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wofati front edge of roof concerns

 
paul wheaton
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So I tried to redraw this from mike oehler's book, and then added my concern.

The dark brown is the wood. 

The black lines are the plastic (polyethelene) sheeting.

The light brown is soil.

The yellow arrows represent sun rays. 

So, the black plastic degrades with UV, right?  So isn't this exposed part gonna disintegrate?  Plus, won't it look butt ugly? 

So for my next post is my idea ...
psp_roof_front_1.gif
[Thumbnail for psp_roof_front_1.gif]
 
paul wheaton
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The first pic shows the rough idea.  The second pic shows a bit of a closeup and some more detail to the idea.

My idea is to not have the plastic wrap all the way to the bottom of this board.  In addition, add several more layers of black plastic where shown.  These would be "sacrifice" layers.  Let the sun have them.  In fact, maybe a layer (or two) of some sort of UV safe stuff (?). 

I'm thinking that all of the non-angle wood would be black locust (should last 100 years in the weather with this sort of use).  So it can take care of drip stuff and it can help shield the plastic from the sun.  It would also replace the look of the plastic with the look of wood.

In the second pic, you can see the plastic getting such a pinch!  The blue stuff represents wood deck screws. 

So ... I have to qualify all of this by saying that I have never built a PSP structure before, and that after reading Mike Oehler's $50 and up underground house book, this one issue made up about 80% of my concerns. 

Will this work?  Is there anything I'm overlooking?

psp_roof_front_2.gif
[Thumbnail for psp_roof_front_2.gif]
psp_roof_front_3.gif
[Thumbnail for psp_roof_front_3.gif]
 
Susan Monroe
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I don't really understand what you're talking about, but can't any exposed plastic be covered with something?  Even with multiple layers, it's just going to degrade one layer at a time, and then what?

Sue
 
paul wheaton
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Well, surely you can use layers of something that is UV resistant. 

I suppose one could cover it with cement.  Or, since it is sort of covered by wood, maybe a bit of cob right there would hold up for a really long time.

 
Dave Boehnlein
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I haven't seen the book, but just to check, are you sure the author didn't recommend leaving the excess plastic exposed only temporarily to allow for settling. I know with liner ponds a common mistake is that people lay down the liner & fill it up. Then they cut the excess plastic and walk away. However, with a pond it can take as much as 18 months before  the liner has fully settled. Therefore, we never cut the excess material off of our pond liners until a couple years have gone by so we don't end up cutting it wrong.

Perhaps the intent is to cut it or cover it with something later. Are there finished photos in the book? Do they show a sloppy poly edge hanging over or do they show a neat & tidy edge?

Dave
 
Steve Nicolini
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The photos are finished.  He pinched the plastic down after wrapping it around the low side of the overhang. 

I think Paul's concern is with the soil settling on the slope of the roof, but I am not sure.

If the soil isn't cultivated, on a 1:3 slope, do y'all think it will erode at all?
 
paul wheaton
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If he were to trim it later, couldn't water get in?  My impression is that he has carefully designed it this way so that water hitting the vertical surface will drip off at the bottom edge.  Apparently he once had a different design and water kept coming in the house.

 
Steve Nicolini
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I think it would allow water in.  Drainage drainage drainage.  Paul, what if you covered that top piece of lumber with soil as well?  Would that help prevent exposure of the roof plastic?
 
paul wheaton
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Steve Nicolini wrote:
I think it would allow water in.  Drainage drainage drainage.  Paul, what if you covered that top piece of lumber with soil as well?  Would that help prevent exposure of the roof plastic?


It would help prevent exposure to the roof plastic, but it would also lead to the wood decaying.


 
Steve Nicolini
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True.  I think what you drew up is pretty good, and would probably work.  I wonder if soil settling would pull hard enough on the polyethylene to tear it. 
 
paul wheaton
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Mike Oehler
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Valid concerns. Yes the sun will deteriorate the poly and yes it is ugly. Former clients of mine have built galvanized sheet metal caps to put over the poly. A lazy man's way (mine)is to just put a temporary sheet of poly over the thing every year. Only takes a little time.
 
Steve Nicolini
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Mike, do any PSP structures you know of have gutters?
 
paul wheaton
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I think if it had gutters it would, by definition, not be PSP.  All PSP roof stuff flows down to soil. 

 
Nicholas Covey
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I wonder if they make vinyl or metal facia covers that are wide enough to cover the entire shoring board.

And it wouldn't need much of a gutter, maybe a vinyl j channel since the runoff would only be what was running off the top of the board itself. It might minimize drip across a doorway or window though.
 
Steve Nicolini
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So if someone used Mike's design and built their home in the dirt, but decided to put up a gutter in one area on the downhill side to collect runoff water, would that be PSP? 

It seems to me that some gutters might help hide the Front edge poly and help collect rainwater too. 

How are gutters so bad?  Just using the extra material?
 
paul wheaton
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It's not that gutters are "bad", it's just that if you have one inch of space where a gutter would be of use, then you don't have a PSP structure. 

That's why the design can be challenging.  That's why the video has that segment that has you do those exercises about getting light in from all four directions.  What gives PSP its super-power is this simple idea:  all water flowing downhill flows into soil.  Never anything gutter worthy.  If you can stick to this principle, you get to have all the bennies that go with PSP.  If you dodge the principle, then you lose your bennies.




 
Mike Oehler
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Seve, I sympathise with your desire to collect rainwter off the roof of PSP but have shelved the idea as probably unworkable. We were going to try to collect roof water off the Ridge House with sand just below the down side of the roof, drain tile at the bottom and a layer of polyethylene below that to catch and contain the moisture so it would run into the drain tile. We gave it up after reading reports of the miserable amount of water earth yields as opposed to a hard surface roof. Since we are planning a pond with the Ridge House we will catch the water off the ponds glazing (it's also a greenhouse with several other functions) and off several metal roofs we are going to install over other sturctures specifically for precipitation catchment. Otherwise you might be able to catch water off a PSP roof by laying out on the roof a layer of polyethylene duting rains and snows then rollong it up again. "Gutters" as such only work off hard surface roofs.
Steve:
A gutter will never work off the drip board and as a cattchment device on the drip bards because they are at the highest pqrt of the roof. You put gutters on the losest part of the roof. Go to a shop that works with sheet metal to have them make you a cap to protect the drip board poly. Take the dimentions to several shops nd get estimates.
 
Davin Hoyt
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Please find this to be the latest design details Paul has communicated with me.
Fascia_Section01.jpg
Wofati Fascia Section
[Thumbnail for Fascia_Section01.jpg]
 
Davin Hoyt
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Please find this to be a supplemental drawing.
Roof_Section01.jpg
Wofati Roof Section
[Thumbnail for Roof_Section01.jpg]
 
Davin Hoyt
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I have updated these drawings...
FACIA_section02.jpg
Fascia Section without Exterior Wall
[Thumbnail for FACIA_section02.jpg]
FACIA_section03.jpg
Fascia Section with Exterior Wall
[Thumbnail for FACIA_section03.jpg]
ROOF_section02.jpg
Typical Wofati Roof Section
[Thumbnail for ROOF_section02.jpg]
 
chad duncan
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paul wheaton wrote:The first pic shows the rough idea.  The second pic shows a bit of a closeup and some more detail to the idea.

My idea is to not have the plastic wrap all the way to the bottom of this board.  In addition, add several more layers of black plastic where shown.  These would be "sacrifice" layers.  Let the sun have them.  In fact, maybe a layer (or two) of some sort of UV safe stuff (?). 


I built a doghouse last spring to try out this style of construction. I built it similarily to your picture "psp_roof_front_2.gif " but I ran the plastic and the covering board down to the bottom of the facia. The 'cap' over the plastic on my doghouse is made of cedar picket fence boards and it is built a little more like a 'U' shape. The cedar on the roof side is in dirt and will eventually rot but it is cheap and it's easier to replace that than the plastic sheet. On the side of the 'U' that is exposed, I expect that to last forever. The 'U' shape allows me to put the wood securely in place without having to perforate the poly. I'm just coming up on one year now and there are no leaks and so far no visible rot on the boards.

If you have horses near you, you can use that manure for the dirt on top and you will have grass covering it in just a few weeks. Grass keeps the dirt in place, dirt protects the poly, the poly keeps the roof dry, dry roof keeps the walls from getting wet and rotting, this eventually leads to a hole in a bucket somehow but I just can't remember how the song goes.
 
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