What is the best way to pour a slab that is 20 ft x 20 ft but has a slope. The grade is 18 inches higher on one side

  • Monolithic pour?
    – Huesmann
    Commented Oct 30, 2023 at 15:01
  • Not sure what a monolithic pour is, but this 20 x 20 slab will be for a greenhouse in which the steel frame is already cementm posted into the area I want slabbed
    – Chris Espi
    Commented Oct 30, 2023 at 15:32
  • You might find it easier to pour 25 4x4 slabs, or 16 5x5 slabs - form up half (checkerboard fashion) pour them, let them set, pull the forms, then just form the outer edges and use the first slabs as screed guides for the second half. Lots of joints, but that should not have a big impact on a greenhouse application.
    – Ecnerwal
    Commented Oct 30, 2023 at 16:31
  • 2
    Does the slab really need that much fall or are you matching the slope of the ground?
    – Criggie
    Commented Oct 30, 2023 at 23:55
  • @ChrisEspi a monolithic pour is a single pour. If you're putting a greenhouse on it, are the bottom panels of the greenhouse gonna be cut at an angle, or is the whole greenhouse gonna be tilted?
    – Huesmann
    Commented Oct 31, 2023 at 13:32

2 Answers 2


Assuming you're planning to DIY, be ready for it to be a lot of work.

The primary method to place concrete on a slope without having it all end up on the bottom is to use "low-slump" concrete - that is a relatively low water to cement ratio mix, which (when placed in a standard "slump-test cone" and tested) does not slump very far.

This is also colloquially referred to as a stiff mix. Because it does not flow much, it takes a good bit of work to get it in place and screeded. With a 20 foot wide slab, you might want temporary screed guides at 1/3 and 2/3 the width of the slab rather than trying to screed 20 feet of stiff mix in one go.


You want a decent bedding below that slab anyway. Dig and fill that so it's level. Build the retaining form you'll pour the concrete into so it's at least close to level, perhaps with a slight slope in the direction you want water to run off in. Pour concrete and float it level with the top of the form, with that drainage angle allowed for.

  • 3
    We don't know the purpose of the slab. If it's a driveway, which is a very common sloped concrete object, the 18" drop at the street end might be a tad inconvenient.
    – Ecnerwal
    Commented Oct 30, 2023 at 14:00
  • A driveway could be 20x20', but that's a bit unusual. I guess if a double-wide garage door was only 20' from the road...
    – FreeMan
    Commented Oct 30, 2023 at 14:52
  • Its for a greenhouse foundation
    – Chris Espi
    Commented Oct 30, 2023 at 15:30
  • @Ecnerwal: I don't see that what I've written us inconsistent with that...?
    – keshlam
    Commented Nov 3, 2023 at 14:15

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