Plastering Tolerances

Log Out | Topics | Search Moderators | Register | Edit Profile

Author Message Dave Metzger Senior Member Username: davemetzger Post Number: 181 Registered: 07-2001 Posted on Tuesday, January 16, 2007 - 11:12 am:    We have a theater renovation project, where we are building an interior partition consisting of three-coat gypsum plaster on metal lath on cold-formed metal framing. Because the plaster will be used for a projection surface, the architect wants a very flat plaster surface. The only tolerance that I can find in ASTM C 842 is for the finish coat thickness to be between 1/16 and 1/8 inch thick. In the Plastering Manual, all I can find is "Finish all plaster, interior and exterior, true and even, within 1/4" tolerance in 5', without imperfections..." Which is a long way from the 1/8 inch in 8' we usually specify for plumbness of a gypsum board partition. So my question is, what tolerances has anyone had experience with successfully specifying for flatness, smoothness, and plumbness of a plaster partition? Nathan Woods, CCCA, LEED AP Senior Member Username: nwoods Post Number: 153 Registered: 08-2005 Posted on Tuesday, January 16, 2007 - 11:29 am:    I think the application of this material in this location is a bit ackward. How are you going to hide control joints? Is your "screen" area less than 10'-0"? There are a few options I can suggest. 1. There used to be a company called Flexirock that made a really good smooth plaster system. I think they got bought out and I don't know what the system is called anymore, but I'm sure it still exists. However, you still need control joints! 2. EIFS - this would seem to be a more ideal surface for you. Larger panels, much flatter & more square, with smooth texture available. 3. Precast or tilt-up concrete. 4. Prefabricated plaster walls, built flat on the ground, troweled with laser screeds, and then hung in panels. still have those joints though... Ultimately, you can specify what you need for tolerances, and call attention to it so that the Contractors really see it and factor that into their bids Richard Howard, AIA CSI CCS Senior Member Username: rick_howard Post Number: 107 Registered: 07-2003 Posted on Tuesday, January 16, 2007 - 12:35 pm:    Assuming that no one will be viewing this "screen" at a wide angle from perpendicular, I wonder how critical the actual flatness would be to the apparent flatness. I think that smoothness would be more important. You could improve the levelness of the plaster the same way you get a flatter floor; ask that longer than normal screeds be used for leveling the brown coat. By the way, USG's spec for plaster work calls out a tolerance as follows: "Do not deviate more than plus or minus 1/8 inch in 10 feet from a true plane in finished plaster surfaces, as measured with a 10-foot straightedge place on the surface." John Bunzick, CCS, CCCA Senior Member Username: bunzick Post Number: 637 Registered: 03-2002 Posted on Tuesday, January 16, 2007 - 01:50 pm:    It would be useful to know how flat a standard projection screen is as a comparison. Maybe one of the screen manufacturers can shed some light on this. I would think good plasterers, using screeds and other techniques, could get a pretty flat wall, particularly if there's only one area requiring this extra effort. Dave Metzger Senior Member Username: davemetzger Post Number: 182 Registered: 07-2001 Posted on Tuesday, January 16, 2007 - 05:13 pm:    That's true, Rick and John. When you look at typical projection screens, except maybe for the tab-tensioned type, they aren't all that flat. Thanks for all the help, people. Marc C Chavez Senior Member Username: mchavez Post Number: 181 Registered: 07-2002 Posted on Tuesday, January 16, 2007 - 05:44 pm:    Once you plaster it. see the attached site for "Screen Goo". I have spec'ed this once - no complaints. But have not seen it work. http://www.theaterthings.com/ Colin Gilboy Senior Member Username: colin Post Number: 71 Registered: 09-2005 Posted on Tuesday, January 16, 2007 - 05:51 pm:    Suggest you go to the manufacturer. Just added them to 11-5200 http://www.goosystems.com/ Ron Beard CCS Senior Member Username: rm_beard_ccs Post Number: 163 Registered: 10-2002 Posted on Tuesday, January 16, 2007 - 06:48 pm:    Dave: The wall (projection) surface is only one of the many issues that one should address when dealing with “theater” projecting. As appropriate to any good design process, basic questions should be asked and determinations made. In the case of media projections, the following questions should be among the first to be asked: 1. What is the size(s) of the image intended? Be sure to confirm whether the size given is the horizontal width or the diagonal dimension. 2. What is the type and technical specs of the projector to be used? ....high-def, pixel count, light output, cinematographer's control (tonal reference) chart, focus, etc. Is the Owner expecting to use leading-edge technology? 3. Prime seating distance (from the torso of the viewer)? 4. What are the overall lighting conditions in the space? Is it a multi-purpose room with windows to the outside which might use a projection system occasionally or a dedicated theater projection facility? Light output and light scatter will adversely effort the projected image. Generally, it is best to frame the area to be projected with darker colors. Once the brightness of the scene, luminance of the electronically converted signal, and the ability of the projection system to transfer images fast and without losses of color or distortion, then the surface onto which the images are to be shown can be fully determined. The level of smoothness of a surface viewed for a distance can be a lot different than one viewed at close range. As mentioned by Mark, www.goosystems.com is a good source to consult. Gerard Sanchis (Unregistered Guest) Unregistered guest Posted on Tuesday, January 16, 2007 - 07:51 pm:    Dave, I believe that there are misconceptions regarding gypsum plaster. It's like brick which expands after it's installed. Plaster expands when it cures - therefore it doesn't need "control joints" in most cases. As to how flat it can be made, the beauty of gypsum plaster is that it can be sanded and re-plastered many times making it very flat; 1/8-inch in 10 feet is easy to get and it's all you'll need for a projection screen. Make sure to specify a projection screen paint. I would not use portland cement plaster because it's difficult to get a flat, smooth surface that doesn't crack - FlexiRock, by the way, was a pre-mixed portland cement plaster, it's now called EnRock and is marketed by Parex. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 255 Registered: 04-2002 Posted on Friday, January 19, 2007 - 02:31 am:    Coming from the land that fills megaplex theaters every night (I lost count at 250 screens within 10 miles of home --- in Orange County CA), it should be noted that large screens are not flat but are curved. Even in high-end home theaters, the viewing surface is sometimes curved (projected images rather than LCD or plasma HD-TVs). Also, the sound system should be considered. There are often speakers behind the acoustically-transparent projection screens in commercial theaters.