Articles | Talking Trade
My Plasterboard has the 'Chickenpox'
Wednesday, 1 August 2018
By Graeme Robertson
Homeowners are often both alarmed and disappointed to see fastener heads ‘popping up’ in their nice new plasterboard walls and ceilings. Looking like little bumps directly over the screw heads, they can be extensive and ugly!
Fastener popping is actually commonly encountered, not only in plasterboard, but also in timber decking, claddings and long run roofing. The quintessential 'squeaky floor' is also most likely the result of fastener popping. It has been around ever since we started building with timber framing.
If the plasterboard has been installed as per our recommendations, any popping is more than likely due to timber shrinkage.
How does it work?
The maximum moisture content of timber framing at the time of lining must not exceed 18%. A building inspector will help check this by testing a few areas. It can be very difficult to get it to this level with New Zealand's cool, wet winters.
After a building is enclosed, timber framing will find its equilibrium moisture content at around 10 - 12% in Summer or 13 - 16% in Winter. When the moisture content within the framing alters by +/- 3%, the crosssectional dimension of the timber framing may vary by around +/- 1mm. Therefore when timber shrinks, the shaft of the fastener pops out the surface of the plasterboard or leaves a bulge of compound on its surface.
The screw is not undoing itself, the timber is shrinking away from the head.Extreme weather conditions (a long hot dry summer following a wet winter construction period) can however exacerbate shrinkage in a new house and thus the extent to which popping occurs.
Pops that occur after at least one month’s heating cycle are a good indication that timber shrinkage is the cause. Ensuring that, once occupied, the home is well ventilated and not closed off for prolonged periods with heavy reliance on heat pumps will help to minimise popping.
The MBIE Guide
The Ministry of Business and Innovation published a 'Guide to tolerances, materials and workmanship in new residential construction 2015' document (available on their website) which makes mention of the following in relation to popped fasteners:
- Popping of fixings (where the outlines of fixings are visible under the finish) that occur over a period of time after handover that does not break the surface and is not visible from normal viewing position. (refer to the Guide for an explanation of this position).
- Popping that is visible from 'normal' viewing position in Level 4 and 5 finishes at handover. (Most new homes are finished to Level 4.)
- Popping that breaks the surface.
Before fixing ‘pops’ the house framing needs to reach its equilibrium moisture content. Given that most ‘pops’ occur following winter builds, we recommend to wait with repairs for a full year, or after the next summer and winter heating cycle.
Additional screws are placed 50mm away from the popped fastener which is then driven below the surface. The mounded stopping compound is removed, and new coats are applied ahead of re-decoration.