Anyone have any tips for trimming foam sheets to create fall (drainage) on a flat roof? The structure is AAC panels and they'll be installed flat. I have an embarrassment of 2x8x2" xps sheets and I am looking to trim them for a fall from 4 inches at the roof's centerline to 1 inch at the gutter. The distance is 11.5 feet, so each 2' sheet would give up roughly half an inch. Alternatives to xps must have similar load bearing properties because I'm building a deck on top of the foam using sleepers. Thanks

EPS roofing insulation manufacturers all have products that come in a number of standard tapers for drainage purposes, but they'll also do custom stuff for a fee.

EPS would be fine from a load capacity for a walking around, or even dance-party load on the deck. But if you insist, tapered XPS is also commercially available, eg: http://www.sarnafilus.com/sarnathermxps-tapered.pdf

Thanks Dana. The friendly neighborhood roofing supply carries 20 psi polyiso. sufficient? A planned container garden will create localized weight issues.

Posted By toddm on 14 Jun 2010 12:06 PM
Thanks Dana. The friendly neighborhood roofing supply carries 20 psi polyiso. sufficient? A planned container garden will create localized weight issues.

Considering that 1lb density XPS is only good for ~15psi, I think you'd be good to go, eh?  

The more typical 1.8-2lb XPS is good to about 25-30psi, (which is overkill.)

Unless you're planting a giant Sequoia or Redwood, your planter isn't likely cause an issue.  20psi is about 1.5 TONS per square foot(!), at which point I'd worry more about stressing the framing than the insulation.  If you were using a 100mm thick half-meter tall uranium pot as the container you might run into localized loading issues, but a damp-dirt & ceramic or wood thingy, fuggedaboudit!

Since you want tapered insulation, I assume your membrane will go over the insulation.

From a basic load/sf standpoint, it probably doesn't matter what you use.  We typically spec HD XPS in applications such as this because the cost differential is low and it is less likely to be damaged during construction or from point loads.  I don't know what type of membrance you will use, but you want a good, uniform, smooth surface underneath it.

If you roll a heavy planter on edge across soft insulation, it will definitely deform.

Bruce

Many thanks, Dana and Bruce. We're gardening in the air to frustrate the deer, but I didn't reckon on raccoons. They're strolling right through the site in broad daylight, paying little heed to us, to dine in the cherry trees. Then they relieve themselves before heading back to the creek, which, of course, is the pits.
The layup is AAC panels, tapered foam, EPDM membrane, sleepers, wood deck, raccoon provisions.