# Putting sand down before laying a concrete slab foundation?



## KingAndrew (Mar 7, 2020)

Let me start by saying that I'm new to this. And to this forum.

My research tells me that sand is the most ideal material to have underneath any concrete foundation.

I'd appreciate some perspective on this: *Would it be worth having a 3-foot-thick layer of sand put down before laying down a slab foundation for a house?*

I'd like to get at least 40 years out of the home without any cracks in the foundation that could allow water intrusion. 

How much of an impact do you believe this would have on structural integrity?


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## mark sr (Jun 13, 2017)

Locally gravel is laid under a slab both to build it up and provide drainage. I would think sand that deep would sink when the concrete is placed [but I'm a painter not a concrete guy]


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## Nealtw (Jun 22, 2017)

When we do foundations they always back fill with sandy soil if it is available on site. No organics, no clay, and no top soil. Not all sand is compactable so mostly they use a mix of like 3/4" crushed right down to sand. If you are going to spread fill and do concrete soon then you will want to compact layers as you fill it up.


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## huesmann (Aug 18, 2011)

For any particle mix to compact well it needs to be well graded. Particles of the same size don't "pack" very well.

I wouldn't use sand for a slab foundation; I'd lay down some geotextile and put gravel on top. Not the rounded pebble type gravel—sharp cut #57 or similar.


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## Nealtw (Jun 22, 2017)

https://www.multiquip.com/multiquip...dbook_low_res_0212_DataId_59525_Version_1.pdf


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## SeniorSitizen (Sep 10, 2012)

Sand is about all that's used in our area whether it be residential or commercial but it* MUST* be a particular class of sand of the many classes available. I no sand enjiner but I'm fairly certain beach sand wouldn't work very well or the blow sand of the Sahara Dessert. In the two photos of piles of sand they are both dumped on what we locals call sand but the native sand seems to have just a little too much clay.


There are a few quarries in the our area and even those can be of different coloration but when testing the end result is nearly equal. The sand pictured is similar to that which might be used in foundry castings. 


In my estimation if this was used in footing trenches in the expansive soils we read about so much in TX the foundation and drywall cracking would be mostly eliminated.


I've no idea why 3 ft. would be required being 12 inches of well compacted is fairly common but you may have a specific reason. If you can locate the correct sand go for it and your ready mix company or concrete contractor may have some insight of this type sand in your area.


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## Bud9051 (Nov 11, 2015)

In addition to what you put under the slab it is important what you do around the slab. IMO the slab needs to be placed well above any water table and no poured in a hole level with surrounding elevations. Basically you want well drained soil all around that slab and a slope that carries water well away.

Bud


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## SeniorSitizen (Sep 10, 2012)

Bud9051 said:


> In addition to what you put under the slab it is important what you do around the slab. IMO the slab needs to be placed well above any water table and no poured in a hole level with surrounding elevations. Basically you want well drained soil all around that slab and a slope that carries water well away.
> 
> Bud


I suspect that would be good advise for any structure with the exception of Venice and a few others around the globe. :biggrin2:


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## Bondo (Dec 8, 2007)

Ayuh,..... 'round here, a foundation footer is poured on _Undisturbed_ hard pan, or possibly a few inches of well compacted crusher run stone, if hard pan isn't there,.....

To me, 3' of sand is the worst possible sub-grade for anything you don't want to move later,....


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## stadry (Jun 20, 2009)

bondo's upstate ny where there's lots of 'hard pan' ( geological term is 'canisteo *loam',,, ixnay on the sand as it is NOT compactible,,, we use 4" standard road base mtl - run of crush, crusher run, gab, gabc compacted- lots of names for the stuff
everyone forgot VAPOR BARRIER PRIOR to placing conc ! stego is the most often spec'd mtl by architex & engineers - 15mil ;-) you won't get a chance to do this the right way again so don't cheap out OR you will face future water issues
IF your soils engineer says reinforcing fabric mat, use it*


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## Nealtw (Jun 22, 2017)

stadry said:


> bondo's upstate ny where there's lots of 'hard pan' ( geological term is 'canisteo *loam',,, ixnay on the sand as it is NOT compactible,,, we use 4" standard road base mtl - run of crush, crusher run, gab, gabc compacted- lots of names for the stuff
> everyone forgot VAPOR BARRIER PRIOR to placing conc ! stego is the most often spec'd mtl by architex & engineers - 15mil ;-) you won't get a chance to do this the right way again so don't cheap out OR you will face future water issues
> IF your soils engineer says reinforcing fabric mat, use it*


You have said that about sand before, I posted a link in post 5, have a look at the bottom of pg 7.


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## stadry (Jun 20, 2009)

*respectfully, you framed houses to a min code rqmnt while i built/repaired roads, bridges, tunnels, airports, & houses,,, if YOU want to build on sand, its your house so have at it,,, if YOU were the builder, you'd have responsibilities after the sale,,, i don't think framers have that exposure,,, i would never advise anyone to use sand*


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## Yodaman (Mar 9, 2015)

Unfortunately whether you like it or not. your concrete is going to crack. The best you can hope for is to keep the cracks at designated control joints. And when it does crack it stays together from rebar re-inforcement.
Personally I like to add in long strand fiberglass along with the rebar. Although it does effect the finish some and cost a little more per yard, I like the trade off.


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## Nealtw (Jun 22, 2017)

stadry said:


> *respectfully, you framed houses to a min code rqmnt while i built/repaired roads, bridges, tunnels, airports, & houses,,, if YOU want to build on sand, its your house so have at it,,, if YOU were the builder, you'd have responsibilities after the sale,,, i don't think framers have that exposure,,, i would never advise anyone to use sand*


I didn't advise that either because he said slab where there is a chance of wash out, but back filling a foundation any day.


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## KingAndrew (Mar 7, 2020)

Awesome information in these responses so far, thanks so much everyone! This is all very helpful.


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## stadry (Jun 20, 2009)

*& we're not fans of fiber,,, still think wire mesh is best choice over fiber even realizing difficulties placing @ correct elevation,,, fiber got its nose in the tent by conc jabonies leaving wire mesh where it absolutely did NOT belong
more important to have a proper contraction joint installed at correct time to prevent random cracking ESPECIALLY in slab homes,,, work w/your designer to ensure contr jnt pattern is hidden by partition sole plates as much as possible*


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## KingAndrew (Mar 7, 2020)

stadry said:


> *& we're not fans of fiber,,, still think wire mesh is best choice over fiber even realizing difficulties placing @ correct elevation,,, fiber got its nose in the tent by conc jabonies leaving wire mesh where it absolutely did NOT belong
> more important to have a proper contraction joint installed at correct time to prevent random cracking ESPECIALLY in slab homes,,, work w/your designer to ensure contr jnt pattern is hidden by partition sole plates as much as possible*


Any particular reason that you advise against fiber?


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## stadry (Jun 20, 2009)

*imo, an unneeded expense - + makes finishing the slab difficult as it results in a 'hairy' surface,,, much our work we do is decorative staining/sealing so fibers are a pita requiring diamond grinding 1st to eliminate them
then again, i'm no fan of drilock as its rarely a solution,,, alcohol IS a solution but not for wtrproofing *


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