Installing TPO Over Modified Bitumen on Brooklyn Flat Roofs
Installing TPO over modified bitumen is a job BST Roofers does in Brooklyn once a moisture scan and core cut show the old roof is dry and under the code's layer limit. Then the mod-bit stays and the TPO goes on top. Shall we test yours?
Four tests decide whether the old roof can stay
Covering wet or rotted roofing seals the damage in, so these four tests come first.
The points a full TPO roof inspection walks through go further.
- Moisture scan of the whole field for wet insulation
- Core cut to count the existing roofs against the building code limit
- Walking the deck for soft or springy spots
- Drains, parapet walls and edge metal, checked for drainage and flashing problems
What sits between the mod-bit and the TPO
Four layers, bottom first.
-
01
Existing modified bitumen
Left in place as the base.
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02
Separation layer or cover board
Usually DensDeck gypsum or high-density polyiso, because asphalt and TPO cannot touch.
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03
Polyiso or tapered insulation
Optional, for added R-value or to pitch water toward the drains.
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04
TPO membrane
Such as Carlisle Sure-Weld or GAF EverGuard, with heat-welded seams, new flashing and new edge metal.
Ways the TPO recover can be built
The roof below decides the method. If the membrane itself is still open, start by weighing TPO against EPDM.
| Method | When it fits | What it adds |
|---|---|---|
| 01Cover board, mechanically attached TPO Our usual build on rowhouses and storefronts |
Dry roof, sound deck, working drains | Quick install, no adhesive |
| 02Cover board, fully adhered TPO | Concrete decks, windy exposures | Smooth sheet, no fastener rows |
| 03Tapered polyiso, then TPO | Water ponds after rain | Slope to drains, more R-value |
| 04Partial tear-off, then recover | Wet patches in a few spots | Wet areas out, dry roof kept |
Swipe to see both columns
What you gain by keeping the mod-bit
Installing TPO over modified bitumen means nothing comes off. There is no dumpster on the street and no night with the roof open over your brownstone, multi-family or mixed-use building.
The white surface reflects summer sun, so the top floor stays cooler. Seams are welded with hot air instead of torched laps. Insulation can go in at the same time.
When we tell you to tear it off instead
Some roofs should not be covered, and we will say so plainly.
- Wet insulation spread across the roof, well beyond a few patches.
- Two roofs already on the deck, or decking that sags or has rotted.
- Drainage that needs rebuilding from the deck up.
Then a full tear-off and new TPO roof is the right job.
Your recover questions
01Can TPO go straight onto the old mod-bit with nothing between?
No. Asphalt degrades TPO on contact, so a cover board or separation layer always goes in first.
02Does the manufacturer warranty cover a recover?
Yes, when the work follows the manufacturer's recover specification and passes their inspection.
03Will tenants or the shop below be disturbed?
Very little. The roof is never opened and no torch is used, so the space below stays dry. The fastening drill is the main noise, during working hours.
04How does the cost compare with a tear-off?
It usually costs less, because nothing is removed or hauled away. Size, cover board and insulation set your figure, as what a TPO install costs on a Brooklyn flat roof lays out.
Book the scan and quote for your recover
Would you like your mod-bit roof scanned, cored and quoted in writing for a TPO recover? Call or send the form.