Essential structural considerations in roof design

Ponding case study
This case history is of a building in Alberta shows ponding failures are not isolated to steel structures and not solely caused by rain. It also demonstrates the possible consequence of upgrading the roof insulation and the need to check the impact of the colder roof.

This building was originally constructed 1968 and has had three additions. The roof structure was constructed level using sloped insulation to provide drainage. The structure system was 38 mm (1.4 in.) wood decking, supported by 89 x 280 purlins, spaced at 2400 mm (94 in.) on centre, connected to 178 x 743 glue-laminated timber (glulam) beams spaced at 4800 mm (188 in.) onto 225 x 826 glulam beams.

The analysis of the as built condition of the roof found the structure met the requirements of the timber design standard CSA O86- 1970 and the National Building Code of Canada (NBCC) 1970. In 2018, using ponding theoretical calculations, the author found the roof was susceptible to ponding failure.

The cause of the roof members’ failure was due to excessive snow and water on the roof. Environmental records indicate the maximum-recorded ground snow depth in the area, in 2018, was between mid-February to the beginning of March. However, historical weather records for the area showed higher ground snow depths since 1968 occurred in 1978 not 2018. The only change identified was the roof was upgraded using R30 insulation in the early 2000s.

The amount of snow depth on the roof was higher than the recorded ground snow depth. The roof system was overloaded when the maximum ground snow loads were mid-February causing some permanent deflections and cracking in the wood purlins. When the snow on the roof melted, water ponded in depressed locations. The additional load due to the ponded water caused the purlins to have a complete rupture failure.

Luckily, the problem was caught before there was a total collapse and the building is again operational with addition purlins supporting the roof deck.

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