Drainage under decking

[Tubby2]

Apart from putting decking on a concrete slab (see my other thread) I will also be putting a timber deck/step 1000mm x 5500mm along the back of my house. I would like this deck to be 100mm or so above the grass level (it could be paved, my wife hasn't made a decision yet).

I was wondering what I should be doing with regards to drainage under the deck. There is a down pipe at the end of the wall that I could easily connect a channel drain to if need be. We have just moved in last week and there is no landscaping so with all the rain we have had lately its just one big mud pile out there. The interesting thing tho is I get to see what happens when we have 400mm of rain on one month. Water tends to run to the area where this deck will go.


Thanks for any help.

[joe greiner]

Grass should slope away from the house at 5% minimum, for a distance of at least 3m; for pavement, 2% minimum. Also 2% for impervious downpipe channel or pipe, same distance. The upper end of the slopes should be at least 200mm below any timber construction except foundations. These are metric equivalents of USA standards, but water knows no nationalities; there may be Oz rules slightly different.

Joe

[Tubby2]

OK. There is a storm water drain in the middle of the back that I can angle the grass/paving towards but under the actual deck do I need to provide drainage?
Do I need concrete under the deck or put 100mm of aggregate and provide a drain? I'm assuming its no good for water to sit under there for any length of time.

Cheers

[Bleedin Thumb]

From you diagram I would slope your excavated subgrade under the deck down to a collection point or central channel. If its a central point ...set in a 300 x 300 pit that you connect to the SW via 90mm line.
If you choose to go the channel way lay an agg line in it to falls and connect to SW line (line the chanel with Geotech fabric first). You could have it falling from both ends to a central T piece junction from what you have described.

Once you have got your deck posts in cover the exposed soil with a 15GSM Geotech fabric and cover with 20mm gravel.

However in any future landcaping try to get falls away from the house so that your drainage under the deck is just a backup because you can't get to it to service it.