Hardwood Flooring Relief Grooves

I am milling a section of hardwood flooring for the entry to my house. The boards being used are roughly 10" in width, and just short of one inch in thickness.
Is there a standard, or a "good" way to determine the proper depth and spacing of relief grooves on the underside of the boards?
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I'm not sure those are relief grooves. I thought they were milling identification marks -- to be able to quickly identify whether two boards came from the same milling run (i.e. flip them over and if the patterns match up they were, otherwise they weren't). I know they do this with cornice molding, chair railing and base boards (I asked my finish carpenter and that's what he told me anyway). However, having said that I don't really understand why you would want to know that for hardwood floor -- there's no pattern on top, it's just flat. Unless it relates to the wood itself (different trees perhaps and so maybe the wood may or may not match??).
For what it's worth, Mike

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Mike in Idaho wrote:

They help the floor lay flat when the sub-floor has imperfections. I don't know that the size or spacing is critical.
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You can look at some factory milled flooring to get an idea.
It is likely that 10" wide is about 3 or four times too wide for good flooring. There are two problems with wide floor boards. First is cupping. I get significant cupping between summer and winter with my red oak which is 3 1/4" wide. Most hardwood flooring is 2 1/2" wide these days.
The other problem is expansion and contraction across the width of the board. Tongue and Groove is used for flooring with only one side nailed and a small gap left on the other side so that each board can move back and forth, in place with changes in humidity. To do that with ten inch wide boards you'd need to leave a gap four times a wide as with 2 1/2" wide boards.
I think you would be much better off saving those boards for a table or desk or something and using pre-milled flooring, or milling narrower boards. You could rip those 10" wide boards to a narrower width and then use them but you may have paid a premium for wider boards to begin with.
What species are you using and how are the boards sawn? You can use wider boards if they are quarter sawn, but still not ten inches wide.
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FF

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Fred the Red Shirt wrote:

In my experience they are nailed tight and as the wood dries gaps develop. I've never purposely introduced a gap and have never seen any flooring installer space the boards. The mallet is used to drive the boards tight to each other and the nailing draws them a little tighter.
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