Massachusetts Ice and Water Shield Code: What 780 CMR R905.1.2 Requires on Your Roof

Here is the requirement, in one sentence. On a Massachusetts roof, an ice barrier has to run from the lowest edge of the roof to a point not less than 24 inches inside the exterior wall line of the building. On roofs with a slope of 8:12 or greater, it must also extend not less than 36 inches measured along the roof slope from the eave edge.

That is it. Memorize the two numbers - 24 inches inside the wall line, 36 inches up a steep slope - and you can evaluate a roofing bid better than most homeowners in this state.

The rule sits in the Massachusetts residential code at R905.1.2, and it applies to asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, and wood shingles and shakes. The barrier itself has to be either two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet - what everyone on a job site calls ice and water shield.

Why "24 inches inside the wall line" is not the same as "one course"

This is where bids diverge, and where the cheap one gets cheap.

The measurement does not start at the edge of the shingles. It is measured to a point 24 inches inside the exterior wall line - meaning inside the heated envelope of the house. So the total width of membrane you need equals the overhang plus 24 inches.

A house with a 12-inch overhang needs at least 36 inches of coverage. A house with a 24-inch overhang needs at least 48 inches. Membrane comes in 36-inch rolls, so on that second house one course is not enough. It is not close to enough.

The 8:12 rule catches a lot of Boston housing

The second requirement - 36 inches measured along the slope on pitches of 8:12 or steeper - is not an alternative to the first. Both apply.

And 8:12 is not steep by the standards of the housing stock around here. Victorians, Queen Annes, antique colonials and plenty of triple-deckers exceed it comfortably. On a steep roof with a deep overhang, correct coverage routinely means two or three courses of membrane at the eave, not one.

It is also worth putting membrane where the code does not strictly require it: valleys, around chimneys and skylights, along rakes on exposed elevations, and anywhere a lower roof dies into a wall. Those are the places wind-driven rain actually gets in. On asphalt shingle roofing jobs we treat the code figure as the floor and the geometry as the guide.

What the membrane is actually fighting

Ice dams are not a marketing invention in this climate. Boston averages roughly 92 days a year at or below 32 degrees, but only about 23 days that stay below freezing all day. That gap - about 69 days a year crossing the freezing line in both directions - is the mechanism.

Heat escaping into an unvented or under-insulated space melts snow mid-slope. The meltwater runs down to the cold overhang, which is not over heated space, and refreezes there. Ice builds up, backs water up behind it, and that water sits against the roof under hydrostatic pressure until it finds a fastener hole.

Membrane is the last line of defence, not the fix. The actual fix is air sealing, insulation and ventilation, which is why we assess attic ventilation and insulation during every replacement rather than selling it separately afterwards. Our guide on preventing ice dams on a Boston triple-decker goes deeper on the building science.

How real is the damage? During February 2015, one Massachusetts insurer reported it was on pace for more than double its prior-year February claim volume, and a Wellesley Hills agency reported a tenfold increase - overwhelmingly ice-dam driven.

The other half of the code: can you go over the old roof?

Sometimes, and less often than contractors imply.

• Asphalt over asphalt: Massachusetts caps you at two layers total. If there are already two, the answer is no.
• Slate, clay tile, cement tile, wood shake and asbestos-cement tile: these must always be removed, never overlaid. There is no version of this where an overlay is legal.
• Any deck with rot or structural doubt: you cannot inspect what you cover. An overlay is a decision to not look.

Even where an overlay is technically permitted, it adds dead load in a state that assigns design ground snow load municipality by municipality - Boston, Cambridge and Salem at 40 psf, Danvers, Ipswich and Gloucester at 50. It also voids most manufacturer warranties, and it means nobody has looked at your sheathing. We tear off. When a bid comes in dramatically cheaper than everyone else's, an overlay is usually why.

How to check that you got what you paid for

1. 1Get it in writing. The estimate should say how many courses of ice and water shield go at the eaves, and where else membrane is being used.
2. 2Look during the tear-off. The black self-adhering membrane at the eave is visible for a few hours before shingles cover it. Take a photo. That photo is worth having in five years.
3. 3Count from inside. Stand in the attic at the eave and measure how far up the slope the membrane goes past the wall plate, if you can see the deck.
4. 4Keep the permit. A permitted job means an inspector had the opportunity to look. See our guide to the Boston ISD Short-Form Permit.

Use this to compare bids, not to argue

You are not trying to catch anyone out. You are trying to find out whether two numbers that look $3,000 apart are actually describing the same roof - because very often they are not, and the difference is exactly this kind of detail. A free roof inspection that ends in a written itemized scope is how you make that comparison possible.

We work Boston, Cambridge and the North Shore, and our estimates spell the membrane out line by line so you can hold them against anyone else's. Get a free roof estimate or call (209) 758-8550.