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Cold feet: building a platform over a concrete floor

 
gardener
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Some years ago, I poured a concrete hearth riser to accommodate a pellet stove. The reason that I did this was simply because it looked nice like a fireplace is “supposed” to look. Now, after years of heating the room to a toasty temperature and still having ice cold feet, I am considering building a raised platform island to delineate a workshop warm zone for carving work and small detail projects.
The 11’ x 13’ raised platform inside the 22’ x 22’ former garage space would be hearth level (about 4”). The legs on the pellet stove are 8" to the base of the fire box. The platform would be made of 2" x 4" framing with recycled tongue and groove floor boards or planks on top.

Questions:
Is the air itself enough of a thermal break between the concrete floor and the planks or shall I stuff some insulation in the gaps?
If insulation is needed over the concrete, what kind is recommended for the spaces in the platform?

Suggestions for building a warmer hearth-level “living room platform” (Google search term) are much appreciated!
 
rocket scientist
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Hi Amy;
I think I would use a large heavy rug on the concrete floor.
I think it would be more convenient than a raised floor.
Adding a wooden floor would be slightly more pleasant than concrete but not really much warmer, and it would make a fine mouse habitat.
 
pollinator
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A thermal break of any kind will make a huge difference.

An air gap is problematic, with convection currents sucking heat back into the concrete and (Thomas beat me to it!) mouse habitat.

Keep in mind that dry wood, even scrounged from pallets, has good insulating value. A solid platform keeps the troublemakers out and a chunk of scrounged carpet on top makes it toasty.
 
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Amy Gardener
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Thank you Thomas, Douglas and Jordan. I’m hearing from all of you that the wood is enough and no insulation is necessary so long as I have some form of break: wood, cloth, carpet, or electric mat. Also, it is important to consider ways to keep critters out. Your observations are a great help.

Regarding the suggestions of flatter surface breaks, my goal of a final height of 4” is twofold: to match the hearth height AND break up the room space to make a relatively clean area where sand does not collect. Currently, the exposed concrete area has evolved into a giant mudroom where boots and sand collect. Rugs don’t work due to the high traffic from outdoors into shop and the second doorway into house: high traffic messy zone. The second purpose of the platform (beyond warm feet) is to corral the transition from outdoors through the shop into a hallway. One end of the hallway leads to the house and the other end of the hall leads to hardware storage closets. Both are necessarily dirty most of the time. Due to the mild daytime climate, power tools are stored inside but used outdoors: the shop doesn’t need to accommodate chop saws and the like.
The platform is meant to be a clean zone island (fireplace and comfortable chair and table) within the large open room.

Since the platform won’t need insulation, and the floor is dry, I could use the space between the floor joists to store pipe, moulding, and small lumber. I don’t expect mice to be a problem but a bull snake might find the tunneling joists interesting. I’ll have to think more about how to block the critters.
 
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A rug is a much easier solution. But, it shouldn't be under stove and be no closer than a minimum of 8" from stove. And, liking artificial or not, rug would be better off if it was flame resistant anywhere near the stove. You can sometimes get a stray spark that jumps out the stove door when you open it. You may not see it, and some time later you'll have a big problem. If you put in a raised wood floor, you just increased your danger. If there is any cracks or seams between the boards, dust and dirt can and will filter down to underneath the wood. Now you have a perfect hidden fire starter from a stray spark.
 
Douglas Alpenstock
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Very true with a wood stove. The OP has a pellet stove. Based on the ones I've seen, I suspect the risk is lower.
 
Jim Fry
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Low risk, high risk, fire is fire. There really is no reason to take unnecessary actions. One of the things I have learned (over these many years) is that things happen. You do everything right, every single time. Except the one single time you don't. Then you fall off your ladder, drive into the ditch, leave a pot to burn on the stove, or start a tragic fire. It's much better to do everything you can to help avoid those momentary lapses.
 
Douglas Alpenstock
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Jim Fry wrote:Low risk, high risk, fire is fire. There really is no reason to take unnecessary actions. One of the things I have learned (over these many years) is that things happen. You do everything right, every single time. Except the one single time you don't. Then you fall off your ladder, drive into the ditch, leave a pot to burn on the stove, or start a tragic fire. It's much better to do everything you can to help avoid those momentary lapses.


I agree 100% with your philosophy Jim. Looking back over the years, I have dodged a lot of bullets. I try to design systems based on "my worst, dumbest, stupidest day ever." It's not paranoia; it's a practical assessment of how many things can go wrong, and setting things up so there is a safety buffer for an idiot lapse or a freak event. This is wise and practical policy.
 
Amy Gardener
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Thanks, Jim, for all the safety tips for the area around and in front of the fire box. I will absolutely protect the wood floor by using a concrete backer-board and tile solution as the surface of the platform such as the one presented in this clever video:



While a rug might be an easier solution, taking the more difficult and aesthetically pleasing journey toward realizing a vision is my path in life: this riser is going to happen even though it might be complicated. Anyone willing to guide readers toward a safer, more functional, and beautiful outcome is welcome to chime in to this conversation. Thank you!
 
Rocket Scientist
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What are the dimensions of your platform and room? Will you be able to orient joists so that long thin materials will be able to slide in and out? If so, I think that is a good idea. One thing I would suggest for this is to make a bit of the flooring at the far end removable so you can reach and clean out the backs of the spaces. Nothing like getting a smaller piece of something stuck down at the end and not being able to get hold of it to pull it out...
 
Amy Gardener
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Glen Herbert writes:

What are the dimensions of your platform and room? Will you be able to orient joists so that long thin materials will be able to slide in and out? If so, I think that is a good idea. One thing I would suggest for this is to make a bit of the flooring at the far end removable so you can reach and clean out the backs of the spaces. Nothing like getting a smaller piece of something stuck down at the end and not being able to get hold of it to pull it out…



13 x 11’ platform
17.5 x 19.5 open room size

These dimensions are smaller than the 22’ x 22’ slab due to hanging a some ~60+ year old hand-built interior cabinets from an adobe home that was gutted and remodeled. I have extra pieces of lumber from the salvage project that would make a decent subfloor. The joists would be oriented on the narrow 11’ side, where a 8.5’ wide hallway runs parallel to a 5’ wide double door to the outside. Given the wide hallway and door, it would be very easy to feed a long extension tool between the joists. Your suggestion to make the flooring at the far end removable is really helpful. Double sided access would mean my fishing tool could be far less than 11’. Thank you Glen!

Anyone have thoughts on joist spacing? Given the sturdiness of the subfloor, (3/4” thick scrap in many lengths), I am thinking of spacing the joists 18” to 24”. I will also support the subfloor using the concrete stem wall (4” rise”).
 
Glenn Herbert
Rocket Scientist
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I would not hesitate to support 3/4" plywood on 24" centers if a little flex is acceptable. 3/4" boards are another matter - they might be okay depending on width and species. Softwood of 6" or less would make me very nervous.
 
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