# Flat Roof Replacement Cost on a Triple-Decker | Magno & Co

Page: https://magnoandco.com/blog-flat-roof-replacement-triple-decker

> Flat roof replacement cost for Somerville triple-deckers: national price ranges, EPDM vs TPO vs modified bitumen, insulation rules, drains and roof decks.

Kitchens, Baths & Exteriors

## Flat roof replacement cost on a triple-decker: EPDM, TPO or modified bitumen?

Flat roof replacement cost for Somerville triple-deckers: national price ranges, EPDM vs TPO vs modified bitumen, insulation rules, drains and roof decks.

**Mateus Magno**
Licensed Construction Supervisor · November 2, 2026 · 4 min read

Quick answer

HomeGuide's national figures put flat roof replacement at $3 to $11 per square foot, or $4,000 to $16,800 for a typical job. Greater Boston usually runs higher, and a triple-decker roof three stories up adds staging, insulation and drain work.

How much does a flat roof replacement cost? HomeGuide's national figures put it at **$3 to $11 per square foot**, or $4,000 to $16,800 for a typical job. Those are national averages, and Greater Boston usually runs higher, especially on a triple-decker roof three stories above a narrow Somerville street.

Below I compare the three membranes I see most, explain the insulation rules and show when a patch is enough.

## Flat roof replacement cost: the national numbers

These are [HomeGuide's national averages](https://homeguide.com/costs/flat-roof-replacement-cost), installed, per square foot:

| Membrane | National cost per sq ft |
| --- | --- |
| EPDM (rubber) | $3.50 to $10.00 |
| TPO | $4.00 to $10.00 |
| Modified bitumen | $4.00 to $7.50 |

The same guide lists tear-off and disposal at $1 to $2 per square foot and polyiso insulation boards at $0.50 to $3.00 per square foot, again as national figures. Here in Somerville, Cambridge and Medford, labor, hoisting and dumpster permits usually push the total higher.

## EPDM vs TPO vs modified bitumen

All three work on a triple-decker. The right one depends on the roof, what sits on it and who will maintain it.

- **EPDM** is a black synthetic rubber sheet, glued or fastened down. It comes in large sheets, so there are few seams. It is easy to patch years later.
- **TPO** is a white single-ply sheet with heat-welded seams. The white surface reflects summer sun, which helps a hot top-floor unit.
- **Modified bitumen** is a layered asphalt sheet, often with a granule surface. It handles foot traffic well. On old wood-framed buildings I prefer self-adhered versions over torch-applied ones.

The code treats them the same on slope: modified bitumen and single-ply roofs need a design slope of at least **1/4 inch per foot** for drainage ([IBC 1507.11.1 and 1507.12.1](https://codes.iccsafe.org/content/IBC2021P2/chapter-15-roof-assemblies-and-rooftop-structures)). Many old triple-decker roofs have sagged below that, which is why I check for ponding first.

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## Insulation under the membrane: the energy code

A triple-decker is a three-family building, so it falls under the Massachusetts base building code, not the one- and two-family [residential code](https://www.mass.gov/regulations/780-CMR-tenth-edition-residential-chapter-1-scope-and-application-amendments). For energy, though, a residential building of three stories or less still counts as residential under the [IECC definitions](https://codes.iccsafe.org/content/IECC2021P1/chapter-2-re-definitions).

- A simple **roof recover** is exempt from new-construction insulation levels ([IECC R503.1.1](https://codes.iccsafe.org/content/IECC2021P1/chapter-5-re-existing-buildings), exception 4).
- If the roof has **no insulation in the cavity** and the sheathing is exposed during reroofing, it must be insulated above or below the sheathing (exception 5).
- Somerville, Cambridge, Boston and Medford have all adopted the state's [Specialized Energy Code](https://www.mass.gov/info-details/massachusetts-building-energy-code-adoption-by-municipality), so I confirm the exact requirement with the building department on each permit.

In practice, a tear-off is the cheapest moment to add rigid insulation. Tapered boards can also fix a roof that holds water.

## Drains, scuppers and gutters

Most triple-decker roofs I work on pitch gently to the back into a gutter, but some drain to an inside drain or through a scupper in the parapet.

- Where a parapet would trap water if the main drain clogs, the code requires a **secondary overflow drain or scupper** ([IBC 1502.2](https://codes.iccsafe.org/content/IBC2021P2/chapter-15-roof-assemblies-and-rooftop-structures)).
- Old drain bodies and clamping rings are often rusted. I replace them with the membrane, not after.
- A leaking downspout down three stories causes as much damage as a leaking roof.

## Roof decks

A roof deck makes the job bigger. The deck usually has to come up so the membrane can be replaced underneath, and the posts must be flashed properly through the new roof. Adding a new deck is a separate permit and zoning question; my [Somerville deck permit guide](https://magnoandco.com/blog-deck-permit-somerville) covers it.

## When to patch and when to replace

- **Patch** when the leak is at one seam, one flashing or one puncture, and the rest of the membrane is still flexible.
- **Replace** when the insulation is wet, seams are failing in several places, or there are already multiple layers.

For one- and two-family homes, the residential code does not allow a recover over a water-soaked roof or over two or more existing layers ([IRC R908.3.1.1](https://codes.iccsafe.org/content/IRC2021P2/chapter-9-roof-assemblies)). I use the same test on triple-deckers. My [repair or replace guide](https://magnoandco.com/blog-roof-repair-or-replace-somerville) has more signs to look for.

**Builder's tip:** Before you sign, ask the roofer to mark where water ponds after a rain and how they will fix it. A new membrane on a sagging roof that still holds water will fail early at the seams.

## Permits

Somerville's [ISD Building Division](https://www.somervillema.gov/departments/isd/building-division) requires a permit for all exterior work except painting, tiling and flooring. On a three-family building, a licensed contractor must apply for it.

## Need a hand?

I manage flat roof replacements on triple-deckers across Somerville, Cambridge and Greater Boston, from the membrane to the drains and roof deck. [Book a free walkthrough](https://magnoandco.com/contact) and I will send a written line-item budget, usually within 48 hours.

## Frequently asked questions

How much does it cost to replace a flat roof?

HomeGuide's national figures put flat roof replacement at $3 to $11 per square foot, or $4,000 to $16,800 for a typical job. Greater Boston usually runs higher, especially on a three-story triple-decker.

Is EPDM or TPO better for a flat roof?

Both work well. EPDM is a black rubber sheet with few seams that is easy to patch, while TPO is white with heat-welded seams that reflect summer heat, so the choice depends on the roof, foot traffic and what sits on top.

Do I need a permit to replace a flat roof in Somerville?

Yes. Somerville requires a building permit for all exterior work except painting, tiling and flooring, and on a three-family building a licensed contractor must pull it.

Do I have to add insulation when I replace a flat roof?

Under the 2021 IECC, if the roof has no insulation in the cavity and the sheathing is exposed during reroofing, it must be insulated above or below the sheathing. Somerville and Cambridge use the state's Specialized Energy Code, so confirm the details at permit time.

**Planning work on your home?** See [everything I do](https://magnoandco.com/services), [how I price work](https://magnoandco.com/pricing) and [the towns I work in](https://magnoandco.com/areas).

**Good to know:** this article is general information, not legal or engineering advice. Rules change and every property is different — confirm the details for your home with your city's building department, or [ask me](https://magnoandco.com/contact). See our [Terms & Disclaimers](https://magnoandco.com/terms).

**About the author**

Mateus Magno founded Magno & Co in 2020. He is a licensed Massachusetts Construction Supervisor (CS-123658), registered Home Improvement Contractor (HIC #220484) and certified Lead-Safe Renovator Supervisor who has renovated and managed over $5 million in projects across Greater Boston.

[More about Mateus →](https://magnoandco.com/about)

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