How to Prevent Cracks in Drywallcomments (6) August 5th, 2011 in Blogs
Taping and finishing drywall is one of those things you can expect to do occasionally as a remodeler, but unless you're a full-time professional who does nothing but drywall you can also expect to run into problems once in a while. Such is the nature of the beast, and such is the experience of Howard, a general remodeler with years of experience.
He rates his drywall skills as intermediate, "by no means an expert," but he clearly isn't satisfied with a job that's amateurish in any way. The ceiling he has installed has developed two problems, as he describes in this post at Fine Homebuilding's Breaktime forum: a hairline crack 18 in. long on a joint, and a "bubble line" stretching 4 ft. to 5 ft. on a butt end that he swears wasn't there before he primed and painted.
He's used 1/2-in. drywall over joists 16 in. on center, with mesh tape and four coats of lightweight all-purpose joint compound.
"In my exerience with drywall I know that if the drywall has gaps at the seams the tape will bubble up but these seams are nice and tight," he writes. "Also, I put a light across the joint before I primed and it looked sweet. I am sort of a perfectionist and it is driving me crazy having these two issues."
Breaktime Business Forum
Questions about how the drywall was hung
At least one poster thinks 1/2-in. drywall is too skimpy for a ceiling. "Being picky, I'm surprised you used 1/2-in. on a ceiling," writes Tyr. "It should be 5/8-in., especially if [the ceiling is] textured," he adds, which makes drywall more susceptible to sagging.
But Liveonsawdust isn't so sure that's the issue. He says 3/4-in. drywall was once routine on ceilings, but adds: "I'm seeing more 1/2-in. lately. But we are using ceiling board (1/2 in.) that is much stiffer than regular 1/2 in. So I wouldn't worry about your 1/2-in. on 16-in. centers, especially with a smooth surface as opposed to a texture."
If there's no general agreement here, there's little doubt that Howard should have staggered the joints on adjacent sheets of drywall, something he says he did not do.
"Butt joints really should be staggered on walls and ceilings," writes Calvin. " More stress and movement on a continuous joint across several boards." That would explain why one of the defects Howard details could run more than 4 ft. where two butt ends meet. Staggered joints limit the length of a continuous crack.
While many of us would make sure that panel ends and edges landed on framing, where they could be securely fastened, Clewless1 offers an alternative.
"One trick I learned from the guy that did my house was to, in as much as possible, never let your butt joints end up on framing members," he says. "Best to let them 'float' and back them w/ e.g. a 3 in. scrap of plywood. This is good because: 1) you don't have to measure and fuss w/ getting a 4-ft. edge to be dead center; 2) fasteners near the edge often break the drywall which compromises the structure of the joint at that point; and 3) the floating joint is actually structurally superior. Movement of framing won't affect the joint and the joint is stronger because you don't have to put the fasteners so close to the edge.
"This makes it a piece of cake to layout a sheet w/out having to worry about having the joint line up."
Choosing the right tape and joint compound
Although Howard typically uses paper tape, on this job he decided to try mesh tape, along with lightweight, all-purpose joint compound. And this may also have contributed to the problems he encountered.
"I only use Durabond to embed mesh," writes Calvin. "Dries harder and adhesive content higher. Same goes with paper, tho I don't think that is as necessary."
NickNukeEm has had the same experience: "I only bed tape using Durabond or general purpose, never lightweight," he writes. "Every article I've ever read about bedding recommends green top mud or DB. Even so, you can get bubbles if you starve the tape by either spreading on too think a layer, or squeezing too much out after bedding. The debate of mesh vs. paper will wage on as long as both are on the shelves," he adds. "If I use mesh--which is rare--I bed it in setting compound, and I believe the mesh tape instructions recommend this, though I'm not sure."
Tyy, however, is certain that only a setting-type compound should be used with mesh tape, and that keeping the consistency of the compound stiff also is important.
"My preferred setting compound is Durabond-90 or -45, depending on the amount of job I've got," Calvin says, referring to different versions of the same product that cure at different rates. "Regular joint compound in a bucket is also fine. You take your chances on the liteweights and bucket toppings."
Geoffrey recommends this strategy: When using self-adhesive mesh tape, use a setting type compound (either Durabond or, even better, E-Z Sand). Apply the first coat over the tape, allow it to set, then apply a thin second coat, and touch up with a final coat as needed.
For paper tape, he adds, "Apply bedding coat of JC, apply paper tape, smooth and clean joint, allow to set-up, apply second coat, let set, apply 3rd coat if needed, sanding in between if needed, but there shouldn't be to much sanding needed."
Trying to pinpoint reasons for cracks and bubbles
To Megspop, the problems could have been caused by the application of too much compound.
"I haven't used paper tape on beveled joints or butt ends in years though I have fixed many, many, many bad tape joints (mostly from DIY homeowners) when paper tape was used," Megspop says. "I found it's usually poor bedding when the paper tape bubbles."
Bubbling can occur when the compound used to fill gaps sags, pushing down the mesh tape as it goes. "Check to see if there is mud or air (empty space) behind the bubble," Megspop says. "Paper tape is easy to removed and replace. Mesh tape, not so much. It's easier to run your knife along the crack or bubble and reapply new mesh. They also make 6-in. tape. I use 6-in. tape when repairing a DIY'ers attempt to do drywall.
"For your crack issue, is it a straight crack or does it vary? There are many reasons for compound to crack. In this case, without seeing it, I would rule out movement and lean more toward heavy compound application (too thick of a coat will crack) or again a gap. But its tough to determine without looking..."
To Boschvaark, Howard's goal of a perfect finish may be a contributing factor.
"Perfectionism may be the problem," Boschvaark writes. "I was a perfectionist when I first started taping, and pressed too hard, trying to get totally smooth coats. Which pressed out too much mud, allowing air in, creating bubbles on the next coat. I have found the opposite problem from you: If I don't have a gap between sheets, I more often get bubbling. These days, I want about [1/8-in.] gap and set paper tape in mayo consistency Durabond. The thinnish consistency makes it easier for me to load mud into the gap (which seems to hold better in the long term, sort of like keying plaster when using lath), and forces me to press more lightly when setting tape, aka I go a lot faster than I did when I was new to taping."
posted in: Blogs, drywall, ceilings
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