TPO & EPDM Flat Roofing in Stanislaus County

TPO or EPDM: which one, and why

Both are single-ply synthetic membranes and both will outlast a built-up tar-and-gravel roof. The choice comes down to how the building is used and how much sun the roof takes.

In the Central Valley we specify TPO more often than EPDM, for one reason: TPO is white and reflective. Summer here runs past 100 °F for weeks at a stretch, and a reflective membrane measurably lowers the cooling load on a building with rooftop HVAC. On a large warehouse that difference shows up on the power bill.

EPDM — black synthetic rubber — is the better call when the roof takes heavy foot traffic, carries a lot of penetrations, or ponds water that cannot be re-sloped away. It is exceptionally puncture-tolerant and its seams age well. On a roof with a dozen HVAC curbs and a technician walking it monthly, EPDM is usually the more durable answer.

Membrane thickness is where low bids hide

We install 60-mil as standard on both systems. Forty-five mil is thinner, cheaper, and is what a lot of low bids quietly assume without saying so. If you are comparing quotes, the mil thickness is the first line to check — a 45-mil roof and a 60-mil roof are not the same product, and the price gap between two bids often is that difference.

What a complete system actually includes

A flat roof is an assembly, not a sheet. From the deck up:

• Structural deck — steel B-deck or plywood, sounded for corrosion and fastener pull-out before anything covers it
• Polyiso insulation — rigid board, mechanically fastened, tapered where the roof has to be taught to drain
• Cover board — gypsum over the iso, for fire rating, hail resistance and something firm to walk on
• Attachment — fully adhered, mechanically attached or ballasted, depending on deck type and wind exposure
• Membrane — 60-mil TPO or EPDM, laps cleaned, primed and heat-welded or taped
• Flashing and terminations — perimeter bar, corners, curbs, drains and every penetration

Skip the cover board and the first HVAC technician across the roof dents the insulation; the membrane follows the dent, water collects in it, and you have a soft spot that becomes a leak two summers later.

Drainage is the whole game

A flat roof is not flat. Code calls for a minimum quarter-inch-per-foot slope, and on a Valley building the practical consequence of getting that wrong is ponding — standing water that never dries, accelerates membrane aging and multiplies dead load. If your existing roof holds water more than 48 hours after rain, tapered insulation to correct the slope belongs in the scope, not a thicker membrane over the same problem.

Recover or tear off?

California allows a recover over one existing layer if the deck and insulation below are dry and sound. A recover is meaningfully cheaper and faster. But if there is trapped moisture in the insulation, covering it seals the problem in and you pay twice. We core-cut and check before recommending it — a moisture survey costs very little compared with tearing a new roof back off.

What it costs

Commercial single-ply in Stanislaus County generally runs $9.50 to $14.00 per square foot installed, depending on insulation thickness, how much tapered work the drainage needs, deck repair, and the number of penetrations to flash. A straightforward recover on a sound deck lands at the low end; a full tear-off with new tapered iso and extensive curb work lands at the top.

Manufacturer-backed warranties of 15 to 20 years are available on both systems when the assembly is installed to spec by a certified contractor. That warranty covers material; our workmanship warranty is separate and covers the install.