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Optimizing profile geometry to reduce the checking, shelling and cupping of profiled softwood deck-boards exposed to natural weathering

University of British Columbia

Wooden deck-boards are susceptible to surface checking and distortion during weathering. Surface profiling is an effective physical pre-treatment to reduce the checking of boards, but it is less effective at reducing cupping, unless it is also used on the underside of boards. I hypothesize that the checking and cupping of softwood deck-boards exposed outdoors can be significantly reduced by optimizing the geometries of surface and sub-surface profiles. To test this hypothesis, I examine the effect of profile geometry and growth ring orientation on the checking and cupping of western larch deck-boards exposed outdoors. Standard and tall rib profiles were significantly more effective at restricting checking than other profile types, particularly when profiled boards were oriented pith-side-up. However, such growth ring orientation made larch deck-boards more susceptible to shelling, a severe defect caused by the separation of growth rings. Confocal profilometry and macro-photography were used to characterize the projection of separated latewood at deck-board surfaces. A negative correlation was revealed between the heights of protruding latewood and growth ring angle to the weathered surface of flat larch boards oriented pith-side-up. The number of shelled lanceolate-shaped tips at the peaks of profiled boards was more common in boards with rib profiles particularly those oriented pith-side-up. I conclude that profiled larch boards should be oriented bark-side-up rather than pith-side-up. Hence, there was a need to develop different sub-surface profiles to distinguish pith- and bark-sides of boards so that boards can be correctly oriented in decks with their bark-side facing uppermost. Pacific silver fir and western hemlock boards were chosen to test sub-surface grooving as an alternative to sub-surface profiling because previous studies had optimized surface profiles for both species. There was an unexpected effect of number of grooves on the cupping of hem-fir boards. Boards machined with an odd number of grooves that included a groove running down the centre-line of the underside of boards were less prone to cupping compared to boards with an even number of grooves. In conclusion, results demonstrate that checking and cupping of softwood deck-boards can be reduced by a combination of surface profiling and sub-surface grooving.

Text

2020-12-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76899

Forestry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/