The City's list of work not requiring a permit refers to replacing roof shingles and to non-structural roof repair — steep-slope, house-scale language that does not name low-slope roofing. Separately, the City lists new roof sheathing as requiring a permit, and making alterations generally. On a drainage job that matters, because adding tapered insulation, cutting in new drains or sumps, or installing scuppers through a parapet can all constitute alterations. We check your actual scope against the City before quoting and file through the City's application portal where a permit is needed.
Our roof ponds after every rain. How urgent is that?
More urgent than it feels, because ponding is self-reinforcing. Ponding deflects the deck slightly, the deflection deepens the low spot, the deeper spot holds more of it, and so on — and on a long-span warehouse deck that loop does not necessarily stop on its own. Beyond the structural side, rainwater sitting on a sheet ages it faster, collects debris that migrates toward the drains, and hides whatever is happening underneath. The first question is why it ponds: a blocked or badly placed drain, insufficient fall built in originally, or deflection that has developed over time. Those have very different fixes and different costs. What they share is that a new cover laid over the same arrangement will pond in the same place.
A drain has blocked and it's pouring. What do we do right now?
If it is safe to get on the roof, clearing the debris off the grate is worth doing immediately — a great many blockages are surface debris and clear in seconds. What we would ask you not to do is send someone up alone in heavy weather, or wade into standing rainwater on a deck whose depth and structural condition nobody has assessed. If it is deep, if the ceiling below is bowing or dripping across a wide area, or if you cannot get to the drain safely, clear the space underneath it and reach us during the storm — the line is answered around the clock. Afterwards, the useful follow-up is finding out why it blocked and whether the overflow would have coped, because a blockage that happened once will happen again.
Sort the Roofing Drainage Before It Sorts Itself
If you manage a warehouse or plant in Oshawa and nobody has run a hose into the drains or located the overflows in recent memory, that is the cheapest thing on your list and the one with the largest downside if it is wrong.
Call (416) 744-7788 or email info@crownroofing.ca. We will test the drains, find the secondary provision or confirm there isn't one, scan for moisture that has already got into the assembly, and put it in writing.
Crown Industrial Roofing · 227 Queens Plate Dr Unit #3, Etobicoke, ON M9W 6Z7
What you can check before you sign
✓ Free spring and fall inspection, written report included.
✓ Licensed, comprehensive general liability insurance, WSIB coverage.
✓ Safety-certified crews, with certificates sent whenever you ask.
✓ Manufacturer-backed warranties on materials, plus a workmanship warranty of up to 15 years that we carry ourselves.