Building a deck close to the ground?

posted: 10/Aug/2015

It is now fairly common practice to build timber decks close to the ground or just over concrete slabs. In these situations sub-deck ventilation is often minimal and as a result the decking will more than likely attain higher than normal moisture contents (MC) which need to be addressed. Apart from ensuring the decking and framing materials used are of the highest durability, the increased MC levels will also lead to potential expansion of the decking well above normal seasonal variation expectations plus cupping due to MC gradients through the thickness of the boards. 

The deck that didn't get it right: example of a 140 mm wide deck close to ground with limited sub-deck ventilation, board gaps totally closed and significant expansion and cupping. 

Peak industry body Timber Queensland have measured the moisture content in a number of decks laid close to the ground or over slabs with MC’s as high as 18 to 20% recorded. If decking is laid at say 12%, then significant expansion can be expected. Depending upon the cover width of the decking the magnitude of the expansion that needs to be catered for may be up to or exceed 5 mm with wider boards being more problematic needing wider spacing at time of laying.

Timber Queensland's Technical Data Sheet 13 (avaliable from your local member of the Accredited Queensland Timber Merchant Network) recommends a minimum gap between boards for decks laid close to the ground of 5 mm.

The Australian standard for hardwood timber decking also specifies an allowable range of decking MC from 10 to 18% so it is just as important with decking, as it is with flooring to measure the MC in the decking at time of supply to ensure the relevant gap is used at installation.

NOTE: Some proprietary deck fixing systems ‘set’ the decking gap at time of laying to well less than 5 mm. 

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