I was talking with one of our clients yesterday. His builder was concerned because constructing the new pole building first, then pouring the concrete slab seemed backwards to him.

Here is the information I shared with the client:

While the preference is to have the building shell completed prior to pouring concrete slabs, at the very least roof should be installed.

Building columns tend to grow “bull’s-eyes” in the presence of pre-mix concrete trucks. A completed building shell is far more resistant to potential damage. Pouring slabs with columns only in place, adds to the risk of one inadvertently being knocked out of plumb.

This blog is not meant to provide the necessary instruction to pour a building slab. Not because the task is beyond a novice’s abilities, although many do contract out this job. Pouring a slab is within most people’s abilities. However, unlike wood framing, which can be corrected if improperly constructed, work on a slab is “set in stone”. Due to this, and the fact so many local codes and practices apply to concrete slabs, I am only going to touch on this subject. If deciding to personally undertake pouring your own concrete slab, I suggest talking with local professionals to know what you are getting into. Have  a building inspector (usually a requirement in permitted situations) or a professional inspect work before pouring concrete. If less than 100% confident, hire a professional to work alongside during the concrete pour.

If in “frost country” a sub-base 6” or thicker should be first placed across the site. To maintain frost-free soils sub-base should be such that no more than 5% (by weight) will pass the No. 200 sieve, and it is further desired no more than 2% be finer than .02 mm.

Prior to pouring, 2” to 6” of clean and drained sand or sandy gravel is spread below where concrete is to be poured. Mechanically compact fill to at least 90% of a Modified Proctor Density, so as not to cause slab to sink.

Install a good vapor barrier (15ml thickness conforming to ASTM E1745) below any interior pour, to stop moisture from traveling up into slab through capillary action. Run vapor barrier up inside of skirt board (splash plank) by six inches. Vapor barrier seams should be overlapped by six or more inches and sealed with vapor tape.

The best insulation product to use under concrete slabs is closed cell polystyrene with a minimum R -10 value. Place directly on top of vapor barrier and below concrete. Insulation board seams do not need to be taped.

If not using fiber-mesh or similar reinforcement additives to concrete mixture, place wire mesh or rebar (reinforcing steel rods) in slab center to add rigidity to concrete to aid in minimizing cracking. Concrete should have a low-water mix.

Local code will dictate such things as slab thickness (usually 4” nominal), wire mesh sizing, gravel or sand layer thickness, and size and rebar location. Many garage or shop slabs also have a center drain. In the event structural engineering for a concrete floor (or any concrete or other masonry footings, foundations, walls, or retaining walls) is required or requested by you, or a building official, a registered professional engineer should be consulted for the design.

On solid walls of building, the pressure treated 2×8 splash plank will serve as forms for pouring a slab. In open wall areas, or across sliding or overhead doors, a 2×4 will need to be temporarily placed as a form.

Prior to pouring a nominal 4” (3-1/2” actual completed) thick concrete slab in building, finished, graded compacted fill TOP will be even with splash plank BOTTOM. If a thicker floor is desired, excavate below splash plank bottom, by any slab thickness greater than 4”. In no case, will the concrete floor top, be even with either top or bottom of the splash plank. Using any other measure for the concrete slab top, will result in wall steel and doors not properly fitting, as well as interior clear height loss.

In other words – after the floor is poured, when standing inside your new building, approximately 3-3/4” of the splash plank will be visible above the top of the slab.  

In the event a professional is hired to finish your concrete slab, most often costs can be reduced by directly paying the local pre-mix company for the concrete. Many offer discounts for prompt payment, so do not be afraid to ask. On a properly leveled site, a pre-mix concrete yard will cover an 80 square feet area, nominally four inches thick.

My last words of wisdom here…my best concrete slabs, were always poured by someone else.  I have learned when DIY jobs are truly DIY, and when to leave them to…a concrete professional.