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Discussion Starter · #1 ·
I have a door project and a large piece of HDPE (high density polyethylene) plastic lumber which might work fit nicely as a subsill. It has the advantages of being UV resistant, never needing painting, termite proof and on hand.


One application that seems possible is where fabricating an door subsill where an extended outside overhang is needed. After hours of Internet searches and talk with manufacturers, I'm advised that sealing under the topping threshold against wind-driven rain can be done with specific caulks, Lexcel and the silicones being named specifically. The bond to the slippery HDPE won't be great--it can be improved by roughening and cleaning the target area or flame treating it--but the mechanical load of the threshold (and better bond to its short aluminum sill) should be enough to keep the caulk in place and working.

Whether new perimeter stucco bonding to it would be a problem is
a question I have not resolved.

If anyone has experience or thoughts here, I would appreciate
both.
 

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Discussion Starter · #2 ·
It might help to say the dimensions of the installed subsill will be about 8" wide x 1.5" thick x 34" long and note that the material is white which will be less prone
to movement than darker colors. How much movement with consequences for the application can be expected from this white piece of HDPE, I do not know. Currently, allowing a 1/4" space for expansion to adjacent studs is my preliminary thought.
 

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Discussion Starter · #4 ·
I think you are way over thinking this.

And I think you might be a homeowner.
Thank you for your reply.

I am a homeowner. But I'm also ply a hand at a simple trade, just not one that allows me to be more knowledgeable about this relatively unusual approach than people who earn their living in the area.

In the process of pursuing the question, I did come across a few web resources that may be of interest to carpenters that deal with PVC materials like Azek and trim. There wasn't a whole lot on HDPE but I've made deductions from reading that may be useful short of
words from someone with superior experience.

The PVC websites are:

http://www.thisiscarpentry.com/2011/10/21/everything-moves/

The table on coefficients of thermal linear expansion for some building materials was informative. HDPE expansion comes in at .000055 inches per inch of material for every increase of one degree Fahrenheit.

http://www.prosalesmagazine.com/molding-millwork-and-trim/getting-the-most-from-pvc-trim.aspx

Hopefully, the above URLs help someone else.
 
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