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DRYWALL IMPERFECTIONS Project Guide

Drywall imperfections can include drywall cracking, seam bubbling or wrinkling, corner bead crush or pop, and nail and screw pops through drywall. Usually, these defects are caused by the simple shrinkage of the wood framing members in the house.

In a newly constructed home, we commonly observe nail popping or screws popping through drywall. This is a very common occurrence in new homes after about a year or two.
Visually, a popped drywall nail or screw can be identified by what is usually a fairly symmetrical, round protrusion from the face of the drywall wall or ceiling surface, approximately 3/8 to 1/2 inch in diameter. These "pops" normally do not protrude from the plane of the wall or ceiling more than 1/32 of an inch, but that is enough to show up as an unsightly blemish.

The cause of these pops is quite simple: the wood studs and joists delivered to construction sites typically have a fair bit of moisture in them, usually about 19% moisture content (MC). The equilibrium moisture content of these framing materials in a heated home is around 10 %, averaged for the year. The reduction in moisture content causes shrinkage in the wood member: the average "green" two-by-four, with approximately 19% MC, is approximately 1-5/8" by 3-5/8" in size, and will shrink to approximately 1-1/2" by 3-1/2" at about 10% MC.

Because most houses are erected quite rapidly, and the drywall is applied to the studs before they have had any time to stabilize and dry out, these studs will dry out within the wall space and will actually dry away from the drywall, causing a gap between the back surface of the drywall and the face of the stud. If pressure is applied to the face of the drywall by a person leaning on it or setting a ladder against it, the nail or screw can pop or push its head through the face of the drywall.

The mechanism for ceiling pops can be related to shrinking of the top and bottom wall plates, which forces the drywall on the wall against the drywall on the ceiling,, again causing the gap between the face of the wood framing member (in this case a ceiling joist or truss) and the backside of the drywall to close, forcing the head of the fastener through the face of the ceiling. Truss uplift can also cause movement of the ceiling, causing popping, as well as other problems. Popping caused by either of these mechanisms usually occurs at the perimeter of the room.

Sudden changes in air pressure, most commonly caused when an exterior door or window is quickly opened and/or closed, can cause the drywall on the ceiling and/or the walls to be pressed into or pulled away from their supporting wood framework. This again can cause closure of any gap between the wood member and the drywall, causing the fastener to be popped out of the face of the drywall. Nearby explosions on adjacent construction sites could cause a similar air pressure change.


Minimizing Nail and Screw Pops
Excerpt: Detailing for Wood Shrinkage
© 1996 Stephen Smulski, Ph.D.


The familiar fastener pop is probably the most common drywall problem that crops up when studs and joists shrink. When first fastened, drywall is driven tightly against framing. But as the wood between the fastener tip, whose position is fixed, and the edge of the framing shrinks, it pulls away from the back of the panel, leaving a small gap between framing and panel. Pressure later applied to the panel face closes the gap, forcing the fastener head to lift the taping compound. Pops are fewer and less pronounced with screws versus nails. First, for the same holding power, screws are shorter than nails, so there is less wood between the screw tip and framing face to shrink. And secondly, it takes higher pressure to force drywall along a threaded shank than it does to slide it along a smooth one.

Pops frequently appear in ceilings near the perimeter because shrinking top plates force ceiling drywall down onto the upper edge of wall panels. Prevent these pops by not using fasteners in ceiling drywall within 16 inches of walls. Pops that appear when outlet and switch plate covers are screwed down, or when interior trim is applied, may be the result of overfastening or misplaced fasteners. You can reduce the potential for pops considerably by screwing and gluing drywall. The Gypsum Association, for example, extends its screw-only on-center spacing for walls from 16 in. to 24 in. when panels are screwed and glued.

Stop the pop that telegraphs through vinyl sheet flooring by using screws or ring shank nails long enough to fully penetrate the subfloor below underlayment, and by slightly recessing their heads as is done with drywall. Annoying floor squeaks result when subflooring and stair treads rub against the shanks of fasteners popped from joists and stringers. Happily, prevention is easy. Just lay down a bead of gap-filling construction adhesive before installing subflooring and stair treads, and the culprit gap will never form. Using ring shank or coated nails seems to help too. Framing floors with drier, engineered wood joists, which shrink minimally after installation, is also a good solution.

Understanding withdrawal
Nail pops occur inside buildings because of the initial shrinkage of the framing. But cyclical shrinkage, swelling, and warping of exterior siding, trim, and deckboards can cause nails to be partially or completely withdrawn from framing. The holding power of nails driven into green wood that stays wet, or seasoned wood that stays dry, is essentially unchanged over time. But the withdrawal resistance of nails sunk into green wood that dries in place, or seasoned wood repeatedly wetted and dried, drops substantially over time. As exterior wood swells in thickness, it pulls on nails in direct withdrawal. The pulling action is amplified in flatsawn lumber whose edges lift as it cups.

Take the sight of nail heads protruding from water-borne preservative-treated deckboards. Saturated during treatment and sold essentially green, deckboards always shrink in thickness after installation, so nails flush with the surface when driven will later protrude. And regardless of whether they're laid "bark side up" or not, flatsawn deckboards almost always cup up as the sun dries their exposed tops faster and to a lower moisture content than their shaded bottoms. Cupping reverses itself when tops are wetted by rain. Repeated reversals can slowly pry nails from framing. I recently investigated a case in which cyclic cupping of flatsawn bevel siding caused nails to be withdrawn completely. Lack of backpriming, smooth nails that were too short, and butt joints that didn't fall over framing contributed to this callback.

Reduce the potential for nail withdrawal in exterior wood by using nails of proper size, ring shank nails, or where appropriate, screws. Apply a paint, stain, or water repellent as soon as possible to reduce cyclic dimensional changes. Choose quartersawn (vertical grain) rather than flatsawn, and narrow rather than wide, siding patterns when possible. Always backprime siding, and use fasteners that penetrate solid wood (sheathing and framing) at least 1 1/2 in. Minimize moisture content and dimensional changes after installation by buying siding, trim, and treated lumber ahead of time and letting it acclimate to site conditions elevated off the ground under a loosely draped tarp. Or consider using the newly available engineered wood fiber- and flake-based siding and trim products, which are touted as being more dimensionally stable and resistant to warping.

New techniques, new trouble
While solving old problems, new technology inevitably brings with it new ones. Such is the case with the two-decade-old truss-rising phenomenon that can cause cracks to open at wall/ceiling junctions during the heating season under roofs framed with metal plate connected wood trusses. An aesthetic problem only, truss-rising is usually associated with long span trusses (>26 ft.) of low slope (<6/12), and attic insulation more than 8 in. deep. Exposed to essentially the same air temperature and relative humidity, top and bottom truss chords have about the same moisture content for most of the year. But during the heating season the moisture content of the bottom chord, smothered in insulation and surrounded by warmer air at lower relative humidity, will drop. Meanwhile, the moisture content of the top chords, enveloped in much, much colder air at higher relative humidity, may increase. As a result, the bottom chord shortens slightly, while the top chords may grow a bit longer. Lengthening of the top chords forces the roof peak higher, while webs connecting top and bottom chords lift the bottom chord and ceiling drywall attached to it. Gaps close once the heating season ends as top and bottom chord moisture contents again equalize.

Though truss-rising can't be stopped, you can mask truss movement several ways. One option is to create a floating corner by holding back fasteners in ceiling drywall about 16 inches from partitions. Then use a drywall clip fastened only to the partition to make the ceiling/wall corner. Or, omit the clip, and hide the gap by fastening corner molding to the ceiling only. Another solution calls for 2x6 blocking to be fastened to the top of partitions, with no connection to the truss. Drywall edges are fastened to the blocking, but fasteners in the field are held back 16 inches from the edge, permitting drywall to flex between the edge and field fasteners as trusses rise. Yet another option is to make partition/truss connections using L-shaped brackets attached to the bottom chord with one fastener that slides in a slot as trusses arch upward. Here too, fasteners are held back 16 inches from partitions. Never rigidly attach trusses to partitions; this could induce bending forces trusses weren't designed to carry, or cause partitions to be lifted off the floor.

Excerpt: Detailing for Wood Shrinkage
© 1996 Stephen Smulski, Ph.D



More about Nail Pops

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* Pro Tips *

Before installing drywall,
be sure wall studding is sufficiently dry. Shrinking of damp studs is a prominent cause of nail pops.

Check moisture content of studs before drywall installation using a
Moisture Meter to help prevent excessive stud shrinkage and movement leading to cracks and nail pops.

You can reduce the potential for pops considerably by screwing and gluing drywall. The Gypsum Association, for example, extends its screw-only on-center spacing for walls from 16 in. to 24 in. when panels are screwed and glued.