Concrete floor -trenching vs pouring

[Jon Snider]

New shop in planning so lots of recent questions. Thanks to all who have replied so far.

For my power tool cluster in the middle of planned 38 x 24’ shop is it better to leave a trench in concrete floor, or just pour concrete over dust collection pipe and electric conduit put in place?

The concrete will be covered by some type of wood floor TBD over small sleepers. Want to keep it as soft as possible or for backs and legs.

My early thoughts are trench is better better since I can just cover that with it’s own section of plywood (assuming I don’t go with more traditional maple type floor) and can access to repair or modify in future.

Thx

Jon

[Jim Becker]

Personally, I'd opt for a trench with a removable cover so that when "stuff happens"...you can get to the duct work or wiring to fix or change it. The trench should be cast with a lip that will support the cover which could be anything from steel plate to plywood. Select the "what" before you pour so that the support lips are sized appropriately for the material. This method requires more effort and may add a little cost, however, but it also provides the most flexibility over time for "under floor" resources.

Good choice on putting down a more comfortable floor surface to work on. If you do the trench(s), it would be pretty easy to attach whatever you floor with to the trench covers so all that's visible is the joint lines under normal circumstances.

[Mike Wilkins]

Having seen shop photos of a trench for dust collection and electrical wiring, I would vote for the trench. A simple 2 X 12 over a cast-in ledge seems to be the favored set up. Easy to access should a clog occur or you want to change the shop layout.

[John C Cox]

Absolutely trench.. That way you can get to it.. And if you ever change hobbies or sell the place - it will have a built in trench to run whatever comes next.. Pour the duct in and it's there till someone breaks out the excavator...

Another consideration is getting water back out of the trench.. I might slope it a bit and put in a well for a sump pump at the low end.... Of course - water will never ever get into it ;) ;) But just in case...

[Malcolm McLeod]

... trench should be cast with a lip that will support the cover...



H... a cast-in ledge seems to be the favored set up.
....

I've a fair amount of experience with 'utility' trenches in an industrial environment. Newly installed they look great, but go look at one 5-10yrs removed from new. (Seems like there was another thread about this several months ago?)

I'd strongly advise any ledge be formed of angle - iron or aluminum, preferably the former. (Spot weld bolts or small pieces of re-bar on the 'back', then fasten it to the trench forms, and cast it into the concrete.) With much traffic of any kind on a purely masonry lip/ledge, you'll get spalling on the edges and soon have more of a ramp than a ledge.

And beware of some mythical 'flexibility', or even serviceability. Flexibility? ....Quick move that trench 6" to the left.:confused::( Not. Expansion? The trench usually gets sized for what's in it at inception, not what you want to add 5 yrs down the road. Serviceability? 'Mine' were intended to allow in-place maintenance on the piping, but in reality this is nearly impossible. Fixing a leak generally entails removing an entire section, repair, then re-install. (Oddly (and sadly), leaks NEVER occur on the top of the pipes..?) So for piping/ducting, be sure you make allowance for getting one section out - whether by unions or some type of slip joints. ...Concrete won't flex at the trench ends.

Working on anything while standing on your head should generally be left off your bucket list. Hate to be the pessimist, but be careful what you wish for...

Lastly, maybe things have changed, but I suspect the price to get these formed and poured correctly will be scary - - YMMV.

[Peter Christensen]

Don't forget to leave room for compressed air and possibly vacuum cleaner piping in your trench. Edge lipping as Malcolm described.

[Jon Snider]

Assuming cost of trench gets too high, as alluded above, is it better to bury S&D pvc or metal dust pipe in the concrete?

[Jim Becker]

As much as I personally dislike PVC for dust collection purposes (mostly be cause of the lack of non-even-number size choices and certain types of fittings), it may be the better choice for "full immersion in a concrete bath". The same goes for conduit to handle electrical needs and conduit to route flexible air lines (PEX product designed for compressed air) out into the middle via the floor...PVC is the best bet for that and make for easy pulls in general.

[Jon Snider]

I’m going back through the workshop subforum in reverse order (and learning a lot), and am on page 50 and found this post

https://sawmillcreek.org/showthread.php?243069-Ducting-channel-in-concrete-slab

[Bryan Lisowski]

If you are still in the planning stage, and want to do a plywood subfloor, why not just raise the floor enough to chase the electric and DC above the slab. I get you may be limited on building height, but let's say you were planning for 12' to the finished ceiling, you could raise the building to 13' to the finished ceiling. To me this adds flexibility in the event you change layouts, etc.

[Joe Jensen]

I started at least one thread on trenches in the shop. It's still seductive and I am getting a price for the adder of trenches to the concrete bid but the builder expects it to be "scary". All trades here are swamped with business and something special like this may add a ton to the cost. I will post once I get a price.

[Jon Snider]

Brysn, Actually woke up with this very idea :). I am limited by height restrictions, but couldn’t I just pour my concrete slab 8” below grade and then build subfloor and floor up to grade?

Thx again to all for help. This is a great forum.

Jon

[Joe Mioux]

Brysn, Actually woke up with this very idea :). I am limited by height restrictions, but couldn’t I just pour my concrete slab 8” below grade and then build subfloor and floor up to grade?

Thx again to all for help. This is a great forum.

Jon


I'm just asking, not suggesting but if one was to pour a slab 8 inches below grade, why pour concrete at all? Wouldn't a vapor barrier with rock on top accomplish the same goal for less money? Just create floor joists/unfinished (very short) crawl space.

[Steve Rozmiarek]

I'm just asking, not suggesting but if one was to pour a slab 8 inches below grade, why pour concrete at all? Wouldn't a vapor barrier with rock on top accomplish the same goal for less money? Just create floor joists/unfinished (very short) crawl space.

I think Joe is right, if you want a softer wood floor, why even pour concrete? Save some $, and just pour some piers so you don't have to truss that big of an area, and you have the wood floor.

If you are thinking about future use of the building for something garage like, that may not be a solution though. If you do go with a concrete trench, the cost is because it has to be poured in steps. The rough excavation will be (depth of trench+slab thickness)=depth of excavation, so you end up with a pretty substantial bit of dirt work and forming to make it happen. After the trench is cured, the slab can be poured. I'd quote it at double the labor of a slab alone, maybe more.

[Joe Mioux]

I think Joe is right, if you want a softer wood floor, why even pour concrete? Save some $, and just pour some piers so you don't have to truss that big of an area, and you have the wood floor.

If you are thinking about future use of the building for something garage like, that may not be a solution though. If you do go with a concrete trench, the cost is because it has to be poured in steps. The rough excavation will be (depth of trench+slab thickness)=depth of excavation, so you end up with a pretty substantial bit of dirt work and forming to make it happen. After the trench is cured, the slab can be poured. I'd quote it at double the labor of a slab alone, maybe more.


Another option would be to pour a foundation and if the need ever arose for the a concrete floor, just pull the wood floor/joists out and pour the pad for the future garage.

[Jim Becker]

I'm just asking, not suggesting but if one was to pour a slab 8 inches below grade, why pour concrete at all? Wouldn't a vapor barrier with rock on top accomplish the same goal for less money? Just create floor joists/unfinished (very short) crawl space.
Yes, this is a possibility, although if it's below grade, you'll have the cost of concrete block walls or similar for the portion below grade...say two courses of 8" block. Local code and general building practices are going to come into play here as well as practical drainage considerations. Joe's suggestion of a crawl space is also worthy of consideration. Unless you're going to park vehicles, you don't necessarily need a concrete slab floor and even if you do have an area for that, you can do a slab only in that area and the crawl space with wood flooring elsewhere. Of course...cost is always a consideration.

[Bryan Lisowski]

Others have answered whether you could skip the slab. The other option was you could use a different truss design which wouldn't affect the overall height, but give you more interior headroom.

[John C Cox]

Jon,
What you must do mostly depends on where you live and the frost line... "Up north" deep foundations are required so they are down below the frost line... So the foundations don't heave...

The second obvious factor is "Do you live on a hill.."

And last it appears you are dealing with local building code requirements which change once you hit 8' "walls".... But remember - most of the time, "walls" don't include foundation height... So an 8' wall starts on top of the foundation crawl space.. Your local codes may vary - familiarize yourself with them...

Personally, it sounds to me like you would do much better with whatever foundation is required to get past the frost line, then add 18" or 2' of crawl space above grade. Install floor joists and floors... All your utilities and dust collector piping can go down in the crawl space... Then - WHEN climate control, an outdoor kitchen, and a bathroom is desired in the future - there is room for all that stuff too.. Because nobody will ever think to add a bathroom or AC. ;) ;). Pour a slab on grade and you are SOL...

I also want to put this idea into your head..... What do you call a sealed, cement enclosed below grade space? A swimming pool... If you can possibly help it - leave "below grade" behind... It will fill up with water and radon and make you miserable... And then you are talking french drains, sumps, sump pumps, radon vent systems, and all that associated stuff... Keep the project above grade and it's vapor barrier covered with gravel...


Brysn, Actually woke up with this very idea :). I am limited by height restrictions, but couldn’t I just pour my concrete slab 8” below grade and then build subfloor and floor up to grade?

Thx again to all for help. This is a great forum.

Jon

[Jon Snider]

Others have answered whether you could skip the slab. The other option was you could use a different truss design which wouldn't affect the overall height, but give you more interior headroom.

Thanks. I’m trying to preserve attic space for finishing room, boat storage, etc but will look into truss or hand rafter options.

[Jon Snider]

I met with Regional Building today with architect, plus friend from Ciy Planning. Some code and hillside overlay restrictions look to point this pretty much one way. If it’s a detached garage then I can only have a limited amount of total combined garage space which won’t work with existing garage. Plus there’s a limit of total “garage” doors. When I told them it was not a garage but a workshop with wood floors they became much happier. When I mentioned maybe floor joists on footers with no slab (and therefore no garage) they became estatic.

So it looks like my choice is clear. Will have a mini crawl space for all ducts, etc.

Oher interesting items from the meeting include setbacks and height limits. My house is surrounded by three “streets”, two dead ends, thus I’m told I have three front yards, all with 25’ setbacks. Plus for a detached structure height limit is 16’ for 4/12 roof and 20’ for 6/12 or greater so this may limit options for loft/attic. Might just go with open roof which will be very nice for height and will be cheaper.

To answer question above, building area is fairly flat. I’m in Colorado Springs.

Thx all for help. Appreciate expertise here.

Jon

[Chris Parks]

It's good to see a solution which is in fact what you wanted and simpler. As for in floor ducting I have suggested the following to several people putting in dust extraction. Run the pipe end to end of the slab so the duct ends are accessible for any blockages that may occur, simply unscrew a cap and put a drain clearing auger through it if needed and end of problem. I would also run a separate conduit for electricals and possibly CAT6 for networking, a lot of stuff will work that way in the future. I am installing a system for a woodworking school that will require CAT6 for control from each blast gate, the VFD has a removeable control pad and that will be in a central position using CAT6 and a master infra red control worn by whoever is in charge of the class. Five years ago none of this was feasible so it shows that forward planning is a good idea.

[Bill Dufour]

If you put in big ducts you have to anchor them down so they do no float up in the concrete. I suppose any size pipes may try to float up.
Bill D.

[Brian Backner]

For an elevated floor, you might want to look around for companies that install and decommission computer server rooms. The facilities they work on typically have all of the equipment set on a raised grid system of steel standards/rails into which 2x2' floor panels fit flush. All the wiring (and there is a TON of wire in these places) runs underneath. A friend rented out one of these spaces for his business- all the servers were gone and when we looked under the floor to see if he could run his compressed air and other things, there were literally miles of cable in the 1,200 sq ft space. One of these systems would have no problem taking the weight of even industrial grade wood working machinery and they're very easy on the knees/back. I'd bet you could buy such a system for little money - most of the ones getting ripped out around the Boston area go into dumpsters ....