I am in the process of building a multi level deck where the second level would be low to grade. As such, i had to dig some trenches (about 10-12" deep) as part of the beams will be below grade. My issue is that since the ground is full of clay, this will cause a drainage issue within the trench whenever there is hard rain (see attached picture).

What can I fill this trench with to prevent standing water as well as not impact the beams from any heaving?trench standing water

  • Are you looking for a short-term solution during construction, or a long-term solution after the deck's built? Short term, grab a bucket and bail. Long term, see the answers... – FreeMan Jul 1 '20 at 13:48

You will need to dig these trenches deep enough so that the supports for your deck are situated BELOW the frost line for your area. No amount of fill or drainage will be sufficient since it's the freezing of the moisture in the ground that causes frost heave.

Get in touch with your local building codes department and get their specifications for how deep things should be in your area.

I'm also not clear what you're doing here with this trench. Generally decks are supported by piers which would be a lot less excavation than what you're doing.

If you actually intend the beams for your deck to be below grade, I think you're going to have issues. You should probably rethink your plans here if it's not possible to excavate the area and grade it so that water drains away from the area.

  • Unless those trenches remain to give free air space around joists, the joists will end up in contact with the ground and even ground-rated pressure treated boards will require replacement much sooner than optimal. – FreeMan Jul 1 '20 at 13:47
  • Thanks for your comment. Correct that i have ground rated PT for the 2x8s, i just worry about water accumlation in the trenches that fall between the deck boards if i do not fill. Ive read only on some people filling with 3/4 inch gravel to shun percipitation... is that an option? – Cameron Ross Jul 1 '20 at 15:01
  • To the comment before, the deck will be supported by helical piles that will be drilled in to frost line. The trenches are for the built up beams that will be partially sub grade given the elevation drop for the lower level deck. – Cameron Ross Jul 1 '20 at 15:05
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    @CameronRoss Please explain "built up beams". Are these wooden, say a couple of 2-by-somethings that are nailed together to make a "beam"? If so, then again I think your deck will fail prematurely due to rotting or just softening of the submerged beams. – jwh20 Jul 1 '20 at 15:19
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    Again, it's less than fully clear what you are attempting to do here. How about posting a drawing of your project so that folks can understand the structure? – jwh20 Jul 1 '20 at 15:20

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