Hello again guys,

Sorry for a sort of repeat question but i think i can sum up my dilemma here.

I am installing 3/4" solid maple, 3.5" wide in a house in which i anticipate a relative humidity percentage change of at least 10% (the house is steady now in the winter art 38% and i assume it will rise to about 50% come summer)

I'm installing some of the floor now (the rest inspring), starting with a 10' x 12' room ( bout 3 rooms this size on this floor will try to install with no transition trims) and am wondering if i should add gaps betwen the boards to anticipate the humidity change and how much if so.

I found this stability chart online:



and what they say to do is multiply this number by the width of the board (in inches) by the moisture content change in the wood. My problem (i think) is if the house changes in relative humidity by 10%, i'm snot sure that it means the board will change 10%.( i do not have a moisture reader and can't seem to find one on ebay ( i know there expensive though and you can't really use em for anything else))(and even if i had one i do not want to wait till summer just to see what the moisture content change would be)

Anyway if anyone knows approximately how much i should gap these boards the info would be greatly appreciated. Or what kind of moisure content change i should expect in a relative humiodity gain of about 10%.

PS Floorguy, KevinD and Gary, thanks very much for all your help, let me know if any one of you is interested in online poker, maybe i can set you up with a little transfer to repay you guys for the invaluale service your providing.

thanks again.

If a 3&1/2" board expanded by 10%, that means EACH board would expand by 1/4" or more. No way that is going to happen unless the flooring in under water. If you're at 38% RH now and only expect a 12% change, I would'nt be overly concerned. IMO, that amount of change is not significant. In the ideal world, interior RH levels should be 45 to 50% constant, yet they rarely are. If you want to be extra safe, try using playing cards between each row of flooring. Or about every 4 or 5 rows, use thin putty knife blades. Either way, you are building some expansion into the floor. The biggest issue is not knowing the MC of the flooring now. If it is high, you may lose some width and therefore, no expansion would be needed. But if it is low, and you expect the RH to change considerably, then some expansion would perhaps be in order. You need to find out the MC of your flooring. Why not take some boards to a lumber yard or a hardwood flooring shop and ask them to measure the MC with their meter. It would only take them a minute or so.Take about 5 boards from different boxes and average the results. Then you will have a baseline to work from. If the MC is low, 5% or so, then adding expansion would be wise because when spring and summer come, that flooring MC will go up some and the wood will need to expand a little. If the MC now is high (8%+), I wouldn't worry about expansion. After you install the wood, it will shrink a little and voila, there is your expansion.

MC can be calculated by using temp and humidity. Temp and humidity can be measured with a thermometer/hygrometer you can buy at Wal Mart for less than ten dollars. Each species of wood has a specific expansion coefficient. To extrapolate how much movement you should expect, you simply plug the values into a simple equation. These values are: expansion coefficient, MC, and board width. It is not very complicated.

thaks for your help guys i will see if i can get my hands on a moisture reader.

HI Chuck,



Can you run this in a sample sceario? For me, I understand better when I can see an example. I'd like to know how to do this. PLEASE!

I'm not a pro. In fact, I've never installed hardwood flooring in my life , but I found this: http://www.csgnetwork.com/emctablecalc.html

I plugged in 70 degrees and 50% RH and got 9.2% for EMC (Equilibrium Moisture Content)
I plugged in 70 degrees and 38% RH and got 7.4% for EMC
The difference is 1.8%

Variation in the width of the board = width of the board x coefficient of the species x variation in the % of moisture content

Keep in mind the variation in the % of moisture content (calculated above) is not the same as varaition in the RH.

In this case, variation in the width of the board = 3.5" x 0.00353 x 1.8 (not 1.8%) =0.022"

You beat me to the punch, Protege.

Keep in mind thi is going to be an average for plain sawn boards. anyway the only info I have is for 2 1/4 plainsawn oak. A ten% change in humidity if kept at the same temp should force a moisture content change in the floor of 1.5% which in a 21/4 oak will cause the floor to expand anywhere from nill to a scant 64th.

The info you have is wrong.

Well if It's wrong then you might want to call NOFMA and the US dept of agriculture and let them know there information is not up to snuff. I diddnt pull it out my tail so please enlighten me.

Dude, I will bet you 50,000 dollars that a 1.5 point increase in MC will make a floor move a hell of a lot more than nil to 1/64 of an inch. If you meant to say one board would expand that much, then nevermind.

BTW: All of these calculations are based on unbound wood. Bound wood will not expand as much as the calculations suggest because the wood has the capacity to compress, thereby assimilating SOME of the increase in size. This will, however, translate into gaps if the wood is forced to compress against itself beyond its level of resiliency. The effect is called compession set.

Still, 1/64"(!) per 2-1/4" board means about 1 inch of expansion/compression every twelve feet. Something worth pondering, methinks.

Sorry Bud, didnt mean to jump on you like that, You are right about most of the caculations you can find out there are not real life scenerios. Too many factors to get a true number. subfloor type. fastner type, nailing schedule, wood specie and composition humidity and temp it's no wonder why people have trouble understanding how everything works together. i have caught myself more than once getting way over people's heads sometimes too much info is not a good thing.
Now here is a real life scenero, My floor in the showroom here shrinks during this time of year significantly. (I can hear people already you need a humidifier" I have one it just is out of water more than not). ANyway its a plainsawn red oak on plywood subfloor with cleats. It reads about 7-8 during the summer and about 4-5 during the winter. That 3 percent change makes this floor shrink leaving a .02 gap between each board on average. MUlitiply that by 60 boards it comes out close to an 1 1/4" overall. Not what this table says but I believe this to be a bit more realistic expectation.

Just mop it real good. Problem solved.