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Update/solution for future reference: I used a laser level set 20in. above desired top the joists and then laid a joist on its side next to where it would ultimately be positioned and then used a ruler to measure the concrete surface to the laser level, subtracted 20in to find the joist height at that position. I did this every 16in down the length of the joist and cut along a line that connected these marks. This method worked very well. This video demonstrates a similar method: https://youtu.be/02mdHbaZW9E


Future viewers should also read and understand OP's later question about ventilation at
Vapor barrier (sheet) over concrete pad below ground level deck?


I'd really appreciate advice on a 'ground level' deck I'm going to build off the back of my house.

There is an existing concrete slab (pink in images) in good condition - recently poured before I bought the house. The slab will be under a part of the deck.

My plan is to have joists (black in images) fastened to the concrete section. Over the section that has no concrete slab, the intention is to use posts supporting bearers with joist in between, secured by joist hangers.

The slab slopes away from the house, but not evenly. There is compound curvature where a 'hump' of concrete feathers out from the door.

Each joist will need to be scribed and cut individually to achieve correct leveling. The thinnest joists (resting on the 'hump' near the door) will be ~2.25", and at the far edge of the slab would taper to ~6.5".

I'm struggling to figure out the best way to secure the joists to the concrete section and level them correctly...

I've considered using 2x8 joists, scribing and cutting the taper for each, shimming them at 24"(?) centers with HDPE shims, then securing them with tapcons at the thin end and use L-brackets and bolts through the board at the thick end? ... OR, perhaps a bearer supported by 4x4 posts in concrete footings at the far edge of the slab with joist hangers, 2x6 joists, then tapcon those joists to the slab where the joist begins to taper and contacts the slab?

I'm sure I'm overthinking some of this, but would really appreciate some help from someone who has encountered a similar situation.

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ECNERWAL - SECTION: enter image description here

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  • Have you priced removal of the slab versus all the scribe work you’re proposing? Feb 22 at 4:15
  • 3
    8 hours with a pencil and a circular saw. vs demolition excavation, cartage, and new footings?
    – Jasen
    Feb 22 at 9:03
  • 1
    Remember to retain at least 1/8" per foot slope away from the house on your deck surface for drainage purposes.
    – Ecnerwal
    Feb 22 at 13:53
  • 2
    Curiosity: What's your goal here? You have a nice slab. Why put a deck with a much shorter expected lifespan over it? Have you considered pavers?
    – isherwood
    Feb 22 at 14:29
  • 2
    @isherwood - Goal is to have a deck. I grew up in Aussie spending a lot of time outside on a deck. It feels like it's missing here. The concrete is sterile and ugly. Pavers won't bring the 'warmth' I want either. I understand the pragmatism behind your suggestion, but it's just not what I'm going for.
    – Texaus_76
    Feb 24 at 21:28

3 Answers 3

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For the area with not much room for lumber, consider making a nice neat linear pile of mortar at an appropriate height and slope to fully support a (pressure treated) 2x4 laid flat (1.5" thick) with the top surface at your desired height, rather than scribing a 2x8 down to 2.25"

As the height required increases, you can add bricks and continue this method, or swap to scribing lumber.

1
  • Hi! Thanks a bunch for the idea. This sounds like a good plan. I added a drawing to the original post showing my understanding of your suggestion. Did I understand correctly? Also, how would you fasten the 2x4 on top of the mortar pile? Thanks! Image FYI - i.stack.imgur.com/Ksf3n.png
    – Texaus_76
    Feb 24 at 21:20
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See IRC R507.6 for joist span lengths. Install blocking between the joists and spaced according to the allowable joist span lengths. Instead of using blocking with the same height as the joists, buy taller material for the blocking. This way the blocking can bear on the slab and carry the joists level.

Under this system you just scribe the blocking material like it's a header along each blocking line. Then you can chop it up into the blocking pieces afterward. The blocking needs a bit of space between its edge and the top-of-framing elevation so that you can fit upside-down joist hangers in there without disrupting the decking surface.

Non-shrink grout or even poured anchor cement below the blocking supports is an option to fine tune the elevation. I would be tempted to install blocking only in every other gap between the joists. That way I could build up assemblies from 2 joists plus their blocking at a convenient elevation with a jig, clamps, etc. instead of fumbling around on my knees all day.

To prevent it from moving around, just anchor some pressure treated 2x2s down to the slab with concrete screws. 14 inch long chunks running parallel to blocking on an 8 ft grid sounds like plenty. Screw through the blocking and into the 2x2s.

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Sounds like a plan, I've done this before, basically where the joists are over the slab treat them like a bottom plate, so that means support them at regular intervals suitable for their thickness. wedges and use tapcons, concrete nails, expanding bolts, or chemical anchors etc to hold them down.

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