Monolithic or 2-stage concrete pour for this small foundation
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
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Replies
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.
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
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
Malcolm, would you mind sharing how you detail your monopour forms so that they don’t move during the pour? I’m wondering also how to keep the wall forms from floating up from the pressure of the concrete below.
Thanks!
Adam
Adam,
We build on a lot of sloped or rocky sites, so we still mostly board form the foundations - meaning the excavation and footings don't need to be perfectly flat.
The footings are formed with 2"x8"s (1) held together by 1"x4"s at 4 ft oc. on the top and bottom (2). These get staked at about 6 ft oc to keep them straight(3). When the forms are stripped after the pour the upper 1"x4"s remain in the concrete.
2"x4" plates are fastened to the upper 1"x4"s (4), and above them 2"x4"s are run at 3 ft oc (5) to attach the 1"x8" boards (8) that retain the concrete. The ties and bars are placed every second row at about 2 ft oc. You form up the exterior wall first, brace it level, then attach a 1"x2" (7) to the top as a level pour strip before forming the inside wall. (6) is a 2"x4" whaler to keep the exterior of the forms straight. It gets overlapped at corners and braced as required.
The 2"x4" plates keep the concrete from pushing out of the top of the footings. There are gaps in the forms between the 1"x4" s, but if the concrete flows though them the mix was much too wet.
Malcolm,
That seems a bit wonky, why wouldn’t you just remove the 1x and attach your board to footing with a couple of tapcons. You would only need one ever couple of feet I assume as all the pressure is shear.
Also you should look at wedge ties as it seems easier than your proposed form method.
freyr_design,
It's for a monolithic pour, so there are no footings to attach the plates to. Everything gets done at once.
Wedges or bars use the same ties. Wedges mean twice the ties as they need to be used on each board. Bars have less risk of shaking out and can be extended past the ties to support the bottom and top boards.
Malcolm,
Oh I see, I thought you were talking about two pour. This is how I would build it:
With a very tall stem you may need to consider additional bracing for footing, perhaps back to trench wall.
Again on very tall you may need to add a couple of splices from bottom board to one above; but generally the stake is adequate to keep bottom edge from blow out.
If you level the bottom board your form work is very fast to stack up and install wedge ties and rebar as you go.
freyr_design,
I'm sure there are lots of ways to do it. I'm just describing the method everyone has used here for decades and that I've used on about thirty pours. The advantage is predictability. I know it works.
Malcolm,
Yes I’m sure there are lots of ways to do it, I was not suggesting you change how you build your foundations, just sharing how I do board foundation.
I will say though that the if it ain’t broke don’t fix it is my least favorite reason to keep doing something a particular way without considering others. Again, not saying your way isn’t the most efficient, just that “everyone does it like that” is a bad reason by itself.
freyr_design,
The method we use evolved over time. I'd say the reason it hasn't changed much in the last decade or so is it has been optimized. Inventing a new system risks missing why things are being done a certain way.
I'm reluctant to say this because I both respect your knowledge and have learned a lot from your posts, but I don't understand how the the detail you have sketched up would work.
- On a mono-pour the top of the footing forms needs to be covered to retain the concrete or it will just flow out.
- Wouldn't it be next to impossible to accurately stake the forms straight and at the same time keep the separation between the stakes on both sides exactly the same distance - which is necessary for the ties to work and the walls remain vertical?
- Bracing alone won't keep the walls straight. You need a horizontal whaler tying the uprights together, which in turn gets staked.
- The only blow-outs I have ever had were footing forms on higher walls where I didn't tie them together frequently enough. Relying on staked footings on a mono-pour is asking for them to move.
- The ties we have are designed for 3/4" boards. Using 2"x material means the stem-walls will not end up standard widths.
We have been discussing this in section, but other problems come up if you move to the plan.
- How do you tie the corners together with stakes?
- How do you get the stakes driven accurately enough so that the board forms can be attached where they butt on long spans?
- How would you step the footings if necessary?
All of these are answerable with modifications to what you have suggested. I suspect the result would evolve be very close to the method I described.
Malcolm,
It does work, it is used all the time in CA.
Here is a video detailing it. https://www.youtube.com/watch?v=cRcGgI4pbpY
Their build up method is slightly different than what I described but the detail is the same.
" On a mono-pour the top of the footing forms needs to be covered to retain the concrete or it will just flow out."
This is not true
"- It would be next to impossible to accurately stake the forms straight and at the same time keep the separation between the stakes on both sides exactly the same distance - which is necessary for the ties to work and the walls remain vertical."
It is very easy with string lines. your can fine adjust with bracing
"- Bracing alone won't keep the walls straight. You need a horizontal whaler tying the uprights together, which in turn gets staked."
It will, I just did a house pour and was almost perfectly square and plumb
"- The only blow-outs I have ever had were the footing where I didn't tie them together frequently enough. Relying on staked footings on a mono-pour is asking for them to move."
If you ever build one you will find it is almost impossible to move with proper bracing
"- The ties are designed for 3/4" boards. Using 2"x material means they stem-walls will not end up standard widths.
We have been discussing this in section, but other problems come up if you move to the plan."
wedge ties are designed for 2x members
" How do you tie the corners together with stakes?"
See the video
"- How do you get the stakes driven accurately enough so that the board forms can be attached where they butt on long spans?"
String lines
I just poured a foundation formed this way with no issues and have formed and grew up forming this way and have never had issues. If i was going to use snap tie system I would use plywood, which definitely has its place, but for board forms I would use this method.
the attached images show this form method, this was an addition so some of it tied into existing but much of it was free floating
freyr-design,
Thanks for the video. It did answer a lot of my concerns about the wall portion. The 2"x material is a lot more rigid than I realized and I can see it working well on stem-walls under 2 ft high - which is what Adam is proposing. What I'm still not clear on is what advantage having to use 2"x material for the forms offers.
I guess we will just have to disagree on the footings. The video shows footings which are basically the same width as the stem-walls above, and that retain their concrete by running the board forms almost down to the ground below, while relying on trench forming. If you need wider footings, which our code and site conditions always require (with reinforcement), my experience is that the tops of the footings need covering, and the forms need connecting.
you'd need to run numbers to figure out what's cheaper. more money for formwork and bracing and longer form-stakes. one mobilization for pump truck. concrete would depend on the number of trucks and short load charges. Malcolm is assuming you are scraping back all soil from the footprint -- if you are earth-forming the footing then you might reconsider whether that's the best approach. but definitely go a little deeper to make your life easier in the future. or do some serious planning about how your plumbing [stepping the footing deeper at point of entry], site water management and future landscape.
I don’t understand why you can’t embed the stab bolts with a two pour? Also I don’t understand the need to leave exposed hardware with a monopour? If you are using board for forms use Simpson wedge tie. Then you just need wood stakes to support wood and diagonal bracing to keep solid.
Thanks Malcolm. Do you ever level the top of the wall form so you can just screed across the top? I’m attempting this now, but finding it’s difficult to build level footings to accommodate level walls. Perhaps just a use laser level at the end and cut the tops of the wall forms level?
Also, I’ve purchased steel monopour brackets to support the steel wall cleats (thinking this might limit water intrusion, over wood 1x4s). But this is not as robust a connection to prevent uplift (there is only a single sheet metal screw holding the cleat to the bracket).
Finally, what slump do you like ideally to keep it workable from a pumper truck, but limit the leaks between forms?
I really appreciate all the help and ideas, thank you!
Adam
Adam, No that's the beauty of the system. The forms can be all over the place as long as you attach the level strip. The other advantage of the 1"x2" is it gives you wiggle room if your forms aren't exactly co-planar with the walls above.
The uplift will only be pressure from the top of the footings. the walls just have lateral pressure. When you pour do the first round just filling the footings and maybe a couple of inches up into the wall forms. That will be a bit firm by the time you do the second round filling the walls to the top. I've never had uplift problems and I've used this method on walls 10 ft high.
Use a 4" to 5" slump.
Malcolm,
In reply to #18 (less scrolling!)
The advantage of the 2x is you just use your framing package for form work and you have almost zero waste.
“I guess we will just have to disagree on the footings.”
Ha I suppose so, all I can say is I’ve done it. For example on the forms from the photos I posted they were dug with an 18” blade and were 6” stems. A few sections had pier bump outs to make the footing 36” and 24” in a few spots. There was no blow outs and the tallest form was about 40-46” tall. If your concrete is mixed properly it is not a problem.
Also unless you need a footing drain I don’t see the point of digging an extra wide trench, which means you can use earth forming for the footer.
This form method works well up to about 4’, above which I would use snap tie
freyr_design,
My practices are no doubt tempered by living in an area so wet even slab on grade houses benefit from perimeter drains. I can see how elsewhere trench footings and omitting them would make life a lot easier.
It's funny how what is seen as essential in one region aren't in another. I don't know if you saw the thread but John Salinger recently described a new residential build in Switzerland with 6" exterior rockwool, and no sheathing or WRB. If someone had suggested that here on GBA imagine the reaction!
Ha wild! Great drying potential I suppose….
A bit of progress! My footings are now done (more stakes to come) built with 2x8s, and planning to use metal brackets as ties, and cleats for the stem walls.
It’s an interesting debate, whether to use 2x or plywood for wall forms. I appreciated the video, the 2x definitely holds its shape.
On the one hand: My stem walls are only 12” tall, so I’m guessing the 3/4” plywood would be adequately stiff. Since this is a one time project, think I could get away with regular exterior grade sheathing (is there a more cost effective product you’d recommend for a one-off foundation?)
On the other hand I’ve got some site milled 2x6 fir (which I could stack on top of the footings), but the thickness varies 1/4” between boards, and sometimes within a board.
Thanks for the knowledge guys,
Adam
Adam,
For some reason (poor reading skills) I thought your walls were in the 2 ft range. Really short walls are a different animal with few of the problems higher ones see. I'd be tempted to use two lifts of the 2"x6"s.
I’ve made a mock up of the 2x wall forms as Freyr design suggested. But I fear my site milled wood is too dimensionally variable (width, depth, warping, etc) and it’s still green so there may be more warping with the concrete. I think the 2x method would work fine if I had kiln-dried wood cut by a production sawmill. As it is, I may purchase some 3/4”exterior grade plywood, apply form release, and rip it to 12” form strips. Then compensate for the thinner plywood with whalers and lots of bracing. Thoughts? Ill post pictures.
Adam,
I know it's late in the day for this type of comment, but a 12" high stem-wall is very short. Once you deduct the exposed portion above grade, it just leaves you 4" cover over the footings. If you are using perimeter drains that's not much room for the rock, or for downspouts next to the stem-walls.
I assume your going to be framing with something, just buy your 2x framing members now and reuse after forms. You do not need form release on 2x, and if you use it on your plywood you basically cant use that plywood for anything else. that stuff is pretty nasty.
Thanks Malcolm!