The floor of a 12x16 shed is PT 2x6 on top of a PT 2x8 mudsill. The bottom
of the floor frame varies from 6" to 12" above the ground due to a ground
slope. The mudsill is on solid concrete blocks. The decking is 3/4 ACX.
Do I need to seal the underside of the ACX with thompsons water seal or
something similar? I've read a couple of differing opinions that floors
will rot from underneath.
Also, is it required to put a layer of stone under the frame? I packed
gravel under the concrete blocks but not the entire width/length of the
Thank you for your time.
What is your climate like? Answers will be different for Oregon vs.
Arizona. I'm in San Diego. My shed is very similar to your but on patio
stones. I didn't water seal the flooring plywood and it is just fine after
one of the rainiest winters we've had since I moved here. One thing that
can help is making sure there is a bit of a gap between the trim boards
along the bottom and the ground. This ensures airflow underneath. Yeah you
might get some squirrels moving in underneath but if they want they'll move
Thompson's products have had a sorry reputation for years starting
with a Consumer Reports test some years ago and experiences of users
in this NG IMO your logic for protecting the ACX is sensible. In
your situation I would likely spray on a coat of the cheapest oil
based polyurethane varnish I could find, Maybe even a PU porch paint.
It should take around 2 gallons for average coverage. Some polyfilm on
the ground to keep the humidity down might be modestly helpful (and
Here's what the NY Code (a minor variation of the IRC) has to say
regarding the first question:
§2304.11.2.1 Joists, girders and subfloor.
Where wood joists or the
bottom of a wood structural floor without joists are closer than
inches (457 mm), or wood girders are closer than 12 inches (305
the exposed ground in crawl spaces or unexcavated areas located
the perimeter of the building foundation, the floor assembly
posts, girders, joists and subfloor) shall be of naturally durable
AC plywood is not naturally durable.
I'm not sure I understand your shed construction - it sounds like the
foundation consists of some block sitting on gravel, which wouldn't
pass inspection around here. Under a certain size the requirements
are as stringent but when you go above that size you have to comply
with the standard requirements of the building code.
It would be cheap insurance to put down a layer of plastic under the
shed and cover it with gravel.
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