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Plaster over plywood?

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I would like to mount plywood on a kitchen wall that has suffered the slings and arrows of outrageous former residents, then cover it with plaster to match existing walls. I have some leftover 5/8" T&G subfloor material with sanded face. Will the plaster adhere on its own, or does the plywood need to be primed or modified somehow? I am concerned about the lack of keying on a flat surface; I suppose I could drill holes in the plywood if needed?

Planning to use Structo-lite base and Champion Quality finish coat, if this helps. The less work, of course, the better.

Bonus question: define rod and darby. Plasterers need not apply.

(post #177830, reply #1 of 8)

We routinely plaster over the interior OSB skin of SIP panels to skip the additional expense of drywall. We tape the seams with mesh tape and Durabond 90, roll on a coat of Stucco Bond (EL RAY product) or there is also a product called plaster weld we used once can't remember the name of the manufacturer. Basically what it does is acts as a keying agent. Seems to work just fine and no delamination after using it for the last 6 years. Also we use Gypso-lite and find it to be a superior product to Structo-Lite in that there are less aggregate chunks.

And isn't a rod what turned that Irish guy Darby into a leprechaun?

(post #177830, reply #2 of 8)

Bob, thank you, you answered my question head one.

How thick is your scratch coat, and do you do just one? (BTW, do you use Durabond on drywall (instead of mud)? The drying time of mud is ridiculous. Or Easysand?)

Sorry, Bob doesn't win the bonus question. But he does have an interesting Freudian imagination.

(post #177830, reply #3 of 8)

It's been at least ten years since I last visited the Rod & Darby in Gunning Shire. Thanks for resurrecting the memory.

(post #177830, reply #4 of 8)

Actually we break all the rules out here in the wild west and only do one coat of plaster over Durabond 90 taped joints on everything. We also smear the 90 in different directions copiously to disguise the joints as the slick finish the 90 provides takes the plaster differently than the OSB or drywall. Makes for a fresco finish that my customers like and is an easy way out for us that we can do ourselves. We try to only float the aggregate which gives a fat 1/8" finish mostly.

(post #177830, reply #5 of 8)

Bob, does the OSB telegragh through? I am interested in this no drywall SIP method. Also wonder about fire code with no drywall.

Someday I would like to build with SIP and would love a list of all the builder tricks used.



(post #177830, reply #6 of 8)


(post #177830, reply #7 of 8)

AJ and Ditto,

The osb does telegraph through the plaster but not in the way you might think. Random flake shadows show through like a stain which go away with a coat of high hiding primer. Now that we do the 90 spread technique described earlier this happens less. As far as fire proof goes the plaster on the sidewalls has some value as a retardent along with its properties as an insulator and sound deadener.
I use 5/8 drywall for the lids. This is for residential use only. For commercial we apply 5/8" throughout to meet code. Just had to double 5/8" a lid to seperate res/com in a recent project.

There are a few tricks to using SIP's efficiently and cost effectively but the one that I personally don't suscribe to is the vanishing act that most manufacturers give the structural elements ie: framing members and headers. The system I use exclusively has all the structural elements that conventional 2x construction methods utilize. I am still able to build $25/sq. ft. under the average cost in my area.

My "trick" is the open mind that i have had since my first SIP job. Worked with two other carpenters on this project and one fought it all the way and the other kind of got into it. Made the job kind of like Hell. Since then I have done many job's with panels and wouldn't build a doghouse without them. Use a crew of 3 or 4 depending on size and some special tools that I have had made or modified. Our latest project is a 2000 sq. ft. residence that will be completed in a little over 4 months that we started in November here at 7500' in elevation. Also set the walls on a large commercial project in that time and a 42 sq. roof system on a log home with the same crew.

We use a boom truck for all roof sets and manage the walls by hand. Tools that we use the most are cordless screw guns (Bosch 14.4), Paslode Impulse Nailers (for 8d ringshanks), Linear Link Saws (for compound angle cutting), Digital Stick Level (72"), Small Emglo and Stick nailer (for 16d), Big Foot 26" circular saw (custom made bad boy) and a prazi cordless glue gun (30 oz.). For roof sets we use Radio Shack voice activated walkie talkies that eliminate the need for hand signals, saves tons of time and give that spaceage look to the job site.

By the way the roof panels that we use are available with 1x8 pine T&G as the interior face. We overhang the walls and create the eaves and soffits already sheathed! This one product alone saves us more time than you can imagine with an installed/finished cost comparable to drywall!

Not that this has anything to do with plaster over plywood but you asked!

(post #177830, reply #8 of 8)

Brian Ewing responds:

"Hello Andrew,

First off. PVA should only be used under, and for, the base coat.

If you want to apply a finish plaster directly over PVA you have to use a setting type compound like EasySand.

Regarding the question about plaster over plywood.

I wouldn't apply any plaster directly over your plywood. The problem you will run into is called the "co-efficience of expansion and contraction".

What this means is the plywood will expand and contract and a significantly different rate than the plaster will. Resulting in the plaster spalling off and delamination. Even with the PVA applied.

The only way to plaster over your plywood effectively would be to also install diamond rib-lath (galvanized metal rib-lath). This will ensure that the plaster will be there for many, many years. This is how we install exterior stucco out here. The house is covered with plywood and
stuccoed with lath and plaster (with the addition of felt paper for moisture barrier). If using metal lath be sure that you do-not use a perlited
gypsum plaster as it will eventually fail...use only a sanded, fibered gypsum plaster (for interior).

Have a nice day!

Brian "