Roofing Roof Membrane question, shingle roof membrane

Roofing Roof Membrane question, shingle roof membrane




We live outside Montreal, Quebec and have a 37 year old bungalow with a 15-20 year old roof. It currently has an exposed membrane around most of the perimeter either 64 or 96 inches wide (2-3 sheets wide). We assume this is an ice and water membrane.

We were told it is in very good condition. The rest of the roof is shingles which are now curled,cracked and decomposing.

We were given 3 estimates with 3 options.

1. Leave membrane and replace shingles.

2. Leave existing membrane, then cover entire roof with another membrane and shingles.

3. Remove shingles and exiting membrane, add new membrane on edges and shingle entire roof. This would involve replacing the wood under the existing membrane which would make it pretty expensive.

Could you please advise?

Thank you,

Mike & Shelley Horlick

ANSWER: If the membrane has a finish similar to the shingles, granular in nature it is rolled roof membrane. it may possibly be a modified bitumen membrane. The only way to now is to remove any bleed out at the seam if it was torched applied. In fact it maybe totally self-adhesive. If you can see a line in between the laps in the finished membrane, you would have a two ply modified with a base sheet underneath.

If the membrane is in good condition, leave it in place. In regards to shingles on the rest of the roof it is possible. However first have a roofer tell you what the slope is. If it is 4:12 or greater (4 inch rise in 12 inch horizontal run) shingles can be applied. If it is less like 3:12 the shingles will have to underlaid with ice & water shield all over the roof. The shingles can be installed but have to have a dollup of plastic cement (size of a quarter) under the shingles. If the slope is less than 3:12 the roll roofing membrane can be placed everywhere.

When the shingles are done ensure the ventilation is adequate in the roof. According to the local building code, you need one square foot of venting for every 300 square feet of floor space under the roof. So 1200 square feet equals 4 square feet ventilation with 2 sq. feet in the soffit and 2 sq. feet in the roof. I assume you have three or four vents in your roof and each vent equates to about one square foot venting.

all the best.

salut An ex-Montrealer who left in 1974.


Side of House

QUESTION: Thank for your quick response.

Roofing Roof Membrane question, shingle roof membrane

We do not know exactly what the slope is but we do know that all of the houses of this model (Hudson bungalow) in our area have roofs completely shingled (no membrane exposed).

Ours seem to be the only one with this visible membrane. We are not 100% sure if this is the same as an "ice and water shield" but suspect it is because the previous owners had a leak before they had the membrane installed.

Our question really is whether to shingle over this membrane or just to replace the existing shingles? One contractor was going to remove all shingles. put down a "membrane ice and water shield" over the complete roof (even on top of the existing membrane) and then shingle. The other contractor was just going to replace the shingles and leave the existing membrane as is.

With regards to ventilation all contractors were going to install 3-4 Maximuns ventilators. Our roof size is 2700 square feet.


Mike & Shelley


The exposed membrane is not ice & water shield as it has no granules and is not recommended to be exposed to the weather. It is a modified bitumen membrane cap sheet. The membrane can be left in place and new shingles installed. If your roof slope is less than 4:12 you will need ice & water shield over the whole deck under the shingles. if it is 4:12 and greater, only a felt underlayment will be needed under the glass reinforced shingles.

If the roof is 2700 square feet, the floor space underneath might be 2200-2400 square feet. If so you will need four vents on the roof at one square foot ventilation each and four square feet of ventilation in the soffits.

The roofer will be able to tell you the slope as the local shingle distributors can provide a cardboard slope indicator to check.

Leave a Reply