Boiler shed - cement foundation or piers - need to decide asap.

[MrEd]

Needs some advice.

My tarm is going in a to-be-built boiler/woodshed. 12x24. In a perfect world I'd like to have a full cement foundation and slab floor, with footers below the 4' frost line. Made a couple of calls and got estimates of around $6000 for this work, which seems high to me (high in terms of my cheapness quotient, maybe its justifiable).

I've built plenty of decks and small outbuildings, but have always used sono-tubes/bigfoot combos and a PT lumber to frame it out near the ground, and I could do the same thing here if need be. Benefit being it will save probably $3500-$4000 and I can do it all my self with confidence and pretty quickly. Downside is 1) struggling with how best to insulate under a 'pole barn' type construction (all the ones I've done didn't require insulation) as well as keep the critters out, and 2) really struggling with having a wooden floor in my boiler room that would then need to be fireproofed somehow. (would a wood floor support a 2" cement slab on top?)

So my question is 1) for someone that is very handy, how hard would it be to build a 12x24' cement foundation with slab floor w/out screwing it up? - I figure if my labor is free, that I can do it for 1/2 the cost and makes it comparable in cost to a pole/pt/footer solution. Would using ICF's make this job easier for a foundation newbie?, or would conventional stick-built forms be easy to work with?

or else 2) if I chicken out and go with what I know - then what is the best construction method that will allow me to well insulate and critter proof the floor in this building? I am picturing 8 sono-tubes with footers, 4 - 12' ground contact 12x12's as beams, and then using conventional PT lumber 2x10's probably, to frame the floor either on top of, or in between, the 12x12's - can I lay rigid pink foundation insulation ontop of the floor joists and put the flooring on top of the rigid insulation? I can't see using fiberglass between the floor joists in this situation. Will animals chew their way thru rigid insulation?

other suggestions? I really need to get this building completed by end of July in order to have my boiler installed in August....

 

[sdrobertson]

I just completed a 24 by 40 polebuilding which I have a 12 by 24 boiler room on one end located in Michigan. I have no idea of your building requirements but I set the poles 4ft down with a bag of concrete around each pole. Two rows of treated 2by6's around the perimeter and a 4 inch slab of concrete for the floor. I didn't insulate the floor as I'll raise the boiler and insulated under it and insulate all four sides of the water storage.

 

[MrEd]

I am probably over-thinking this, as I am prone to do, but I just can't figure out how long term a slab, without a proper foundation, isn't going to heave and crack over time. I am sure in Michingan you have cold winters like in New England. DO you feel your choice of building will stand the test of time? Trying to imagine what even a 1 inch "heave" will do to my chimney and rigid piping in the boiler room.

 

[sdrobertson]

Here in Michigan we have two types of concrete - concrete with cracks and concrete that will at some point crack. As far as a inch heave, I'm not that concerned about that. The heave will come from lots of water in the soil that freezes and expands. Good fill properly compacted and proper drainage of the building site will stop this. As far as if you do get an inch of heave, it will be extremely slow and you could design your piping around this. If you use a metal chimney it should "slide" the inch through the collars at the ceiling and roof line.

 

[Ron Lloyd]

MrEd,

If it’s any consolation I’ve had no heaving or cracking in a 24’ x 72’ pole barn I had built about 15 years ago. 4 bays are open on one side and 2 bays are enclosed with garage doors. The slab is only in the garage end. The only issue I’ve had is water and slush dripping from the vehicles, refreezing, and causing the surface to flake off.

Like sd said proper attention must be paid to compacting the crushed stone and to drainage to prevent cracks and heaving.

Ron

 

[MrEd]

When I said cracking, I really meant those big structural cracks that go along with a heave and failure of a slab - no concerned about hairline cracks.

But you guys are making me feel better about just going with a slab for the floor and have the structure itself supported by a pole construction.

With that in mind, what would the basic steps to ensure a good firm base, and proper drainage to maximize the life of my slab?

 

[sdrobertson]

It really depends on the location. You want a good base of sand, gravel, or some sort of similar material. You can rent a soil compactor and pack the dirt really well. What you want to end up with is a firm base to pour the concrete on that will drain any moisture. Most of the moisture you will get depends on the water level in your area. Dig a hole now where you want to build and see if you get any water. You also want the dirt around the building to slop away so that you move any water from the roof away from the edge of the building so that it won't migrate under it.

 

[trehugr]

The excavator just got done here. Dug up a 24X20 area, filled that with about 10" of gravel, he dug my 80' ditch, 4' deep all for $600.

Will pour 20X16 slab 5" thick for $500. The slab will float, so if it moves, everything moves. Pex is foam in ditch method coming up through the concrete in a 4"PVC tube about 6" in length. A little strain relief on the pex.

All concrete cracks, don't worry about it

 

[sdrobertson]

(insulated slab under tarm & tank? rest via however. sauna tubes can heave)

I meant that you could insulate under the boiler and storage by raising them but above the slab.

 

[AndrewChurchill]

I had a 24 x 36 floating slab poured this Spring for my new garage. My father used to have a Concrete Construction company and he said to put #4 rebar in the foundation tied in 16" squares instead of wire mesh.

He said of all the slabs he poured doing it with the #4 rebar he never had one crack, other than a few hair line cracks which can't be helped.

It cost me an extra $500 but I'm going to rest a lot better not having to worry about big cracks.

 

[55Razor]

I have a Solo 30 in a 8 x 10 shed I built myself, and a 16 x 24 workshop, both with cement floors 2 years old, with no cracks yet and poured the same way. I poured 4" concrete over compacted sand with a 4" rat-wall about 18" deep dug down around the outside, and 6 x 6 re-wire laid in and bent down into the ratwall. Where the boiler sits, I also laid in a couple of pieces of 1/2 rebar criss-crossed for added strength. This is a common method for around here, seems to work well, and passes inspection.

 

[Dave T]

I have good bank run gravel I removed the top soil and poured a 10'x10' 8" thick slab of 4000# mix right over the gravel for my OWB 4 years ago it has been exposed to the elements from day one..I have a hairline crack in one spot this slab was made just like a sidewalk with broom lines and used a chopsaw to score the surface for controlled breakage.. Drainage is no issue with my install.. Good luck It'll be a breeze if you have to rent a HOE do it,big money saver DAVE