What is the most cost effective and energy efficient solution to re-roof a 1950's MCM home?

Asked by Amy
Walnut Creek, CA

This is a northern CA home built in 1954 with slab on grade foundation (no radiant heat) and no insulation in walls. It has a low sloped roof. Eichler style with many windows. Re-roof is needed b/c there is dry-rot on tail ends of beams and fascia. The roof currently has (possibly) 2 layers of T&G. We are on a tight budget (short sale and a lot of other improvements needed). Thinking Compliant Cool Roof modified bitumen roofing system, 2"in. rigid insulation, and fascia gutter system. Would an additional inter-ply membrane be necessary for this assembly? Other option is a foam roof, but is pricey and roofers that bid did not think it is necessary to remove existing roof - though they will for a price... Do we have any other options? Thank you so much in advance!! :)


Polly Osborne, FAIA, LEED AP

Answered by Polly Osborne, FAIA, LEED AP

Los Angeles, CA

Polly Osborne Architects

February 29, 2012

When I think of Walnut Creek, I think of the “hot” side of the SF Bay. And when I look at the data, I see that my impulse is right. 

Although the summers aren’t usually too hot, the temperature can get up in the triple digits, so a cool roof is certainly the way to go.

  • Two inches of rigid EPS insulation, more if the lines of your house allow it architecturally, is also an excellent idea.
  • I probably wouldn’t do the foam roof, if your budget is tight, although with your style of house, the increased energy efficiency would be a big reason to consider it.
  • EPS rigid insulation has an R-Value of about 5.6 per inch, whereas foam is closer to 7.14 per inch.
  • The Foam is continuous, thereby preventing energy loss through cracks and framing and can even be made agri-based.

I don’t profess to know much more than that about foam roof systems as I haven’t had the budget and condition appropriate to use them yet. Suffice to say, try to get as much insulation installed in as continuous a manner as possible, while maintaining the architectural integrity of your home. Wrapping your roof completely with a sheet of waterproofing before any of the roofing assembly is applied will counteract energy loss through framing and gaps.

Interply membrane?

Assuming you are using a modified bitumen roof, the question of whether an additional interply membrane is an excellent one, which should be directed at the manufacturer of your roofing system.

If you have materials that are not compatible between the old and the new roof, between insulation and waterproofing or even within the new bitumen matrix, your lovely new roof could fail.

Originally, this house was probably done with a typical built up asphalt roof. That asphalt may not be compatible with the new modified bitumen roof, or even the insulation, so if the old roof isn’t removed a sheet that separates old and new, a sheet specifically recommended by the manufacturer, would help with long-term roof survival.

Torch-down or cold applied bitumen

You did not mention whether your new modified bitumen roof was the torch-down or cold applied type.

In the torch down variety, which is open flame, that the heating process must be safe and approved by your local building and safety department.

  • There is also a certified roofing torch applicator program from the National Roofing Council and you would want to verify your applicator is certified.
  • Insurance companies also are concerned about torch applications, so take a look at your homeowner’s policy as well.

The Cold Adhesive, Cold process roof membrane have been used in the US for over 25 years. These adhesives are also known as: “cold process”, “cold adhesive”, “solvent-based adhesive”, “cutback adhesive”, “mastic”, “cement”, “glue”, etc. They are commonly available.

  • The same compatibility issues apply.
  • A chemical bond occurs between the modified bitumen sheets and the compatible cold adhesive so that even if a small interply void exists, a blister won’t grow unless the internal pressure overcomes the bond strength.

In general, bitumen roofs have superior bonding than the old built up roofs, but they still use asphalt, which is combined with fibers, fillers, and stabilizers to form the “solids” portion of the membrane assembly.

VOC content of cold adhesives

The use of cold adhesive is less invasive, but solvents sometimes have a high sulfur content or off-gas VOCs (volatile organic compounds.)

  • California has strict VOC regulations and manufacturers have responded and continue to respond as regulations get stricter, so the formulas are often being adjusted.
  • There are also some zero VOC adhesives are available.
  • The cost is higher and the long range performance is about the same, although some super low or zero VOC products have shown to have reached full strength more quickly than their high VOC cousins.

There is even a solvent-free adhesive available that can be installed in occupied spaces with the most sensitive of odor restrictions.

Bottom line

The bottom line, of course, is the warranty. Get the longest warranty you can.

I love the Eichler Style, case study houses, and ranch houses that are part of the California architectural heritage. It is great they are again being appreciated for their elegant, rational style.

Now, if we could just wave a magic wand and get them all energy efficient, they’d be perfect!


Credit for details of bitumen roofing to: "Performance Properties of Interply Adhesives Used with SBS-Modified Bitumen Membranes," by Tim Kersey, Kirk Goodrum and Jeremy Turner, presented at the 11th Canadian Conference on Building Science and Technology in Banff, Alberta., 2007.

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