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byronkw 09-16-2010 12:54 PM

flashing posts protruding thru a wall cap
we are designing/building an exterior (both sides) wall enclosing a exterior deck space with 4x4 wood posts extending 2' above the wall cap to support a pergola - i'm looking for a good solution to flash where the posts come thru the wall cap

Michael Thomas 09-16-2010 01:09 PM

Can you post links to 1) a wide angle view of the (parapet?) walls with the cap penetrations and 2) a close up of a post penetrating a cap?

byronkw 09-16-2010 01:36 PM

my apologies - i should have made clear that i am just starting contruction and am looking for a solution before i get too far ahead of myself

Michael Thomas 09-17-2010 11:25 AM

In my experience it is extremely difficult to provide a durably watertight seal at the penetration of a roof membrane or wall cap by any type of wooden structural member, for example because the wood will absorb moisture, it will expand and contract both seasonally and shorter term at a difference rate as surrounding materials, it's difficult to provide a durable sealant bond to a weathering wood surface, and for many other similar reasons.

It other words, this type of penetration is an inherently Bad Idea.

byronkw 09-17-2010 12:51 PM

your comments mirror my own concerns - and the only system i thought might (?) work would be to countersink a flashing into the posts just above the top of the wall. of course that flashing / post intersection would be subject to all of the forces you describe too...

the follow-up question would be what mechanism would you suggest to suspend / support the pergola above the wall? use steel posts thru the cap? mount the posts on the outside of the wall?

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