Hi,
We are building a modified Timber framed house, and want to build a 1-1.5″ thin slab radiant concrete floor ontop of the 1st floor which will be wood framed. We will eventually tile some of though perhaps not all of the concrete surface, but for now it will remain uncovered (and maybe stained). The underside of the subfloor will be the exposed ceiling in a living space in the basement floor below so we don’t want to use plywood. We are building with our own spruce and fir that a local sawyer will be milling for us. We hope to use straight-edged planks (perhaps 4 or 5 quarter?) ontop of 6×8 joists (we’d rather forgo the expense and hassle of bringing the boards to another mill which can handle shiplapping or T&Ging the edges). For a variety of reasons, we are trying to use as little sheathing as we can manage. I’m thinking to use tar paper over the planks to prevent the concrete from bonding to or leaking through the subfloor. What I am unsure of and can’t find the answer to is:
1. What is the maximum allowable deflection in the boards to prevent any signicant cracking? L360? I can’t find any tables that answer this question. What I’m trying to figure out is can we structurally get away with using straight-edged boards, and what is the maximum span that will safely avoid cracking?
2. If we use fiber in the concrete, will this add to, subtract from, or not impact on the plankings span strength? What if we don’t use fiber?
3. If we have to, we could shiplap the edges – how does this change the equation?
Thanks for what ideas or help you can provide,
Howie
Replies
Howie,
If your joists and mainfloor subfloor are exposed in your basement, where will you run all your plumbing, electrical, gas lines etc? We are building a log home and our second story will have exposed log floor joists with 1x6 t&g going over them and that will be what is exposed on the main floor ceiling. However, we are planning to build a subfloor with 2x6 running perpendicular to the log joists and plywood on top of that in which we will run plumbing and electrical. It won't have any gypcrete or concrete on top, just the finished flooring. That is how we are managing to "hide" things for our second floor, but there isn't much up there.
2. I would worry that the tar paper might smell in the basement once it was heated by the radiant heat.
Disclaimer, I'm just learning all this stuff as we are building our home so I'm not one of the experts here, but looking at all the electrical and plumbing that run through our floor joists in our basement, I can't imagine trying to keep that system hidden. It just seems like a lot of work. Wouldn't it be easier to build the subfloor traditionally and then cover the basement ceiling with 1x tongue and groove after all the mechanical systems are in?
Hopefully others will have more helpful information instead of just questions.
Hi Paula,
Thanks for your thoughts. As far as mechanicals, the exposed part of the floor (the basement ceiling) will only have living space under one protion - the plumbing and electrical will be running in the utility spaces elsewhere. What we are trying to accomplish is a radiant slab on the 1st floor (the basement ceiling) as simply as possible, which means using our local sawyer (who doesn't have T&G milling capability) to make straight edged boards. I meant to say felt paper, not tar, for the lining, so I don't think smell or outgasing will be an issue. I'm fine with the look of the straight edged boards even if they shrink (which they will some). If we want to we can eventually finish off the basement ceiling in the living space, but for now we're hoping to keep it open. I know it is more commonly done with shiplap or T&G boards, or now 4x8 sheathing, but we're trying ot use as much of our own wood as is reasonable. Again, thanks for the feedback.
Howie
You need to do some research. I think that you need insulation under the radiant slab. That means either use sheets of foam between the slab and sub-floor or insualtion under the sub-floor, where it will show.
Hi Bill,
Thanks Bill. I have been looking into this issue, and am not certain about it yet. I realize the heat will transmit down through the subfloor, but it will be into well insulated but not always heated space. Without the insulation, the heat control gets more complicated. We will look into this more before we decide.
However, this doesn't impact on my primary question of how to calculate the subfloor board sizing and joist spacing to prevent complete cracking of the slab. Any thoughts out there?
Howie
If your sawyer makes them nice and straight for you, could you t&g the small amount you need yourself, using a router?
-- J.S.
John,
Actually, there will as much as 600 sq ft of slab on wood, which seems more than makes sense to go this route. Thanks for the thought, though.
Howie
I would be surprised if anyone here can guarantee a crack free slab under these conditions. First, slabs are prone to developing cracks. They require control joints for this reason. Second, you are putting tubing in the slab which reduces the thickness of the slab where you have the tubing. Given this, I would recommend that you set a wooden grid to be used as screed boards and left in as control joints after the slab is poured. In lieu of that, pour a thicker slab
You will do fine using straight edged boards as a subfloor. The slab can distribute the weight over adjacent boards better than any t&g would do. Just make sure that the joists are sized and spaced for the total load of the finished floor. Tar paper will work as a bond breaker.
Hi,
Thanks for the feedback. When you say pour a thicker floor, how thick a pour would you imagine needing given 5/4 boards on a 3' span? Is this an impossible span even with mesh in the cement?
Howie
> The slab can distribute the weight over adjacent boards better than any t&g would do.
I thought he wanted the look of t&g, because the underside of the subfloor will be the ceiling of the room downstairs.
-- J.S.
> I thought he wanted the look of t&g, because the underside of the > subfloor will be the ceiling of the room downstairs.
Hi John,
Well, yes and no. I'd rather just use straight edged boards, and not worry about T&G, but yes, it will be exposed in parts below, which is why I don't want to use plywood.
Howie
Hi
Please note right away that I am a homeowner not a builder, but we did build our own home.
We have a 1.5 in. slab on plywood with radient floor heating. We used 6mil plastic to line the entire floor, not tar paper. It worked great. We have cracks in a few door ways, but expected that( no control joints). We stained the floor and it looks great and is very easily cleaned and maintained. We did not use mesh. We did all the work but the pour ouselves. The joists are beefed up to 12 inch on center and and 12 inch instead of 9 inch ( manufactured joists.(
It is over a crawl space and we have not insulated the bottom yet.( We WILL)
It was still easy to heat.
Jill
Thanks Jill,
I realize that plywood would probably be the better bet, but I'm still looking for possibly using our own milled boards. Sounds like you have a good floor system though.
Howie
Have you considered any of the radiant floors that don't use a poured covering? I'm not an expert, I just watch a lot of home improvement shows on TV. Seems to me you have a good plan for your wood. I would try to change the form of the heater rather than questioning something you already figured out. Can you get more wood and sandwich hydronic tubing between it? Where you want tile, could you put cement backer board on your plank subfloor and use the electric radiant heat that just goes right in the thinset?
Check this out: http://www.wattsradiant.com.These guys talk about all kinds of hydronic and electric solutions that don't involve cement. I'm not trying to endorse their product, it just shows options.
I only know them because I bought a piece of the electric SunTouch floor for a tile bathroom from them. The site looks different than I remember. I figured out a way to buy it online, though, without the hassle of some authorized dealer and that nonsense.
Hey BReese!
Thanks for the website tip - I'll check it out. I haven't considered cement backer board, but have seen it referrenced often - not sure if it makes sense but I'll do some research. As far as electric heat, we are off the grid - this doesn't seem to be a reasonable option for us - I wonder if it is a reasonable option for any but the smallest applications...
Howie
Sorry for the distraction from your original question, but how does one do an "off-the-grid" RFH system? I would have thought that the circulation pumps would be reasonably power-hungry, and there would be too much resistance in the floor tubing to just use convection. Obviously more feasible than electrical resistance heating, and I guess I can see it if you are running a generator (although the folks I know doing that are doing so are not running them 24/7). An interesting concept...
Don H.
Well, yes the pumps do eat some electricity, and I've never done radiant floors before, but as I understand it, if you design your loops short, with larger diameter tubing (5/8"), and remove as much of the flow restrictions possible, than you can size the pumps very small (~15w), and witht the slabs floor, one can run the pumps less frequently, and if you really do a good job, and get lucky, the hot water will thermo-siphon. But in any case, that size pump is vwery manageable off-grid, especially if we use a hotwater heater instead of a boiler to supply the system, there is actually a fairly low power demand. As far as running the generator, that is what the battery storage is for...