The L-shape stretch of the 1m Cantilever Roof Cover of my house failed during one night two weeks ago. It was about twenty years old. The structure was basically triangular wooden structure nailed to an anchored wood on the brick wall, 3m above the floor. It also pulled down the lower part of the metal rain water down pipe at the corner The anchored wood is still intact on the wall.
Why the structure failed after twenty years?
The triangular truss is generally made up of Selanagn Batu timber, a good tropical hard wood, not easily worn off or attacked by insects. But the purlins for supporting concrete roof tiles and ceiling are of inferior quality timber. Some of them show signs of insect attacks and deterioration.
The rain water down pipe passes through the corner of the L-shape roof cover where water leakage had constantly wet the good quality hardwood of the anchored horizontal wood as well at the critical cantilever supports at the corner. The damp wood had encouraged the termites to thrive and together with the inferior timber purlins, had weakened the structure.
It only needed two to three trusses to fail and the rest was just progressive,pulled down by sudden large dead load from other part of the roof.
Many nails are still intact, pulled out by sudden increase of tension forces.
Lessons to learn:
(a) Cantilever structure is generally a risky structure and have to make sure every part of it is of best material and highest workmanship,
(b) galvanised steel structure is preferable in view of the high raining condition and termite problem. Timber is now as expensive as steel.
(c) Never allow rainwater down pipe to pass through critical structural supports as it tends to leak after some time.
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