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. 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....-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.