Return to index: [Subject] [Thread] [Date] [Author]

RE: Lt wt. conc topping for exterior decks

[Subject Prev][Subject Next][Thread Prev][Thread Next]
For what it's worth, I asked our helpdesk to comment on Chuck's recommendations.  My initial thought was Marine Grade plywood may be an "over-specification" (particularly since it's under a light weight concrete).  Although some may think "Marine Grade" implies that it is automatically preservative treated, this is not true.  I then reviewed TT-008 (referenced below), and Marine grade is listed for decks where maximum smoothness is desired.  Note that preservative treatment is also recommended if deck is expected to be repeatedly exposed to moisture.  Venting the underside might help when plywood is exposed to moisture, but pressure preservative treatment should be considered.
Thomas D. Skaggs, Ph.D., P.E.
Senior Engineer
APA - The Engineered Wood Association
7011 S. 19th Street
Tacoma, WA 98466
ph: 253/565-6600
fx: 253/620-7235
### start helpdesk comments ###
Marine-grade plywood is made entirely of Douglas-fir or Western Larch. It is not treated with any chemicals to enhance its resistance to decay. If decay is a concern, Marine-grade (and other plywood grades) should be pressure-preservative treated to an appropriate standard. Reference APA Technical Note Q220 for detailed information on pressure-preservative treated plywood. Available online at:
APA Technical Topic TT-008 provides recommendations for wood structural panels in outdoor deck and balcony applications. You can access TT-008 online at:
If you have additional questions feel free to contact the APA Help Desk Tel: 253-620-7400 or mailto:help(--nospam--at)

From: chuck utzman [mailto:chuckuc(--nospam--at)]
Sent: Tuesday, June 14, 2005 10:29
To: seaint(--nospam--at)
Subject: Re: Lt wt. conc topping for exterior decks

In addition, I always make sure the plywood substrate is sloped to drain. Water will always penetrate the concrete & you don't want it to pond on the w.p. membrane. But eventually the membrane will leak, so marine grade ply. & venting are highly recommended as well.
Chuck Utzman, P.E.

BCainse(--nospam--at) wrote:
Others have weighed in on the concrete slab issue so I won't but one thing you don't mention is ventilation of the space under the slab.  I have seen numerous instances of joists completely rotten through because the joists spaces were not well ventilated. Concrete does crack allowing water penetration (even with "waterproofing").  A continuous line vent at each edge seems to work well by allowing air flow and a place for any trapped moisture from direct penetration and/or condensation to rapidly exit.
Bill Cain, SE
Berkeley CA
In a message dated 6/13/2005 5:13:34 P.M. Pacific Daylight Time, gmadden(--nospam--at) writes:

I’m designing a house overlooking Napa valley for my best client (he bought the house and is doing a massive renovation). Part of the project includes about 2000 square feet of exterior decks and terraces over two levels.

He wants to use concrete topping slab over plywood sheathing over wood framing. I have designed everything for a 2” light weight concrete topping slab.

He is specifying waterproofing over plywood and 5/8” gyp bd. w/ skim coat of stucco on the underside.

He asked me about using a fibermesh type of topping slab and I told him that my experience with that has been with metal deck filled slabs on the interiors of steel framed buildings.

Does anyone have any comments, warnings, etc… on the above assembly. I know two people on this list aren’t great believers in this stuff and I greatly respect their opinions, so I’m cautious about saying go for it.

He obviously wants minimal shrinkage cracking and is concerned about the sun beating down on the finish surface topping slab. Maybe a sealer over the slab is a better solution?

I hope that made sense, I’m not too sure…hopefully someone can share their past experiences good or bad