White membrane under metal shingles?

I am getting ready to replace part of my old wood shake roof with metal shingles, but I am worried about this end wall pg.photos.yahoo.com/ph/jatranslator/my_photos (about 20’ long) where the 6 in 12 pitch roof abutts against a short vertical wall, creating like a flat valley (pitch in the valley: about 1/4" every 3’!
I added the gutter (seen on the left) to collect runoff from the flat roof because I have had small leaks in this valley area when it rained hard. My guess is that the water pooled in the valley, rose a few inches and started leaking under the shakes. The gutter and a new coat of elastomer on the valley floor has taken care of the leaks, but now with the new metal roof coming I am wondering what would be the best way to do this area.
Because the valley has almost no pitch, I am considering installing a 36" strip of white membrane (ALPSAM50) all along this valley, as an underlayment for the metal shingles. This should create a water-proof basin along this canal so that even if the water rises 6-8" in it, there should be no leakage from under the metal shingles. Does this sound like a good idea or is there a better approach?
The ALPSAM50 has only a 15 yr. warranty vs. 50 yr. for the metal shingles, but used as underlayment in this shaded corner I hope it will last longer.
Thanks a lot, Jack


You could frame it so the valley will drain.

Thanks lefty. Yes, I had thought about installing a transition triangle deck (wide in the back, coming to a point in the front), but I may only be able to achieve a pitch of about 5-6" over the 20’ length. It would improve drainage some, but it seemed like a lot of work for very little drainage gain. If the membrane lasts, I would rather leave the framing as is. My main concern is: would the hot metal shingles damage the white membrane under them? Or would it be better to use an Ice & Water underlayment? I have seen them spec’ed up to 250degF.

Is that spray polyurethane I see on the flat roof?

You could line the valley, or you could install new metal along the valley with soldered joints. If you go the new metal route, do as lefty suggested and re-frame the valley so it properly drains. As for your ALPSAM50, I’m not sure what type of membrane that is, so it would be hard for me to give you any advice there. I wouldn’t use the Grace Ice & Water Shield, but if you go with ice & water shield I would probably use Tamko’s Metal & Tile. You could also install Sarnafil along the valley area, which of course can be heat-welded onto Sarnaclad metal if you need to install metal flashings.

Yes, Cerberus. That is SPF. Thank you very much for your recommendations. I started to look at those 2 data sheets TAMKO’s M&T, and Sarnafil, and they look really good.

not to take anythin away from other members.

are we sure cerberus isnt some computer roofing program, that answers
has anyone givin him that number and letter test to see if hes a real person typin.

just real heavy for a human.


I agree with everyone in framing the valley… frame it to slope 1/2 in 12 and youll be ok .

kinda like this cricket i made but one sided :


Hey Jack,
When we run across this problem on tear offs(, or new construction), we often times use a fully adhered EPDM membrane. You can run it up the wall far enough to be covered by the edge metal from the upper flat deck. Then you run it up the pitched roof 3 to 4 feet. Install your shingles approx. 1 to 1 and a half feet out of the valley. In Alaska we have snow sitting on the roofs for 7 to 8 months a year and this application works really well.

Thanks for all the great ideas. At this time I am leaning towards using the self-adhering TAMCO Metal & Tile underlayment, plus a valley formed from Trim Coil metal. It should look good (same color as the metal shingles), hopefully as durable as the shingles themselves, and provide 2 layers of waterproofing.
I will try the “half-cricket” idea at the far end, as far as possible. The underlayment would go up the short wall on the left and it would extend up the right slope about 30". Likewise, the valley metal would be folded on the left about 6" to go up the wall, and it would extend 16 1/4" up the roof deck on the right . Water should never rise more than 6" in this flat valley (it is open at the front end).
One doubt remains though: Would metal expansion be a problem? What would it do to a 20’ long valley with fasteners every 18"? It will be in a shaded corner, mostly covered by the first course of shingles. On sunny days the corner would get perhaps 4 hrs of sunlight max. An EPDM mebrane, as suggested above, would not have expansion problems, but how long would it last? Thanks.


EDPM will last as long as the roof you are installling.

I have never tore off any rubber that the material itself has failed.

I have to agree with Lefty and Roofboy, EPDM might be the best idea. With the Tamko Tile & Ice you will have multiple laps that are peel and stick, where as with the EPDM you could install it without (or nearly without) any seams.

As I stated before, in lieu of EPDM you may also want to consider Sarnafil, although the EPDM is probably going to be cheaper.