Wrap and Twist in Formply
AS 6669 provides a 30mm flatness limitation for formply sheets. To measure flatness,
formply sheets are placed concave down on a flat surface and any bow or twist exceeding
30mm above the flat fails the product Standard.
In addition, sheets must be able to be pushed flat using “palm pressure”, of approximately
15kg.
Formwork plywood supplied to the market with flatness outside the limitations of AS 6669
is non-compliant, should not be used and returned to the manufacturer.
However, experience has shown that most bow, twist and distortion in formwork plywood
is the result of weather exposure, not manufacturing problems with the plywood.
The phenolic surface film is not “water proof”; it only slows the uptake and release of
moisture. Given sufficient wetter or drying, moisture will penetrate or move through the
surface film. The ingress of moisture causes swelling across and along the sheet, which in
turn causes bow and twist.
It is the nature of timber and timber products to distort when subjected to changes in
moisture content. If the change in moisture content is large, bow and twist is unavoidable,
regardless of the quality and grade of the formwork plywood.
The period of October to January each year is particularly savage on exposed formwork
decks, especially in locations from Sydney to North Queensland.
The weather conditions usually involve very hot days followed by thunder storms or heavy
rain. Such conditions drive moisture into the outer veneers of formwork plywood. It is
hardly surprising that most issues with distorted plywood occur at locations from Sydney
North in October to January each year.
In most cases, distorted plywood will flatten when the moisture content reduces or
stabilises following a few days of dry weather.
