wood framed floors and welding - what for a top layer?

nitsuj02532 · Apr 12, 2012

I'm in the middle of finishing planning and getting ready to start building a shop/shed. It's going to be 16x24, and has to have a wood floor, i would absolutely love to pour a slab and build it on that, but due to high ground water issues and what the winter can/will bring, and my lack of an unlimited budget, i cant do the concrete floor, so i'm going to pour footings 4' deep and frame the floor out of PT lumber, keep a gap under it so it can breathe and doesnt trap moisture, going to do a layer of PT plywood first to ensure my floor doesnt rot out on me. I do a decent amount of welding and i would like to keep the ability to do it inside, i'm trying to figure out what would be a good option for a durable surface that i can still cut and weld inside with out worrying about burning the place down.

I've thought about maybe doing a double 3/4 plywood floor (running sheets in opposite directions) then pouring an inch or two of concrete on top of it, maybe even put wire in it so it doesnt crack as easy, but im wondering if thats my best option or if i could get around it and do something else, or if there is a better option

Buckgnarly · Apr 12, 2012

Wood and LOTS of welding does not sound like a plan in any way shape or form...... :dunno:

Garage Flooring · Apr 12, 2012

nitsuj02532 said:
I'm in the middle of finishing planning and getting ready to start building a shop/shed. It's going to be 16x24, and has to have a wood floor, i would absolutely love to pour a slab and build it on that, but due to high ground water issues and what the winter can/will bring, and my lack of an unlimited budget, i cant do the concrete floor, so i'm going to pour footings 4' deep and frame the floor out of PT lumber, keep a gap under it so it can breathe and doesnt trap moisture, going to do a layer of PT plywood first to ensure my floor doesnt rot out on me. I do a decent amount of welding and i would like to keep the ability to do it inside, i'm trying to figure out what would be a good option for a durable surface that i can still cut and weld inside with out worrying about burning the place down.

I've thought about maybe doing a double 3/4 plywood floor (running sheets in opposite directions) then pouring an inch or two of concrete on top of it, maybe even put wire in it so it doesnt crack as easy, but im wondering if thats my best option or if i could get around it and do something else, or if there is a better option

For starters, I would go to your local lumberyard for some value engineering of what you are looking to do. The first thing I am concerned about is pressure treated plywood and lumber in general has a tendency to warp and twist. Talk to them about using a foam sill sealer, pressure treated plate and then discuss what type of plywood is going to be best. The do make a fire rated plywood and there are systems such as cement board over plywood that you may be able to get a fire rating from. You may want to check with your local ProBuild. I also have resourses regarding calculating the ability of a floor to resist heat including formulas, etc. If you get to that point and you need it, let me know.

The prep that needs to be done to the floor beyond that is going to depend on what you use and how you use it. If you go with an epoxy some our brittle and WHEN your floor moves may crack. Others are very flexible.
Another option would be to go with a PVC Garage Floor Mat which meets DNI standards laid out by auto manufacturers and use a welding blanket or other form of protection in the area you intend to work. We have customers who work above PVC floors and even tiles without damage but they are not welding directly on the surface.

nitsuj02532 · Apr 12, 2012

Buckgnarly said:
Wood and LOTS of welding does not sound like a plan in any way shape or form......

right, which is why i want to maybe figure out a way to do a subfloor of wood and then something on top that will be resistant to the heat.

GarageFlooringLLC said:
For starters, I would go to your local lumberyard for some value engineering of what you are looking to do. The first thing I am concerned about is pressure treated plywood and lumber in general has a tendency to warp and twist. Talk to them about using a foam sill sealer, pressure treated plate and then discuss what type of plywood is going to be best. The do make a fire rated plywood and there are systems such as cement board over plywood that you may be able to get a fire rating from. You may want to check with your local ProBuild. I also have resourses regarding calculating the ability of a floor to resist heat including formulas, etc. If you get to that point and you need it, let me know.

The prep that needs to be done to the floor beyond that is going to depend on what you use and how you use it. If you go with an epoxy some our brittle and WHEN your floor moves may crack. Others are very flexible.
Another option would be to go with a PVC Garage Floor Mat which meets DNI standards laid out by auto manufacturers and use a welding blanket or other form of protection in the area you intend to work. We have customers who work above PVC floors and even tiles without damage but they are not welding directly on the surface.

Fortunately, from the building perspective, my father-in-law is a licensed builder and has been for about 25 years so for the structural design i have a good resource, hes the one that actually suggested looking into pouring a thin slab on top of the plywood floor to obtain heat resistance, were just trying to figure out what is going to need to be done, and then designing the actual structure around it to support the load...just not sure what my thin set concrete options are and whether its my best choice or not

I know the plywood twisting could be a problem, but depending on how heavy a floor i put on top of the joists, i may have have joists close enough together to attach it, that the twist wont be too bad, especially if i lay a second layer of plywood, opposite direction and maybe even use an adhesive to bond it together, unfortunately there is no probuild near me (about 2 hours away). I cant imagine theres much that ill be welding right on the floor, i can only think of a handful of instances that i have had to do it on the floor in the past, i do have a few steel tables i use when welding, but im sure cutting i will have slag and red hot metal hitting the floor.

i'm not too worried about keeping the floor pretty, as i am trying to avoid catching the place on fire, im planning on putting up sheet metal on the walls in the area ill be welding the most as an extra precaution to keep anything flying away from the wood surface as much as possible

magnusk750 · Apr 12, 2012

You want to make a wooden floor because of possible problems with moisture in the soil? Doesn't really make sense to me, even if p t and boat plywood is used :headscrat

nitsuj02532 · Apr 12, 2012

magnusk750 said:
You want to make a wooden floor because of possible problems with moisture in the soil? Doesn't really make sense to me, even if p t and boat plywood is used

I have real high ground water. if i do a poured slab i'm going to have to put drainage below and a sump pit and pump, if i dont im going to have to take the chance of losing my floor and possibly even shed every winter from frost heaves, the water table is high enough that a wet fall/winter and a good freeze could become a problem, sorry i should have been more specific about that before, doing footings is realistic, doing a slab on the ground isnt

Garage Flooring · Apr 12, 2012

nitsuj02532 said:
Fortunately, from the building perspective, my father-in-law is a licensed builder and has been for about 25 years so for the structural design i have a good resource, hes the one that actually suggested looking into pouring a thin slab on top of the plywood floor to obtain heat resistance, were just trying to figure out what is going to need to be done, and then designing the actual structure around it to support the load...just not sure what my thin set concrete options are and whether its my best choice or not

I know the plywood twisting could be a problem, but depending on how heavy a floor i put on top of the joists, i may have have joists close enough together to attach it, that the twist wont be too bad, especially if i lay a second layer of plywood, opposite direction and maybe even use an adhesive to bond it together, unfortunately there is no probuild near me (about 2 hours away). I cant imagine theres much that ill be welding right on the floor, i can only think of a handful of instances that i have had to do it on the floor in the past, i do have a few steel tables i use when welding, but im sure cutting i will have slag and red hot metal hitting the floor.

i'm not too worried about keeping the floor pretty, as i am trying to avoid catching the place on fire, im planning on putting up sheet metal on the walls in the area ill be welding the most as an extra precaution to keep anything flying away from the wood surface as much as possible

Sounds like your in good hands. Ask him about cement board as opposed to pouring cement. Should be less expensive and less likley to crack. Also, keep in mind that do to a process called pyrolysis the ignition temperature of wood goes down when exposed to high heat, even when it is not directly in contact with flame. In some cases this can cause wood to ignite at lower temperatures with no spark --just high heat.

jack stand · Apr 12, 2012

If the $$ spent on a framed PT floor (or less) were put towards a foot of clean stone to raise your bldg elev. (allowing ground water movement through the stone) allow a conventional slab? Equal or less cost, and you get your weld proof floor. I'd still be concerned of abundent moisture and very damp air under the framed floor.

gussteve · Apr 12, 2012

i'm with jack on this one ,why not build up the elevation with gravel or dirt and gravel so all the water around the structure gravitates down.or just pour half the shop now and the rest later when you can save for it.

gumbudah · Apr 12, 2012

Thin concrete, seamed durarock, diamond plate floor, cementicious floor leveler, tile, brick, patio pavers (maybe the cheap 6x12x1.5" or so kind), Some sort of grippy galvanized steel tiles, oooh a glass floor (lay down a good layer of thin set, smash up all your beer bottles, and throw em in, jagged side down of course), rusted old plate steel (rusty and old for grip and to be cheap). I think if it was mine, id try to use small tiles (6" or smaller)...

ConCretin · Apr 12, 2012

We regularly place 1 1/2" regular weight concrete overlays over traditionally framed wood floors for radiant hydronic heat. It would be cost effective and mitigate the risk of welding over a wood floor.

We double plate the wall framing and fill to the top of the bottom plate. I recommend using wire mesh - not so much to prevent cracks but to keep them together.

rasit · Apr 12, 2012

Plywood floor and then cover it with something like Durock cement board.
http://www.usg.com/rc/data-submittal-sheets/panels/durock/durock-cement-board-submittal-CB399.pdf

Garage Flooring · Apr 13, 2012

rasit said:
Plywood floor and then cover it with something like Durock cement board.
http://www.usg.com/rc/data-submittal-sheets/panels/durock/durock-cement-board-submittal-CB399.pdf

My advice exactly. I am not sure why in this application the thin layer of poured concrete is a consideration.

nitsuj02532 · Apr 13, 2012

LLWillysfan said:
We regularly place 1 1/2" regular weight concrete overlays over traditionally framed wood floors for radiant hydronic heat. It would be cost effective and mitigate the risk of welding over a wood floor.

We double plate the wall framing and fill to the top of the bottom plate. I recommend using wire mesh - not so much to prevent cracks but to keep them together.

that sounds like an awesome idea...what do you do to the subfloor, if any anything, and what do you use for a mix? just a regular bagged 3000psi mix? you use it as is with the mesh or add some aggregate to it? id love to know more about exactly what you are doing with it cause it sounds like its going to be my lowest cost, longest term option.

rasit said:
Plywood floor and then cover it with something like Durock cement board.
http://www.usg.com/rc/data-submittal-sheets/panels/durock/durock-cement-board-submittal-CB399.pdf

does this usually hold up a long time? also, what about the joints, do they generally end up causing cracking or hold together?

nitsuj02532 · Apr 13, 2012

jack stand said:
If the $$ spent on a framed PT floor (or less) were put towards a foot of clean stone to raise your bldg elev. (allowing ground water movement through the stone) allow a conventional slab? Equal or less cost, and you get your weld proof floor. I'd still be concerned of abundent moisture and very damp air under the framed floor.

gussteve said:
i'm with jack on this one ,why not build up the elevation with gravel or dirt and gravel so all the water around the structure gravitates down.or just pour half the shop now and the rest later when you can save for it.

I forgot to mention, getting concrete to where the shop is going to be is going to be tricky, i can get cars and trucks back there no problem, i think the yard would be too soft for a concrete truck to get back there and leave it parked for a half hour, nevermind the trees, the 'cleared' part of my yard is my nice new septic's leaching field and i have a feeling i would screw it up having a concrete truck parked on it, measuring it out distance wise its too far to do extension chutes, would have to either get a truck with a pump or rent the equipment to pump it. when they put my septic in they had a 14 wheel dump truck parked back there and it left some good divits (about a foot deep) in the yard

Morrisman · Apr 13, 2012

Lay a couple of thickness's of 3/4 ply then tile the wooden floor, just like a million people the world over have done with bathroom floors.

My wife even has garage floor tiles on her kitchen counter-top, and on the stairs, as they are tough and the colour goes all the way through, so a chip doesn't show as much.

jack stand · Apr 13, 2012

nitsuj02532 said:
I forgot to mention, getting concrete to where the shop is going to be is going to be tricky, i can get cars and trucks back there no problem, i think the yard would be too soft for a concrete truck to get back there and leave it parked for a half hour, nevermind the trees, the 'cleared' part of my yard is my nice new septic's leaching field and i have a feeling i would screw it up having a concrete truck parked on it, measuring it out distance wise its too far to do extension chutes, would have to either get a truck with a pump or rent the equipment to pump it. when they put my septic in they had a 14 wheel dump truck parked back there and it left some good divits (about a foot deep) in the yard

I see! It's still under 5 yards of concrete at 4". A rented skid steel could move the stone and the 'crete, but it's a PITA. I've seen a light weight concrete used on a condo job for noise and fire reasons. It was called gypcrete, and I 'm not sure if that was a slang term or a product name. I think that you might have trouble getting a 2.5 yard load of it, (or any specialty light weight concrete) and if you can it might be costly for such a short load. The cement board is looking more reasonable but I wonder about sealing the "joints" that you know the welding sparks will surely find there way into. Maybe you would set them in a thinset type tile mix that would "squeeze" into the voids between the seams and you could trowel them flat? And like LLwilliesfan said, remember to double the bottom wall plate.

ConCretin · Apr 13, 2012

An 1 1/2" overlay would take a little over 1.5 cy of concrete. You and a couple friends could wheelbarrow it in an hour or two.

No need for a special mix such as lightweight. Regular concrete only adds about 7.5 lbs per sf.

little cowboy · Apr 14, 2012

---a pumper truck is an option to a heavy truck on the lawn.---

Interesting question and great answers. But if it was my floor, I'd invest in drain rock and install weeping tile under the gravel, say on 10' centers and have a sump. PLUS I'd extend the drain field to the out side if I could. ( I am assuming you have clay soils ) The gravel rock would be a foot thick

Then I'd pour a floating slab, well reinforced, and I would consider infloor heat (depends on where you live).

Wood basements work well using PTW. However the wood is vertical. .and the floors I've seen, have been conc.

But the other reason is, I once calculated the cost of pouring a concrete retaining wall vs conc where I live.. and the costs were about the same. So for my money I'd go conc...or at least conc interlocking brick.

Good luck and keep us posted..

BTW...conc and water is not a huge problem, just needs to be managed. Another suggestion would be to talk to a structural engineer as well, for procedures used in your area

Sureshot · Apr 14, 2012

Do it right, do it once. I would rather the concrete in contact with water than wood-pressure treated or not. You could likely get a tractor mounted mixer or one of the electric ones and do it right on the spot with a bunch of buddies. Not sure what resources you have available.

Seems backassward to build a wooden floor then cover it with cement IMHO.

wood framed floors and welding - what for a top layer?

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