Carving polyiso sheets?

   / Carving polyiso sheets? #1  

Sodo

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I have a flat roof that has sagged to hold about 1/2 inch of water but is no longer watertight. It's an add-on, done probably 30 years ago, has torchdown on it. I'm studying the idea to build in a slight slope with PolyIso foam sheets. I know that you can buy sloped PolyIso sheets in several different slopes, but the roof shape is not just square and I can't figure out which sheets to buy as I'd like it to slope in x and y direction. I can figure out some sheets but it seems like I'll have to shape some myself.

?.provided there is a sensible, efficient way to shape PolyIso sheets? Is there a way to do it? Is there a hotwire cutter or some tool? Does a sanding disc cut quickly? I like the aspect of getting more insulation from the "slope", but using PolyIso seems to be getting complicated.

If nobody has experience shaping PolyIso, I'm inclined to just taper a bunch of 2x6s and sheet the slope with OSB.
 
   / Carving polyiso sheets? #2  
There is a type of electric hot knife for cutting foams and for gouging in ICF walls.

130 Watt Heavy Duty Hot Knife

That's what your talking about.

I think some kind of ripped board would be better; say a 2x8 ripped from full height on one end (7.25") down to say 1" on the other.
 
   / Carving polyiso sheets? #3  
There is a type of electric hot knife for cutting foams and for gouging in ICF walls.

130 Watt Heavy Duty Hot Knife

That's what your talking about.

I think some kind of ripped board would be better; say a 2x8 ripped from full height on one end (7.25") down to say 1" on the other.

there is a point of having knife to hot, and a knife that is to cold.

to hot, and you sufficate yourself out from the fumes, and you get a lot of melting.
to cold, and you use extra muscles to try and get it through the hard board insulation.

it can be a tad difficult, to get a nice clean cut with a hot knife, the thicker you get. (wavy-ness)

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scoring... take a razor blade / knife set to max out of the knife. and just run down one side of the hard board insulation. then give it a few taps with your hands. and the foam will bust off right at were you ran the razor knife down. it gives a fairly clean cut, the little bit left, is normally ok pending on your measuremets, to simply place and slightly tap the hard board insulation into a fit location.

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table saw, circular saw, band saw, all can be used, and just treating the hardboard insulation like a piece of plywood.

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companies that do, spay foam, for insulation, generally have a larger blade / knife that they can place up against studs and like in a home, and take large chunks of stuff off at a time to leave a nice flesh result of the spray foam.

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sanding hard board insulation = very very messy! stuff is so light, that just a slight breeze sends it every were. and before ya know it entire house and yard is covered :/

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cut hard board insulation (rough it in) then use spray foam to fill in the cracks, and within a couple minutes of putting in spray foam, cover with the "metal insulation tape" and it tends to give a nice seal proof setup. and keeps everything contained and looking nice.

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spray foam = never try to clean it up when it is wet. no matter what it is on = just making a bigger mess.
wait till it has timed to cure, 24 hours or a couple days. once it has cured, then go back and scrap it off / cut it off, etc...

paper towels = a must, at least for me, and a plastic baggy ya get groceries in, to shove used paper towels in. having a helper during spray foam, can make things go much more quickly, and much more neatly.

i always use "great stuff, in red can / closed cell spray foam", always hold can upside down when spraying, never turn can on it side, or at an angle while spraying. this will give you the longest lasting can of spray foam, and you get more out of it this way. expect some cans, not to work, right out of the package. use entire can once you started using it, not worth trying to save it for the next day or a few hours later in the day.
 
   / Carving polyiso sheets?
  • Thread Starter
#4  
I did not envision having trouble "cutting" PolyIso sheets, the task is to "taper" four full 4x8 sheets from 2" down to 1/2", and the 4th one in two directions (x and y). Didn't think about spray foam, but I'm not trying to seal anything, just making a sloped surface for TPO sheeting.

Good feedback on sanding & the mess, I didn't really want to get into that. Ryan you put a lot of time into that informative post, I appreciate it.

Was hoping for some easy silver-bullet method but I'm starting to think the extra insulation is not worth the trouble. Tapering a 2x6 might be faster, because each rip gets me two stringers. I was thinking an 8-foot 2x6, 4.5 inches on one end down to 1 inch on the other, at 16 inch spacing. The roof I have to slope is about 10 x 18.
 
   / Carving polyiso sheets? #5  
2x6, mark it out, table saw it, install it, be done with it.

if ya got some fancy curves, card board, duct tape, scissors.

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a old 2 man lumber saw might work for how you want to taper, but sounds a little ugly trying to smooth it out once you did a rough cut with it.
 
   / Carving polyiso sheets?
  • Thread Starter
#6  
2x6, mark it out, table saw it, install it, be done with it.
…...a old 2 man lumber saw


I do like classical tools ! :thumbsup: Someday I will build a timber-frame cabin (with mostly modern tools).

But a pencil and an old skilsaw with a patched cord is about as classical as this job will get.
 
   / Carving polyiso sheets? #7  
We needed to have some blocks of EPS foam to block out openings in concrete pours. In the past, that usually meant buying sheets and cutting them to size. If the we had to go thicker than the available sheets, that meant cutting multiple pieces and gluing them together. Then I stumbled onto the people below and found we could order any size or shape for less than we had been paying for material by the sheet. It is too far from WA to work for you, and may not be the right material for you but it is an example of what can be found if you keep shaking the bushes.

What is EPS?
 
   / Carving polyiso sheets? #8  
Why not buy some sloped insulation from a flat roof supplier?
 
 
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