Running Duct under Slab... Good Idea or Bad Idea???

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Forum topic by jcalberto posted 03-20-2013 03:18 AM 8166 views 0 times favorited 34 replies Add to Favorites Watch
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9 posts in 2705 days

03-20-2013 03:18 AM

Hi Guys,

I am building my house and in a few days concrete will be poured in. My plans is to install 7” duct in the slab where my workshop is going to be. The benefits doing this that I have come up is that I will hide all the duct work with the exception where it sticks out of the slab for each of my stationary tools. The other benefit is that I will save ceiling and wall space. Now the questions that I have is this a good idea and has anybody here done this. Also I prefer using metal duct due to its low resistance coefficient but if I do will it corrode over time due to moisture in the slab or should I use pvc? Last, I am thinking that for the main run underneath the slab I should use 7” diameter pipe to allow maximum cfm at the tool end. Would this size be fine?

Thank You,

34 replies so far

View Mainiac Matt 's profile

Mainiac Matt

8548 posts in 2526 days

#1 posted 03-20-2013 03:36 AM

You’ll need a pretty hefty blower to get the velocities required to keep chips entrained in the air stream with 7” pipe. And build up of deposited debris in under the slab piping doesn’t sound like a good idea at all.

I personally think its a great idea to put it under the slab… but I would only use PVC.

Let’s just say that I don’t think you’ll need to worry about grounding the exterior….

You’ll do well to put a couple cleanouts in…

And you’ll want to have your shop layout planned out in pretty good detail, inorder to make sure the risers are located in the right spots.

If you don’t back fill with good material (stone) and don’t compact well enough, or don’t have sufficient depth of fill over the pipes, you’ll likely get a nice crack line in the poured floor running right above your pipes.

I put in the 4” black flex hose around the inside perimeter under my basement slab, just in case I needed to mitigate for radon later on…. and can tell you exactly where the pipe is…. right under the cracks :^(

-- I yam what I yam and that's all what I yam

View Fred Hargis's profile

Fred Hargis

5171 posts in 2691 days

#2 posted 03-20-2013 01:30 PM

I’ve reworked my ductwork probably 8-9 times over 10 or so years, once was for a move to a different house and shop. That’s what I think may be the biggest downside to putting it in the slab…it’s permanent. You will add tools, upgrade tools, or just change the shop layout. But if I did put it in the slab, it would be PVC….

-- Our village hasn't lost it's idiot, he was elected to congress.

View HorizontalMike's profile


7770 posts in 3111 days

#3 posted 03-20-2013 01:49 PM

Since you have the fortunate ability to build from the ground up, I would suggest adding a ~6’x6’ slab on one side of the to-be-shop slab and then build an attached room for your DC system. Then duct the shop from there. I would also consider having/building the shop with at least 10ft ceiling (I have 12ft) for head space and/or ducts.

I think Fred is correct in suggesting that shop layout is an evolutionary process and likely to change in the future. Keep your options open…

-- HorizontalMike -- "Woodpeckers understand..."

View MonteCristo's profile


2099 posts in 2386 days

#4 posted 03-20-2013 11:15 PM

7” ? That’s pretty darn big and an odd size to boot. Better have one honkin’ big collector to keep the air moving fast enough. As someone else mentioned, being able to clear clogs is essential. I don’t think rust will be an issue. Even if it were, you’d just end up with a nice concrete lined hole anyhow !

-- Dwight - "Free legal advice available - contact Dewey, Cheetam & Howe""

View LukieB's profile


966 posts in 2527 days

#5 posted 03-20-2013 11:36 PM

I personally think it’s a fantastic idea, and dream of one day doing it myself. I agree that PVC is the better way to go, and that backfilling properly is important. And I think 6” would be plenty big for a main line.

Hope it works out good for you, keep us updated along your journey!

-- Lucas, "Someday woodworks will be my real job, until then, there's this"

View 1yeldud1's profile


301 posts in 3239 days

#6 posted 03-20-2013 11:53 PM

I see you are from Texas. In some parts of the country moisture under the slab could cause problems if it was to gain access to inside the pipe. If I was to install duct work for a dust collector under the slab here in Missouri I would have to us a lot of fore thought to keep moisture out of the pipe (ground water as well as the pipe sweating). Possibly having the pipe insulated or having the pipe “sloped” to a area of the pipe that i could pull put a drain plug in. Something to think about.

View johnstoneb's profile


3054 posts in 2370 days

#7 posted 03-21-2013 12:43 AM

While putting the ductwork in the slab sounds like a good idea. It’s permanent and can’t be worked on without tearing a floor the 50’s a lot of houses were built on concrete slabs and the plumbing was in or below the the 60’s those pipes had reacted with the concrete in some areas and in others the water was slightly acidic and had eaten holes in the pipes repairs were almost impossible.
You may have your shop layout plan now but that is evolutionary. I am building a 16×24 shop right now and I have changed my shop layout 3 times in sketchup and still not sure that is how it will be when I actually put the tools in the shop. What about adding new tools.

-- Bruce, Boise, ID

View longgone's profile


5688 posts in 3506 days

#8 posted 03-21-2013 01:04 AM

I have no idea how many people put their tools in one spot in their shop and kept them there forever…My shop is 5 years old now and I have added tools and rearranged it several times. My ceiling is 10ft and the ductwork is no problem at that height. I even have some ductwork running along a wall. I sectioned off a 5×8 section of my shop as a sanding room a couple of years ago and extended ductwork into it. Easily done with it running at the ceiling.

Personally, I would never want my ductwork in the slab…

View jcalberto's profile


9 posts in 2705 days

#9 posted 03-21-2013 02:23 AM

Hi Guys,

Thank you all for your inputs. I originally got the idea after seeing a workshop setup where the duct work was done underneath the workshop floor. The only difference is that in that workshop the floor is suspended off the ground but in Central Texas all the floors are made of a solid concrete slab with no crawl space. If I were to go with PVC I am not worried at all it reacting with the concrete or the road base back fill dirt that they are pouring to build up my foundation. The PVC I was planning on using is the same PVC that is used in all the slabs here in texas for the waste water which I hope is made to not decay due to bad reaction with its surroundings. Now, I was thinking of 7” pipe after reading an article from Oneida where it recommends not using 4” pipe until you get closer to the tool otherwise you end up cutting the cfm by half. I believe in their example they said it was better to use 6” but I thought maybe upping it by 1” more would be better but maybe this is too much.

Now the thing that has me worried after reading your feedback is the potential loss of flexibility to moving my tools later on. I already have most all the major big stationary tools so after sketching them and placing them on sketchup I felt really confident where I want to permanently place them but I guess by nature one always gets tired of one setup. Regarding leaving holes in the slab if I sell well I am not too worried either. This is a attached workshop that I am building separate to my attached two car garage. I don’t plan to move ever but if I do, new buyer can always cover with temp plate or if he/she likes pour concrete.

Here is a image of my workshop sketchup.


View jcalberto's profile


9 posts in 2705 days

#10 posted 03-21-2013 02:29 AM

So I was going to run the pvc underneath the slab starting from the two state dust cyclone across diagonal towards the band saw. Ass it passed across, it will have branches that will come out of the slab next to each power tool.

View woodbutcherbynight's profile


5676 posts in 2606 days

#11 posted 03-21-2013 03:34 AM

I would rec not believing you “know” where everything will go forever. As others have suggested you change as you go. Case in point. I bought a mini lathe 10 years ago, made a nice workbench with lots of room to work. Who would know that my Father would give me a second minilathe, with a second set of all the same chisels I already had? So much for plenty of dedicated space with lots of room to grow I spent alot of time working this out to have two lathes side by side and still have not added the extra port for dust collection.

HorizontalMike +1 He suggested modifying your building to have a 6×6 extra room. I have such a setup and getting the compressor and dust collection out of the shop is a big plus. Having a system you can expand and repair is a big consideration. Heck who knows you could be sitting on the next discovered tectonic plate??? (laughing)

-- Live to tell the stories, they sound better that way.

View NGK's profile


93 posts in 2108 days

#12 posted 03-21-2013 04:30 AM

Some good ideas above: However, here are some more points to consider. It it’s not too late, put your dust collector motor and impeller OUTSIDE the shop, even if it’s just covered with a roof. That gets the NOISE out of the shop. Have the impeller outlet return to the shop so that you don’t lose “heated” or “air-conditioned” air. Regarding noise, the greatest source of noise is not the motor itself, but the “exhaust” from the impeller. And that is the precise location for a “muffler”. Many companies sell them, including PENN STATE and the reduce the noise by 50 percent.

No big difference between metal piping or PVC below the floor in the concrete. As one responder said, even if the metal were to rust and decintegrate, you’d essentially just have a concrete tube. Cracking can be eliminated or minimized by (1) digging the “hole” 6-7 inches deeper where the tube will run, and (2) by running rebar across it on top about every 2 feet.

As another suggested, leave access to the underground portion for clean-out in case slow cfm ever contributes to a plug-up. You could even leave a cable between two different clean-out ports and drag a chimney-sweep or other device through that area as needed. Most likely to happen if air flow is reduced with (1) too many ports open or (2) the chip barrel fills up and reduces air-flow in the filter area.

Don’t worry about the diameter being too big, especially ear the motor. I’d start with whatever diameter is by the impeller. Then as you get farther and farther from the power unit, reduce the diameter until you have the typical 4 inches at each machine. Avoid sharp bends. Use Y-fittings instead of T-fittings. Use gradual reducers instead of sharp-cornered reduces. Use long sweeping elbows.

Yes, no initial plan will be perfect as your shop evolves. Plan the lay-out so that one wall has machines that do NOT need dust collection. A jointer, for instance, really doesn’t create dust—it makes chips—therefore you don’t need dust collection there—just a box. I don’t feel a drill-press needs dust collection. And maybe not a bandsaw.

And while you’re thinking of underground stuff—run some electrical wiring through plastic piping in the concrete. I have one such line centrally located where I located my table saw—a four-way box with both 220 and 110. And I have another one with multiple outlets near my assembly table. You might want a floor-sweep, too.

My shop is 32×32 and I have never had a plug-up. Ceiling is 8 ft, 8 inches with most piping on the perimeter of two sides, where ceiling and wall meet. One branch angles off from near the cyclone over to the table saw where I have a down-draft sanding center as part of the outfeed of the table saw. Air is also connected to my radial arm saw, two edge sanders, a router table, a 15-inch planer, a 12-inch drum sander, and a 37-inch double-drum sander.

View sprucegum's profile


324 posts in 2195 days

#13 posted 03-21-2013 07:08 PM

I have a friend who has his shop in part of a old Dairy barn. He put his ducts in the gutters (concrete trench that the cow s&*t drops into he then covered the gutters with some plywood panels. It makes a nice system and it is very easy to change and repair. I see no reason you cannot use the same system. The easiest way to make the trench bottoms is pour them before the slab goes in, just make the bottoms a few inches wider than you need them then you can set the slab forms rite on top of them. Some blocking between the form boards will keep the concrete from pushing your form around provided you fill both sides equally

-- A tube of calk and a gallon of paint will make a carpenter what he ain't

View joeyinsouthaustin's profile


1294 posts in 2270 days

#14 posted 03-21-2013 08:51 PM

+10 to the gutters Idea…..Also add an 8” or bigger line connecting the gutters to a central point it the middle of the room. Think of this as a chase you can run flexible DC hose and electrical through if your shop changes. Also it is much easier to pull flex line out of a chase to clear a clog, than pulling pvc out of the slab :)
Carefully mapped it would also be easier to drill down into the chase in the case of a big future change.

-- Who is John Galt?

View MrRon's profile


5190 posts in 3441 days

#15 posted 03-21-2013 09:14 PM

I think 6” is large enough. If space is a concern, you can run the duct thru the wall and run it along the outside. I would use PVC. I don’t like ductwork, wiring or pipes that can’t be accessed. Ducts in the ground, should they break, would allow ground water, if any to get into the DC system.

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