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CONSTRUCTION KNOWLEDGE  >> FINISHES >>

GYPSUM & PLASTER
 

1. What is Gypsum Board?
2. Why So Many Problems with Gypsum Board Finishing?
3. Why Recycle Gypsum Board?
4. What is Plaster?
5. How does Sound Transmission Work and Why are Resilient Channels so Amazing?
6. What Public Domain Documents are Available for Further Study?
7. Tricks of the Trade & Rules of Thumb for Finishes Basics:

What is Gypsum Board?

Gypsum Board is also called Drywall, Gyp Board, Rock Lath or Sheetrock. With paper on both sides and a gypsum plaster center, the gypsum board product gained popularity because of ease of installation, compared to plastering. Skilled drywall mechanics achieve  reasonably plumb, square and true walls quickly.

The traditional gypsum board panel size was 4' x 8'. But many more options are now readily available. Lengths of 10' and 12' are now common, as are 4' - 6" width, which are used for 9' - 0" ceiling heights. Type X gypsum board varies from regular gypsum board because Type X has noncombustible fibers added to the gypsum to improve fire resistance.

The tapered edges along the length of the gypsum board allows drywall finishing compound to be placed without creating much of a raised surface between the adjacent boards. Due to these tapered edges, a more true (or straight) wall can be built. Since the ends of gypsum board are not tapered but are square, it's desirable to eliminate end butt joints when possible. When building rooms with dimensions 12' or smaller, gypsum board can be hung horizontally (4' wide boards for an 8' ceiling height and 4' - 6" wide boards for a 9' ceiling height) with the appropriate length board to not have any butt joints. Even in larger rooms, hanging the gypsum board horizontally has become more popular, because the butt joints can be staggered, which seems to create less of a visibly noticeable joint.
 

Why so Many Problems with Gypsum Board Finishing?

Over the years, I've had many discussions and problems with the quality of gypsum board joints. Often a wall looks good with the gypsum board hung and the 3 coats of joint compound applied, sanded and wet sponged. Later in the construction project, that wall will look poor with a semi-gloss paint and direct lighting which illuminates all the imperfections. When standing with an Owner, looking at a finished wall and clearly seeing all the joint locations, it's difficult to defend the quality of the work. Here are a few things I've learned:

1. Discuss how the gypsum board will be hung prior to the Drywall Contractor even stocking the job, agree on where the challenging areas will be for quality control.
2. A window at the end of a long corridor or lights near the edge of the ceiling that wash down over the wall both highly accent any drywall finishing joints. So if you can't avoid those light conditions, make the Drywall Finisher aware of how the lighting will occur. Also, make the Owner aware that these lighting conditions tend to make gypsum board walls look bad.
3. Prior to painting the wall, require the drywall finisher to take a bright portable light and check the entire wall for finish quality. When in doubt, add another layer of joint compound and widen the joint.
4.  Try to avoid the use of semi-gloss paint, use a washable flat or a scrubable flat paint. If the Owner or Designer insists on a semi-gloss, clearly explain to them  that they may have to live with the appearance of more pronounced drywall joints.

The effect of the type of paint and lighting conditions has caused so many issues over the years that the Gypsum Association produced GA-214-96 Recommended Levels of Gypsum Board Finish that defines various levels of work for the drywall finisher. The normal three coat finish is called a Type 4 finish. They also define a Type 5 finish for harsh lighting conditions or glossier paints, in which the entire board is coated with a final 1/8" of joint compound to create a truly flat surface.

As in so many other areas, the excellent Construction Supervisor anticipates the problems that may occur with drywall finishing and addresses them with the Owner and Drywall Contractor prior to the start of the work.

Why Recycle Gypsum Board?

In the past few years, we've been recycling the gypsum board on some of our projects. Probably 15 to 20% of gypsum board ends up as scrap on a normal construction project. This scrap typically gets put in the dumpster and hauled to the landfill. We've found that for about the same cost, we can keep all the gypsum board scrap in a separate pile and have a drywall recycler come and pick-up the material. While this takes a bit more effort to coordinate, it seems to be the right thing to do. Also, this construction waste recycling helps the project be defined as a "Green Building Project".
 

What is Plaster?

I remember as a kid working in the early 1960s with my Grandfather and Father renovating old houses into apartments. My Grandfather would show me the horse hair plaster on the walls. I could see the hair fibers in the walls; the original builders had added the hair to the plaster to increase the tension strength for durability. The plaster had been applied to wood laths, about 3/8" thick x 1 1/2" wide spaced with a small gap to the next lath. 

Historically, Plaster of Paris was from a large gypsum deposit found in the Montmartre district of Paris. The name was shortened to plaster. Where I live, there aren't any gypsum deposits, but there is plenty of limestone. So years ago there were lime kilns that burned the limestone and created quicklime, that was sold as a plaster product to build interior walls.

These days, gypsum plaster or cement plaster still get used occasionally on construction projects (though gypsum board gets used far more often due to speed of installation). Plaster projects often are placed over a steel mesh lath that is fastened to the wall or ceiling. The normal plaster project includes a scratch coat, a brown coat and a finish coat.

How does Sound Transmission Work and Why are Resilient Channels so Amazing?

Building occupants often care deeply about how much sound gets through their walls or floor/ceilings. So the building industry developed a Sound Transmission Coefficient (STC) rating system. If the sound level in one room is 70 decibels and 20 decibels gets over to the adjoining room, that would be a STC of 50. So the amount of decibel level decrease from one space to the next is the STC rating. The decibel scale is logarithmic, which means that an increase in 10 decibels will sound twice as loud. An increase of 20 decibels will sound four times as loud. Here are some common decibel levels:

1. Weakest sound able to be heard                0 decibels (dB)
2. Rustle of leaves                                         10 dB
3. Quiet whisper in a Library                         30 dB
4. Average office noise level                          50 dB
5. Normal conversation at 3'                         60 dB to 70 dB
6. Normal piano practice                               60 dB to 70 dB
7. Truck traffic                                              90 dB
8. People shouting                                         90 dB
9. Gas lawnmower at 3'                                100 dB
10. Electric circular saw at 3'                          110 dB
11. Loud rock concert                                    110 dB to 120 dB
12. Jet engine at 100'                                      140 dB
13. Loudest sound possible                             194 dB

So a 2 x 4 wood stud wall with 1/2 gypsum board on either side has a STC of 35. In apartment buildings, these are called "paper thin walls". Adding fiberglass batt insulation raises the STC to 39. Just adding resilient channels to the one side of the studs jumps the STC to 46 (the equivalent of a 2' thick concrete wall). Additional layers of gypsum board can further increase the STC to over 60.

The secret to this excellent sound reduction has to do with decoupling the two wall faces. If the sound energy on the sound source side of the wall is used in vibrating the one wall face, rather than just vibrating through the whole wall and into the next room, then the noise doesn't get transferred. The resilient channel works like a spring in a car, creating vibration isolation. Neither a hat channel nor a zee channel creates this same dampening effect.

The following link http://www.ta-inc.com/newshtml/rc.htm has an excellent further explanation about sound transmission and resilient channels.

What Public Domain Documents are Available for Further Study?

1. The US Air Force offers a basic training course for becoming familiar with installing gypsum board entitled AFQTP 3E3X1, Module 21.
2. The US Navy offers an excellent training document for learning about the various types of plastering and how to actually do each of them. The 18 page guide is a plaster excerpt from the Navy NAVEDTRA 14044.
    

Tricks of the Trade & Rules of Thumb for Finishes Basics:

1. Resolve gypsum board butt joints prior to the board being delivered to the job.
2. Resolve gypsum board stocking locations prior to the board being delivered to the job.
3. Discuss wall lighting and glossiness of paint with the Owner in regard to drywall finishing.
4. Consider a gypsum board scrap recycling program.
5. When building walls and ceilings, always think about fire, sound and heat transmission.
6. When installing resilient channels, make sure the screws through the drywall don't go into the studs.