GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Monolithic or 2-stage concrete pour for this small foundation

AdamPNW | Posted in Green Building Techniques on

Hi all, (zone 4 marine, 12” frost depth)
I’m trying to decide which is better in this circumstance: a monolithic pour or 2-stage (footings then stem wall). 

The foundation plan for my (28’x44’) home is a 12”wide x 8” deep footing, with a 12” tall stem wall. Problem is, my engineer has spec’d a few hold downs with a minimum embedment of 18” (SB 5/8×24). 

so, I can either do a monolithic pour and embed to the correct length (all the way into the footings in this case), or use he can spec a shorter length hold-down but this will require extra reinforcement in these areas. 

To help me decide which is better, I’d be interested to hear any general thoughts about monolithic vs 2-stage pours with regard to labor (formwork), and concrete longevity. 

I’ve read here on GBA (and recently, a best practices manual from BC housing Research Center) that being able to waterproof the top of the footing is best practice for water management, which is important in my marine zone. I am especially wary of water intrusion on my build since my first floor will be slabless (earthen clay floor over a 15mm barrier). 

Additionally, a monolithic pour would require wall brackets (or other hardware) that are left exposed and may lead to future water intrusion. 

Am I overthinking this?  Thanks!
Adam

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Adam,

    A 12" high stem-wall is vey difficult to work with. It only leaves 4" of fill between the top of the footing and grade, meaning that any exterior slab or patio will be directly on top of the footings, and there is no room for drain-rock if you are including exterior perimeter drains. It also leaves nowhere for the downspouts, or sewer connection. So the first thing I would suggest is excavating deep enough for a stem-wall at least 18" high.

    The main problem with monolithic pours is as you say, the inability to include a capillary break between the footings and stem-walls. But that is only useful on buildings with crawlspaces or basements where that moisture can diffuse into those spaces. With slabs (or earthen floors) on grade, the capillary break can be at the top of the stem-wall.

    1. AdamPNW | | #2

      Thanks Malcolm, good points. So, if the moisture issues can be solved, do you think a monolithic pour works out to less labor? Do you think the exposed wall brackets (and the risk of future water intrusion at those sites) would deter you?
      Adam

      1. Expert Member
        MALCOLM TAYLOR | | #3

        Adam,

        I've almost never poured a foundation separately. The concrete volume is the same, but you have to pay for two deliveries, and two pumper truck visits (or laborers to wheel).

        I use pt wood for the cross members on the top of the footing supporting the stem-wall cribbing, and yes they get left when the forms are stripped. There may be some minor water intrusion after a few years, but this occurs at the top of the footing, which should be well below the level of the slab in either a properly detailed crawlspace or basement. If those slabs are protected by a vapour-barrier and capillary break of clear crushed material, I don't see it as a problem.

        One advantage it has over separate pours is the absence of a cold-joint between the two, through which water can also move.

        One potential disadvantage is that if the forms move during the pour, you are more likely to end up with an out of level stem-wall than if the wall had been formed up on top of already hardened footings

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |