Need some opinions on putting in a slab for a building

[Jeff]

I've been cutting some oak trees here this winter to clear the spot where I want to put a future building. It will be a storage/shop/ pigroast pavilion.  The building will have over hangs on two sides large enough to have a half dozen picnic tables in a pigroast rain emergency.

Anyhow, what is the consensus on just burning the stumps down below grade and putting a slab over top, or do I need to get them completely dug out. I plan on using reinforced fiber concrete. There is no top soil to speak of and the solid type is pretty much gravel. This area actually was bought at one time to be a gravel pit, but the gravel vein did not go deep enough to make economic sense, so it was subdivided off.

[Planman1954]

Hi Jeff:
Well, first of all, I realize you are way up North, and I'm way down South, but I'll bet the same applies there, as here. Before someone builds a house slab down here, it is advised to DIG OUT the root system. My brother is a home builder, and the few cracked slabs he has encountered involved areas where trees had been removed. It seems that the stumps were removed, but the extended root areas settled with time. Now, we typically dig out a large hole where the roots were and backfill and compact the fill as we go for good slab support.
That being said...with the light load that you will have (simply a roof) I personally would not get too worked up about this UNLESS the are where the tree was is directly under a perimeter support footing. Then I would zero in on it, and dig it out well. I don't know what your frost line is up there, but you need to get below it a few inches to prevent heaving of the slab in the future.

[Gary_C]

The answer will probably depend on the water table in the soil where you are going to build. The water probably moves fairly well underground with that gravel in wet times but if it has some spots where you are going to build that hold water or water flows underground at times you can have frost heave problems.

Also you should consider good gutters and drainage for the roof to keep the water away from the slab.

[Jeff]

Frost went as deep as 8 ft+ this year in some places I have heard. You just are not going to get below frost with an unheated building.  You can stick a hose in the ground here and never create a puddle.  Also, just twenty feet away from where the building would be, there is about a ten foot drop to the road, then on the other side of the road, there is an 80' drop again.  No drainage problems here at all.

[Gary_C]

Then as long as you do not have any terrain nearby that is much higher in elevation than you, it should not be a problem to leave those stumps just below the ground.

[barbender]

I'd get those stumps completely out, Jeff. From my experience (I did prep work for asphalt driveways and concrete slabs) any organic material left behind will come back to bite you down the line. Large rocks near the surface were always dug out as well, unless they were simply too large to move. I just drove by a drive way that I did about 7 years ago, I remember there was a rock in the base I couldn't get out without hitting a phone line that was run down the middle of the drive. I had to leave it, now you can see the round hump in the asphalt where the rock has been heaving. Basework is no place to cut corners, IMO.

[Gary_C]

any organic material left behind will come back to bite you down the line.

Any organic soil left under a slab will shrink when it dries out and yes, cause problems when rocks and other compacted or solid materials do not settle. But unless the stumps are rotten and punky and especially with oak, they will not be a problem under a slab. In fact they will be preserved.

And with concrete, you can tolerate minor spot settling with the use of reinforcing rod, wire, or fiberglass in the mix.

So leaving wet or uncompacted top soil behind could cause problems, but not the stumps.

[Autocar]

Just my opinion I would cut the stumps ground level and put my forums down with wire and pour the thing.

[isawlogs]

 http://www.barntoolbox.com/images/foundation-monolithic-lr.jpg

this is how I did my garage\shop floor, I did not put 6'' but rather 3'' for the floor with rebar and no fiber added. It has held up with out any major cracks so far, it does have hairline cracks in it, but it did go through 6 winters of no heat in shop.

Jeff you really should take the top soil\grass out, have this on gravel, ( reread and saw you where on gravel )  : :P  )  the roots won't have any effect on the slab.

[snowstorm]

I've been cutting some oak trees here this winter to clear the spot where I want to put a future building. It will be a storage/shop/ pigroast pavilion.  The building will have over hangs on two sides large enough to have a half dozen picnic tables in a pigroast rain emergency.

Anyhow, what is the consensus on just burning the stumps down below grade and putting a slab over top, or do I need to get them completely dug out. I plan on using reinforced fiber concrete. There is no top soil to speak of and the solid type is pretty much gravel. This area actually was bought at one time to be a gravel pit, but the gravel vein did not go deep enough to make economic sense, so it was subdivided off.

no never. dig them out. gravel to fill and compact

[BEEMERS]

if you leave them under slab they will rot away causing the ground under slab to settle.You can use a lot of wire and rebar to offset this but at what cost? plus,you have the time and effort getting them below grade.I'll be more than fair on a price to make them go away....also..Dancers have a backhoe a minute away.I think I know where you live,I remember seeing the forestry forum truck there.
We can prep for the slab at same time..as far as compacting,put the hose to it..waters the best compactor. But make sure its well compacted or you defeat the purpose of removing the stumps. saturate completely and compaction is done.

[Jeff]

I called and left a message for Lee a couple weeks ago, and he never got back to me. I was going to try and trade him the big pile of cut split firewood I generated for digging them out.  I wonder if he is okay? I noticed after that he hasn't been burning wood. His wood pile is still almost full and no smoke has been coming out of his boiler in the garage.

I live on Bischoff, the log-look house last house on the end of the road, south side. I probably will need some fill too as the land has a little slope to it there.  I've also got to find out if the township is going to give me any grief to where I want to put it. Its on a different property description than the house.

[barbender]

I can't speak from experience as far as having a slab over stumps, as I have always removed them. I have fixed asphalt driveways where stumps moved and made potholes. We had a driveway where we put in the standard 6" of class 5, well compacted and everything. What often got us was what the excavation contractor who had been there before us had hid/buried so they could make a quick buck. Well they covered up lots of trees and stumps when they did the site prep, we don't know that. A year after we paved that driveway it looked horrible, spots settled, holes, humps. The owner knew it wasn't our fault but I was still ashamed of that job. Now Jeff, you can cover up those stumps, it MIGHT be fine. Red-mix being around $125/yard in my area, I wouldn't chance it. Even well drained soil has a constantly varying amount of moisture and we all know what wood does with varying moisture, not to mention the effects of frost. Also, if you dig them out, rent a plate compactor and fill the holes in 6" lifts, compacting in between. Water doesn't cut it IMO. I did this type of work for 10+ years, and I got to fix a lot of other peoples shortcuts.

[21incher]

I had a 30 x 40 addition put on a pole barn 3 years ago the builder used the fiberglass reinforced cement for the floor and told me it was stronger then wire reinforcing. Well this year 5 feet in from the north wall is a large ragged crack running the length (40 ft) of the building. The expansion cuts are 10 ft squares. So after seeing this I would not recommend the fiberglass reinforcing over wire unless you plan on cutting expansion cuts every 5 feet in a cold climate.

[kczbest]

Jeff, I am a contractor by trade and I recommend that you NEVER leave any organic matter under a foundation. Anything organic under your slab will not be preserved but will rot and cause you problems. Also the "fiber" used by most concrete batch plants is not rated to replace remesh wire. Google "Strux 90/40". It is the only fiber that I am aware of that can replace wire in a foundation. In the grand scheme of things it wouldn't cost much more to do wire and Strux 90/40. Here in Mississippi the Strux 90/40 costs about $8 per yard more than the worthless cheap fiber mixes. Just my opinion but if it is worth doing it is worth spending a little extra to get it right the first time.

[concretecutter]

dig it out and compact it well, use rebar in concrete never use mesh no matter what someone tells you i cut and remove slabs for a living 99% of the time concrete is pored and the wire is just laid on the ground and never even makes it in the concrete because you walk on it or never pull it up right plus even if it is done right there is no strength to wire just use rebar and save the head-ace down the ro

[BEEMERS]

Did you try the young Dancer?i think his name is Steve?anyway,let me shoot you a price on it stumps gone,any necessary fill in leveled to within a half inch using laser transit. Not disagreeing with barbender..not enough water does not get it...but enough water does..soak it...and you are sitting on the easiest most compactable soil conditions there are. If you want to be sure..get a plate compactor. How big are these stumps anyway? Plus whoever does the slab if you don't do it yourself should take the time to compact it to protect themselves.

[Jeff]

It may be a year before we actually put the slab in, so I know it is going to compact on its own. There is also an irrigation system surrounding the spot, so once we start that up, it can get soaked. 

Beemers, Let me know when you have time to run by and I'll make sure I am here.

The largest stump, the tree is here:
https://forestryforum.com/board/index.php/topic,66831.0.html

Oaks usually come out here pretty good. Sistered maples, that's another story. I don't have any of those this time and glad of it.

[thecfarm]

I would buit it up some too. Get the water away from the slab. It sure won't come up any.

[Corley5]

Dig them out, remove the topsoil, fill to the desired elevation and pour the slab next year after adjusting the grade if needed.  Slabs are too expensive to take chances with.

[BEEMERS]

Will late morning tomorrow work? call me 539-3211..Its funny,you have members here from all over the world,and I live 5 minutes away! Have we ever met? if not then its time.
Id love to see that museum too.

[Jeff]

Not sure about late morning yet, We committed to go babysit for a neighbors 3 foster kids in the morning and I'm not sure what time they get back from work. Tammy already went to bed, so I know better than to go ask her. :D   I'll call when we get free. It might be closer to early afternoon though, but there is a chance it may be earlier.

[Brucer]

When I built this place, I carved out a spot for the garage and used all the fill to level a parking lot (nothing is flat around here). I left the stumps on the part of the hill I was going to fill, figuring they would help retain the soil. Piled soil on top of the stumps, packed it down with the D7 every few inches, put a soaker hose on it after I'd got a foot or so of fill compacted.

After a dozen years the whole thing settled (evenly) about 4" where the fill was thickest. So I asked the local contractor to bring in a load of gravel for me to re-level the parking lot. When he backed in to dump his load, the rear wheels on one side dropped a foot . After we got him out, we found a huge hole that had been bridged over by a couple of feet of earth. It was enough to support my car and truck, but the fully loaded dump truck just punched right through.

It turns out that those buried stumps had rotted out and each winter a bit of backfill dropped down into the hole. In effect, the hole was gravitating upward, just waiting to catch something.

So I'd go along with the recommendation to pull the stumps.

[Gary_C]

When you are putting in any concrete slab, the best recommendation is always to remove all organic material and replace with a compactable base material or to place the concrete on well drained and undisturbed soil. Plus you should provide good drainage under the slab so no moisture is held there to freeze and cause problems with freeze/thaw cycles. And you need to consider the cost of removing all organic material against the risk of failure. So it always comes down to a decision weighing the risks against the costs.

So based on having oak (white oak especially) stumps in non-organic and well drained gravel base, I am saying the risk of any significant failure is very low. And the risk of further decay of those remaining stumps in the absence of significant moisture and oxygen is also extremely low. Plus even if those stumps did decay further and get soft, a fibre reinforced concrete can easily bridge a few soft spots in the base after curing. Especially with light loads.

And I would also point out the risk of extended settling of a base material with fines placed over a gravel sub-base is also very high, especially with a coarse round rock gravel. So there is a long term settling risk with either solution. I have seen waay too many compacted bases that settle and leave hollow cavities under concrete slabs.

So look at the costs and choose the best way you can afford, keeping in mind you are just using this slab to hold up some heavy weight guys, hopefully not all standing in one spot and not your heavy truck or dozer.  ;D

[petefrom bearswamp]

I used the method in Marcel's post for my mill slab, no insulation but added re mesh in the slab body.
Poured in 2001 or 2 and no problems yet.
When my house was built in 1993 my contractor buried a lot of organic stuff in my front yard.
The ground has been settling ever since and i have to fill the holes that appear.

[Bricklayer51]

Will the building be sitting on the slab, or the slab on the inside of the walls?

[Jeff]

I've not determined the method of construction yet. I just know when the time comes that I'm at the point I'm able to do swing it, I want the site prepared before hand as much as possible.  For now, that means the trees and stumps gone and possibly the grade set. My main focus is taking care of these things when the possibility of oak wilt infection is at the very lowest. I have predominantly oaks here and would not want to see them wiped out.

[scsmith42]

Jeff, I am a contractor by trade and I recommend that you NEVER leave any organic matter under a foundation. Anything organic under your slab will not be preserved but will rot and cause you problems. Also the "fiber" used by most concrete batch plants is not rated to replace remesh wire. Google "Strux 90/40". It is the only fiber that I am aware of that can replace wire in a foundation. In the grand scheme of things it wouldn't cost much more to do wire and Strux 90/40. Here in Mississippi the Strux 90/40 costs about $8 per yard more than the worthless cheap fiber mixes. Just my opinion but if it is worth doing it is worth spending a little extra to get it right the first time.

^^^^ I concur about organic matter removal (stumps, topsoil, roots) and not depending upon fiber to replace metal reinforcement. Would also recommend mechanical grade compaction and a low slump concrete mix when you pour...

[Jeff]

Not to much worry about top soil here. The only top soil to speak of gets brought in.

[Stephen Alford]

  The general practice here is if there is a source of good shale it's dug out then the stumps and organics go in that hole , the good shale is placed and compacted where the slab for the structure is to go.   In recent years there are  more weather events were water is running on top of the surface, so the slab site is always elevated. 
  The new idea (at least to me) when its used as a "shop " pertains to heating it.  Trenches are dug and non perforated drain pipe is laid. Plastic drums go on the far end of the pipe.  Fans are put in the drums and blow air through the pipes that come up in the shop.  As I understand it this will keep the shop temp at around 7 c.   Good luck with your project,looking forward to pics.   

[BEEMERS]

Ive done home sites,garage slab preps by the hundreds in this area..we'll get it right..especially if you let it sit for awhile and get a few good rains.Plus roots in your area don't go to deep theres nothing good to get into down below so they tend to stay closer to surface..which spreads oakwilt further,given a greater chance of grafting.
that being said will keep depth of disturbance minimal.
25 years of this and ONE problem...had a basement and attached garage job while I was backfilling basement  I told the Block layer/concrete guy that Id lay the fill in for free because I was already there.He directed me and I dribbled the sand in with an excavator as he needed..then he and his
laborer knocked it down and compacted it in lifts less than 12"
the fill was perfectly compactable sand but it was really dry and told him several times that I didn't like it and it needed water..it was almost dust.well was dusty.They did what looked like a perfect job.What do I know right? Im the dirt guy and they are the concrete guys...any way a year later the slab settled 6 inches against the basement wall and the rest was a mess.
All I did was dribble sand in as directed..for free...and argued that they needed some water with compaction,so why a problem for me?They said that I compacted it!!! I about crapped but a few threats to life and all was well..they tore it out.And replaced it.
Sorry bout the rant..Hey Brucer..I liked your post..there is a couple subdivisions here around Cranberry lake..little lots,little cabins built in the 60,s .They buried all the stumps from each lot right in the front by the road on each lot!All my life Ive been filling them in and I mean some you fill in and come back a few  years and it settled more and fill it again.I mean some are 3 foot deep.
Every year we get calls to haul fill and if it goes to Cranberry lake subs that's probably what its for...I make it real clear to my guys..reguardless of what the home owner says..do not drive through the hole..if they want to fill the hole..back up TO it and dump.Back up IN it and youll be sitting on the frame....yes I know from experience

[Reddog]

Jeff, let me throw out another option for a surface.

How about using 3/4" crushed lime stone with fines. Put 3" deep or more depending on budget.
Put a perimeter of pressure treated around the outside to confine the material.
Then water and plate compact. turns to a poormans slab after a few rains.

[Corley5]

A neighbor put "slabs" in a couple out buildings by spreading a layer of Portland cement, working it in with his rototiller and then soaking it with a sprinkler.  It made a hard surface  :-\

[barbender]

Beemer, I don't mean to say that water shouldn't be used. Just that it shouldn't be relied on for the sole means of compaction. But Beemer is right, dry sand will not compact. On our crew, we used class 5 gravel for most everything. It's easier to compact correctly, but then the company I worked for owner all the travel pits too. If I am putting in a slab base for myself, I'd use gravel except forf the last less than 1" of grade. That's just so I can screed the grade off after compacting the gravel. On the fiber issue, there are different fibers that can be added that add different strengths. For the most part, they add crack resistance (for small cracks) and they can also help keep outdoor concrete from spalting. Steel mesh also only helps with crack resistance, it doesn't add any actual strength to the slab. If you were to try spanning stumps, you better add rebar. That's what gives concrete the ability to span any distance.

[Planman1954]

Be sure to use concrete wire mesh (6x6x10/10). One fellow said it does no good, since many finishers let it lie flat. He is right. But the solution is to always use clip chairs. These elevate the mesh to the center of the 4" concrete, and all is well! Put the mesh in. ;D

[Rockn H]

A neighbor put "slabs" in a couple out buildings by spreading a layer of Portland cement, working it in with his rototiller and then soaking it with a sprinkler.  It made a hard surface  :-\

Soil Cement, the poor man's shop floor:)   I've even see the military use it alone for runways.