View Full Version : Footers/foundation on Rock...

06-08-2004, 06:31 PM
Hello All!

I'm building a new home with a full basement. During the excavation, we encountered a nearly level rock slab about 4ft down under the entire excavation site. I considered this a blessing because we are near a small, flood-prone creek, so I figured we would just build the house up a bit higher for flood protection. However, building on solid rock prohibits me from digging footer trenches as I had originally planned. The excavator hit this stuff and just couldn't get through it. As far as I can see, the footers will now have to be formed up. What would be the best way to do this considering that I will have to find some way to get stakes into the rock below to build the forms?? I want to use 2X10s to form the footers, but I am now wondering how to fasten them to the ground... I tried driving rebar into the ground to use as stakes, but can't get them into the rock. Maybe I'll need to drill holes for the rebar?? HELP!!

I am also considering laying down a 4"-6" base of crushed stone over the entire dig before doing the footers. My reasoning for this would be to allow me to level off the site and would allow me to elevate the footers above the rock which would give me some room to run the basement floor drain lines under the footers. By the same token, this would also give me some extra room for the perimeter drain pipe to remain below the footer level. The footers are to be 24" wide X 10" thick. The basement stem walls will be 8" wide poured concrete. Does this all make sense, or would you suggest another approach?

Any other considerations for building on rock are also most welcome!!

Thanks so much!!


06-09-2004, 04:03 AM
I think you're on the right track with your construction methods. Placing concrete on rock is always an issue. Your idea of placing 4-6" of crushed stone will allow you to take care of most of the issues with stone. Although with the crushed rock you will have some issues of getting your form stakes in the correct place. I wouldn't be too worried about having your footer perfect - at 24" wide you'll have plenty of bearing along the top to be off a little bit. As long as you're pretty close to square you shouldn't have any problems. Measure twice - cut once :)

06-09-2004, 06:08 AM
Thanks for such a quick reply Rich!

This is a great forum and I appreciate the input!

Affirmative on measuring twice, cutting once!

Take care and God Bless!

Curtis D Clemson
07-06-2004, 07:57 AM
Can you please elaborate on the issues here as it seems that rock would make a good substrate for the footers. Thanks.

07-06-2004, 07:11 PM
Hi Curtis!

Not sure exactly what you are looking for, but I'll try and update a bit.

Due to the rock base we hit during our basement excavation, we had to form our footers up on the rock. Although mostly level, there are some low spots. My initial idea was to apply a 4"-6" base of 3/4" crushed stone onto the entire dig to give me a more uniform, level surface for the footers. This would also allow me to cast the footers up off the rock to allow me to install perimieter drain pipe without having to cut into the rock to keep the drain pipe below the top of the footer. The same goes for my basement floor drain lines as well.

Since my original post, I've changed the game-plan slightly. Instead of covering the entire dig with crushed stone, I'm only going to place it into the bottom of the forms and along the sides. I've got some water seepage into some sections of the forms along the hillside on the rear of the site. So, I am switching from 3/4" crushed stone to the smaller crushed stone (called "Run of Mines" here locally) that has a lot of rock dust mixed in. This stuff packs down much better than the 3/4" stone and almost sets up like concrete when mixed with the wet residual clay soil left on top of the rock base after the excavation. If you look at the last picture I've included with this post, you'll see the pile of stone that I'm using. Notice how well the pile held it's shape even when I had to cut into it to place a section of my forms. If you were an artist, you could almost create sculptures with this stuff!

I still have some work left to do in getting the majority of clay muck out of certain areas of the forms. However, once that is done, I'll level up the forms, lay in the crushed rock, and pour the footers.

Here's some pics of the site... It rained all weekend, so there's quite a bit of water on the rear of the site. Also, note that I used 1/2" rebar stakes driven into 12"-16" deep holes I drilled into the rock base to secure the form boards. I also used conduit clamps to fasten the boards to the rebar. This setup allows me to independently level each form board and will make form stripping very quick and easy.

Hope I cleared it up for you. If you have any specific questions or if you see anything I could do differently please let me know! (ALWAYS open for suggestions!)

Take care and God Bless!

\o/ Paul


07-06-2004, 07:42 PM
That's quite a footing you have there. It looks great though. My only suggestion (and you might not be there yet) is to put spreaders across from form to form - it helps it hold together much better than without them. Make sure they are on top of the forms so as not to interfere with the footing concrete. They can be removed while you're finishing.

07-08-2004, 05:25 AM
Hey thanks Rich!

Yes, I am going to install spreaders. We are just finishing up leveling the forms today and will install them shortly. It does seem to be a fairly unique layout. Its actually in the shape of a "cross" which was very appealing to us as a Christian family. I'll post some of the drawings of the floorplan at the bottom so you can see why the footers are so elaborate.

I'm probably overbuilding a bit with my foundation, but to ME (and I hope this reasoning is correct) the majority of residential home structures are basically the same (from a builder's standpoint) from the sill plate UP. You can always remodel and redecorate with different windows, trim, wall treatments, upgraded appliances, etc... But generally, once the foundation is in the dirt, you're STUCK with it and it had better be RIGHT.

Incidentally, I was going to make a separate post about the water (and resulting mud deposits) I have in my forms along the backside of the site (see pics in this thread). I am very concerned about it and would ideally like to have dry forms before I pour the footers. However, more than one person has told me not to worry about the water and to go ahead with the pour directly into it. They say the concrete will disperse the water out of the forms and I can deal with drainage AFTERWARDS. This is my first construction and I want to do it right.

Am I being too cautious by wanting to take more time (and effort!) to resolve the water problem FIRST? You may recall that my forms are sitting on a rock bed or ledge. Also, I had previously discussed with you in another post the fact that I was considering putting crushed stone into the base of my forms. The reasoning would be to help leveling the site and to get my footers up off the rock a bit to allow more clearance for the perimeter and other drain pipes. At this point, I am trying to decide whether to just scoop out as much of the clay MUCK from inside the forms and just go ahead and pour the concrete into the water, dealing with draining it later OR to go ahead and dig (or jackhammer) a trench around the outside of the forms to install gravel and perimeter drain piping before I pour the footers.

Another concern I have is this: If I use crushed stone in the beds of the footers will I potentially have problems with water telegraphing UNDER them through the crushed rock and into the basement floor area?? In regard to this, one person locally has suggested to install perimeter drain pipes on the INSIDE of the footings in addition to the outer ones. Ever heard of that? Overkill? Or a good idea?

I'd just like to get this house GOING, but I don't want to make a critical mistake at the foundation level either. What do you think?

Thanks so much!! God Bless!

\o/ Paul


07-08-2004, 06:47 AM
I would probably place the footings.. water or not. The advice you received on the water is good - the concrete will, for the most part, displace the water. I've poured some 30' deep caissons and had the concrete push water up the whole way.
As far as the stone/gravel in the forms - it will save on some concrete but probably won't help much for the water infiltration. I would suggest placing the footings right on the rock with no gravel. If you have the drain piping run a little higher than the footing I wouldn't worry too much about it. If you feel like you'll get a better installation with the gravel in the forms - go ahead. Looking at what you've done so far I think you've got some common sense.. that'll get you farther than anything else. Either way (rock or not) you should be just fine.
In one instance we did put a perimeter drain to the inside of a foundation wall. The section was lowered down for a future theatre.. and we felt it was so much farther down (almost 4') that the exterior perimeter drain may not pick it all up... so we added a sump pump and an interior drain line. I think it would be overkill in your situation. Make sure your water/damp proofing is good and you may think about putting some type of sheet drain board across the footing and under your perimeter drain just to make sure.

07-08-2004, 09:07 AM
I have also installed drains under slab. On one job we found a natural spring in the middle of the excavation. The house is three years old now and no water problems.

07-08-2004, 05:25 PM
Thanks so much for the info!

Rich, not to belabor, but could you elaborate a bit on the sheet drain board and how that works?

Thanks again!! And God Bless!!

\o/ Paul

07-08-2004, 05:49 PM
It's essentially a dimpled plastic sheet (3' wide) with a mirify fabric on the dirt side to allow the water in without being clogged by dirt. The plastic side goes against the concrete (dampproofing) and it runs down the foundation and is then turned out to where the perimeter drain is.
We use a different product but I couldn't find a website for them.. this site is essentially the exact same www.tuff-n-dri.com

07-09-2004, 03:01 AM
It works basically the same as an airspace behind sidings. It provides a clear path for the water to migrate down to the drains. In my humble opinion it should be part of our building codes.

07-09-2004, 05:00 AM
Looks like a great product. I'll be checking it out much more closely!

Thanks again guys!

I'm sure I'll have more ???'s as I move ahead!

Take care and God Bless!!

\o/ Paul

07-23-2004, 05:34 PM
Sounds like you have things in hand but -- FYI there is an interesting product for footings on rock that make it easier. http://www.fastfoot.com Also provides an protective plastic membrane that helps keep water from seeping into or through the footings.

07-24-2004, 01:13 PM
Thanks for the info Twil! Yeah, it's too late for me to use it now, but after looking at the website I'm now wondering about PONDING and RISING DAMP...

I was scheduled to pour footers yesterday, but had to postpone due to T-Storms. Since I'm sitting on a rock ledge and had to put crusher run into my forms to level up the low spots, I'm thinking that water running past my perimeter drains and under my footers could definitely be a source of Rising Damp.

I'm wondering whether perhaps I should line my forms with 6mil plastic as a barrier to keep the water from wicking up into my walls from the footings?? Anyone ever done this or have any ideas about it?

Another issue....(To me, the dumbest questions are the ones you DIDN'T ask before it's too late, so here goes...) I'm wondering how in the world do you SLOPE a perimeter drain???? I mean, it follows a LEVEL FOOTING all the way around the house, right??? How much slope, assuming I can do it, should I need on the drain pipe??

\o/ Paul

07-24-2004, 05:34 PM
I think the 6mil plastic idea is a good one. You can leave enough over the forms to be able to do (with a little extra work) most of what fastfoot does to protect from rising damp and water seeping through the footings. I don't see how it could hurt. If you protect the plastic from being punctured by backfill & drainage stones and seal it to what ever you use for foundation water proofing you should have a sealed footing/foundation.

Not sure about the other issue. I'm looking at using fastfoot because I'll be building on rock and the rock doesn't have much slope. The water will likely pond in some areas but the membrane/plastic will hold it back until it raises enough to flow out the drain tile. There will likely always be some ponding there and that's why I'm looking to do something to prevent penetration of the footing and raising damp.

07-25-2004, 11:50 AM
Hi – just searching for info…

We will be doing the same thing soon in cottage country. I visited a well made home and it seems he was told to let the rock breath. In other words – water comes in and is routed out or simply aimed at a sump. He was told not to try and block or seal it from the inside. This home is about 20 years old and has no cracks in the plaster.

The place is used only for ½ year. Interesting even with the heat off and temperatures below 0 F the rock will not freeze. They can leave bottles inside all winter with no damage.

He runs 2 de-humidifiers and never opens a window or it would get very damp.

So it’s good but you need to do what the rock wants and you need to find examples of similar situations not a guess or your lovely home may be uncomfortable.