Replacing Tar + Gravel Roof with TPO Roof (Albuquerque: how much, houses)

[rybert]

Ok handy men and women,

My landlord is replacing the roof on our home. She places a lot of confidence in my design decisions based on my academic background. So, I have been given the task of steering the decision of what kind of roof to use to replace the existing tar and gravel.

I've decided since working in architecture these few short years, I prefer a single-ply thermoplastic (TPO) roofing system. It comes with an added benefit of some rigid insulation being installed underneath the sheets of TPO to give it an extra insulative value. So, that decision is made.

Here's the hard part. As a LEED AP, I understand what needs to be done to best insulate a home or guard it from solar heat gain. Instantly when thinking about what color roof I would want, white popped into my mind. It has the best solar reflectance out there. Here's the catch, Albuquerque has a lot more heating degree days than cooling:

Albuquerque, N.M.: 4,361 HDD versus 1,211 CDD Buildings Energy Data Book

Which means (if my math is correct) heating equipent runs 3.5 hours for every hour cooling equipment runs. It would seem that choosing a white roof would give me a substantial "winter penalty" over a darker color.

Does this sound right to anybody? Would choosing a darker roof color (tan possibly) be a wiser decision?

[Zoidberg]

I would imagine that if you corrected for wear issues (hot roofs tend to break down faster than cool ones in the summer), a black roof would make a heckuva lot more sense. Cooling with a swamp and using fans is a significantly cheaper way to condition than heating with gas.

Swamp cooling might prove problematic if the roof retains enough heat though; once you're having to use true A/C you've pretty much eliminated that advantage.

You will end up with much more solar input in the summer than the winter; this will take place not only from more hours of sunlight but also the angles involved (and the tendency for parapets to cast a long shadow). Perhaps erring on the light side would end up being better. But there's probably math you can do.

Could also every spring go up on the roof with silvered tarps and anchor them down with cinderblocks.

Too bad no one's invented cost-effective and non-toxic gravel that's reflective over 20 degrees C and absorptive below that. I think we'll see cost-effective fusion before we see that.

[GregW]

BTW - What is the roof structure made of and how is it held together? Is it properly drained? Can it be walked on? Do you own the place?

I suggest the major concern is not winter heat gain (very little on a flat roof at low sun angles) but summer gain and the longevity of the roof. I suggest the lighter the better. I also suggest insulating the roof membrane from the actual roof.

[rybert]

The home has Refrigerated Air.

The roof structure is a traditional plywood with felt/vapor barrier substrates and tar + gravel. It drains well. It can be walked on, but the home has several small leaks (near parapets mostly). We are renters, but have been tasked by the landlord to aid in the decision making process. We will both win with an efficient roofing system.

Thanks for some the input... things are not always as they seem.

[Mike Horrell]

We have a white, membrane type roof on our home. From what we understand,
the original gravel roof did not seal well and tended to leak so the previous owners
replaced it with a membrane of the thicker, commercial grade type.

I'm not a housing engineer, (nor have I ever played one on TV), but if the roof
is even moderately insulated, just how much heat transfer gain would you get in
winter? I wouldn't think it would be very much so wouldn't it be better to go with
a light roof and gain more reflectivity in summer? It seems to me that's when a
dark roof would actually have a real effect and it would be an undesirable one.

[rybert]

It may be a non-issue that I'm blowing out of proportion.

[GregW]

Go with the white roofing membrane. It will be cooler inside and will also last longer.

[Poncho_NM]

If you price out Tar and Gravel vs the Membrane system, you might see why many homeowners still go with Tar and Gravel because it seems a lot cheaper. A good many Tar and Gravel jobs are done improperly, old crap not taken off, did not use three layers of Hot Tar, etc...

I helped do a few Hot Tar roof in Florida and Maryland over 25 years ago. It did not turn out to be a viable career for me...

I would go with a membrane. The rigid insulation is a good idea. But when it is all priced out your landlord may be shocked. I wonder what color is really popular here? I see a lot of Aluminum colored roofs on very old flat roofed houses. Wondering what the professional roofers recommend and why...


Rich

[rybert]

We're expecting the price to come-in around $6k. It's a 1300 sf house with a ~400 sf garage.

[ohyes]

6k for a single ply membrane? sounds really cheap to me buyer beware! I wouldn't trust it! Had a single ply with two inches of insulation it was twice what you quoted!