NAIL & SCREW POPS IN DRYWALL
We are often called to investigate the cause of deficiencies in houses which are alleged to have appeared suddenly, and which are blamed on construction on the street, nearby explosions, etc. These deficiencies can include drywall cracking, doors that jam, nail and screw pops through drywall, etc. On the subject of the drywall pops, these are often pre-existing defects, caused by the simple shrinkage of the wood framing members in the house.
If you live in a recently constructed home, you probably are all too familiar with nail popping or screw popping through drywall. This is a very common occurrence in new homes after about a year or two, so unless you live in a log house, or a house with interior plastered walls, you likely have these unsightly marks on your walls and ceilings.
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 mass produced 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, as the diagram below shows.
If pressure is applied to the face of the drywall by a person leaning on it, the nail or screw can pop or push its head through the face of the drywall. Voila` - you have a pop showing your 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.
Finally, 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 could cause a similar air pressure change.
So, in a claim situation, when nail or screw pops are listed among the deficiencies allegedly caused by some external event, consider the mechanisms which typically cause them.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2