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Poured foundation on solid rock

Bill5's picture

I am building a simple addition to a summer cottage. Since I want to keep to the floor level of the existing cabin I will end up with foundation walls only 8" to 30" high on solid Canadian granite. In some areas my floor joists will be only 6" above the rock. I need advice on how to do concrete forms across an uneven rock surface. I have recently seen good articles on forming footings in Fine Homebuilding but these assume that a backhoe has dug a hole that it basically flat for you to start with.

A secondary problem is how to build a floor and insulate from above as you go. In at least half the floor area that I have there will not be enough space to crawl under the floor to insulate so I will have to do it from above as I frame up the floor. This cottage has no road access and thus all digging will be by hand and all concrete will be mixed with a small mixer. The crawl space (what there is of it will be unheated, but the floor will be well insulated.

I would appreciate any advice that anyone out ther can give me.

Big Bill

(post #93711, reply #1 of 25)


We had a similar problem with our summer cottage in Newfoundland.  We went down as far as we could before hitting ledge and then built a series of wood boxes about 2'x2' by about 4" high.  We then put smaller boxes on top of those (about 1'x1' by the appropriate height to tie in with the main house).  We ran rebar from the rock to near the top of the second set of piers and then poured the concrete.  We wound up with around 8 piers.  Now, the height of each individual pier depended upon where we hit ledge but I don't think that any were less than 18" high.  Not sure if this will work if you only have 8" to work with in some spots.


(post #93711, reply #5 of 25)

Thanks for the good advice. I really appreciate it.

(post #93711, reply #12 of 25)

Forming over rough and irregular rock can be a challenge but remember this. .......It makes no difference how uneven the forms are as long as you pour level and in steps.

You don't need a footing to start with. The rock is stable enough to start your walls on.

With a mixer you will be doing small batches so you can form and pour small sections at a time. 1 rebar in the top of the pour will be all you need to tie it all together.

With regards to your insulation. Use chicken wire mesh and staple it to the top of the joist and leave it sag in the middle between the joist to form a cradle for your FG insulation.

Then you just lay the friction fit between your joists as you cover with decking and the chicken wire keeps it in place.


(post #93711, reply #13 of 25)


Some great ideas! Thank you so much.


(post #93711, reply #15 of 25)

But, Gabe, if you just use chicken wire, how will the chickens get in to peck out the bugs and mice that get in there? :-)

(post #93711, reply #16 of 25)


Obviously you haven't seen the size of the bugs we have here on the shield. They're Boone and Crocket quality and standard chicken wire slows em down enough for us to get a shot off.


(post #93711, reply #17 of 25)

Oh, yeah, I see you're in Ottowa area. I hear the mosquitos will drain a cow dry there.

Why do you call it the "shield"?

(post #93711, reply #2 of 25)

You could probably just scribe a piece of plywood or OSB to roughly follow the terrain, then brace it with a straight piece.  This would act as your 'dam' at the bottom of the form.  Any remaining gap could be stuffed with plastic sheeting (or bags) and wedges or filled with spray foam (can easily be trimmed on inside).

Honestly, since you are doing hand-pours in lifts, there will never be much pressure at the bottom of the forms, and you can further control leakage by running a stiffer mix on the bottom as needed.

If you want to insulate as you go, the inside 'dam' could be made of rigid foam.

I would think you would want to unsulate the inside of the foundation walls and the rock 'floor', as opposed to the wood floor (which can be tricky).

Best of luck.

Edited 10/20/2003 11:10:08 AM ET by csnow

(post #93711, reply #3 of 25)

We had a similar problem with Maine granite. (Younger than Canadian, but probably as hard!)  We built the basic forms out of 2X10's, temporarily braced them in a horizontal position, then scribed PW skirts to fit the irregular contours of the granite.  The skirts were never more than about a foot deep (from the 2X to the ledge).  If they need to be deeper, it might be a good idea to step the forms.  Rented an impact hammer, drilled holes in the ledge to pin the foundation concrete to the rock with rebar.

You are right about all the "how to" books showing a foundation being poured in what looks like a suburban lawn.  Doe anyone know a building guide for rougher terrain?

(post #93711, reply #4 of 25)

Thanks very much for the good advice. What you describe is just about what I planned to do. It's nice to know that someone else has done it this way without a problem. Thanks again.

(post #93711, reply #8 of 25)

When we form over irregular ledge,it is easy to brace and stabilize the tops of the forms but the ply/osb bottoms need anti-blowout help. We always have metal strapping from shippments so we lay it on the rock, sliding it under the forms, and bend it up and run a sheet rock screw through. Since the strap metal is hard, a drill helps sometimes.


Excellence is its own reward!



Oh Well,

We did the best we could...

(post #93711, reply #9 of 25)

There you have it folks:

Piffin recommends sheetrock screws for securing concrete forms!

; )

Any jackass can kick down a barn, but it takes a carpenter to build one.

"Doubt is not a pleasant condition, but certainty is absurd."

~ Voltaire

(post #93711, reply #10 of 25)

Actually, in the application I mentioned, a little old footer, a four penny ring shank will work just fine. SR screws is overkill


Excellence is its own reward!



Oh Well,

We did the best we could...

(post #93711, reply #14 of 25)

Funny timing that you should mention banding as a form material. 

We just formed our entire foundation wall using a heavy duty version of perf banding material as form ties.  The banding was inexpensive and plenty strong.  We screwed it to walers with, you guessed it!, sheetrock screws.  Held just fine except for one spot where older son had only put one screw instead of two.  It popped, but only left a little bulge on the outside where it will be buried.  I didn't have to say anything because younger son raked older son over the coals for it before I knew what happened! 

Any jackass can kick down a barn, but it takes a carpenter to build one.

"Doubt is not a pleasant condition, but certainty is absurd."

~ Voltaire

(post #93711, reply #6 of 25)

As for insulating the floor over inaccessible places: after framing the floor and before decking, fasten 1x2s onto the sides of the joists at their bottoms. Drop in plywood (hardibacker?) and then lay in insulation. Hopefully, someone will have some suggestions regarding breathability and condensation control.

(post #93711, reply #7 of 25)

I'm gonna offer a real high tech solution here ...

1)  Set in place as wide a form board that you can use, that will run the full length, without running into interference from the rock.  Maybe it's only a 2x6.

2)  Bank up dirt where you have gaps below the form.

3)  Pour.

However, your local inspector may also want you to place re-bar pins at certain intervals to hold your concrete in place, or he may want to see a minimum thicknesss (height) of concrete.  Better check first.

Formerly BEMW at The High Desert Group LLC

Edited 10/23/2003 10:20:34 PM ET by Bruce

The High Desert Group LLC


(post #93711, reply #11 of 25)

On the "insulate as you go"...I've done this twice, once on a 16 X 32 addition, and once on a 20 X 24 cabin, the addition was beam on grade, and the cabin on pier and beam:

Lay the first six or eight joists up in the air with the band joist out on saw horses out over the edge, with the other end and band joist supported by your foundation wall or beam.  Slip full 4X8 sheets of OSB underneath the joists or lay it down on your supports first.  Crawl underneath and gun-nail ring shanks through the OSB into the joists.  Nail a treated 1X4 over the seams.  Then SLIDE the assembly toward the cabin, adding more OSB and joists as you go.  Exactly how you do this depends on which way the joists run in relation to the cabin.

On the 16X32 addition, the joists ran the 16 foot way, so we kind of built the floor system in sections, balanced the sections on saw horses as we went and used a 'couple of come-along jacks to lower the sections down to the beam.  Stuck the joists that landed on the band joist joint in later, with the whole assembly still pushed out a bit so we could nail the joists off.

On the cabin, we had three beams running the 20 foot way, perpendicular to the joists, which were really two 14 footers overlapped over the center beam. We nailed the OSB up from underneath as we went, sliding the whole floor along to its ultimate location on the beams.  The last bit required blows from a big "persuader" to snug it up.

The whole procedure sounds shakey but it's amazingly simple and fast.  Once the floor is in place, put your insulation in and nail the subfloor down.  Upside is a minimum number of seams to seal as compared to putting ledgers on each joist with OSB rips in between.  Downside is that you need to hustle to get your walls and roof up to protect the now moisture vulnerable insulation in the floor.  On the cabin, I got a big tarp and nailed the edges to the floor platform, sticking a 'couple of sawhorses underneath to give the tarp some pitch.  Left it sit for a week with a couple of rains but no damage.  We got the roof over it next weekend. I suppose you could put building paper/VB  between the joists and OSB and/or joists/subfloor.

The addition was small enough and we had a big enough crew so we got it buttoned up with paper on the roof same day.

I wouldn't try it on anything bigger than 20 X 24 though.  Seems really well suited for small additions.

(post #93711, reply #18 of 25)

Geology is a hobby of mine. I know that some of the oldest rock in N America is exposed there. Is the term "Cambrian Shield" in common usage there? How much do you think people around there appreciate what it is? Around here, in my experience, people don't seem to much know or care about geology.

(post #93711, reply #20 of 25)

Truth be known, no, unfortunately most people have no appreciation for the history of most formations that shaped our lands. To too many people, pre-cambrian or pre Fonzy is all about the same era.

But in all fairness, it's hard to appreciate the pressures that were required to create such a landscape.

Can you imagine the pressures generated by a glacier moving across a continent. No man made creation could even come close to this phenomena.


Our mosquitos are soooo big that a camper on a cot is an appetizer on a ritz.

(post #93711, reply #21 of 25)

We have glacial landscape here, too, except it's at the opposite end of the conveyor belt than where you are. We've got all your topsoil plus a lot of boulders from up your way. On the other side of the state (eastern IL), it's not uncommon to see gorgeous boulders in peoples' yards. They've been dug up (the boulders that is, not the people) while excavating for a basement.

All that wonderful clay and topsoil that we have is, as you and probably few others know, ground up Canadian granite. I'd love to have seen those days.

Thanks for the reply. Is the word "skeet" as in skeet shoot an abbreviation for "mosquito"?

(post #93711, reply #19 of 25)


That's nothing.

I was in Alaska years ago and overheard 2 mosquitos talking, one says to the other "should we take him home for desert or eat him here?"



(post #93711, reply #22 of 25)

Hi Bill. We form to uneven bedrock often. Plumb down from your building lines, measure 3/4 " back and use a rotatry hammer to drill holes roughly 4' apart through a 2x4 and pin it to the rock with a short stub of rebar. The lenght of 2x4s will be dictated to the severerity of the undulations in the rock. Nail lenghts of 1x6 vertically to the inside edges of the 2x4s and nail a 2x4 further up to tie in the tops. Of course the concrete will be poured level and in steps as required but by using short pieces of 1X6 it is very easy to scribe fit your forms. The inside wall is down the same way and the forms are blocked and braced as needed to keep things plumb and rigid. Works very well and once you fine tune your own system it goes quite quickly.


(post #93711, reply #23 of 25)

Hi Jas.

Just a note to thank you and all of the others who have posted answers to my question. This is the first time that I have ever posted a question to any internet site and frankly I am overwhelmed by the response. That so many of you would be so generous with your time and experience is really amazing. Thanks again to you all.


Poured foundation on solid rock (post #93711, reply #24 of 25)

this posted thread is just what i was looking for.


thanks guys

Foundation (post #93711, reply #25 of 25)

used to more. Form as close as possible with solid footers then finish up with chicken wire.

a great way to solve this problem