Will a large area ¾ inch solid wood oak floor really expand up to ¾ inch in the “width” dimension (clearance allowance at the walls)? If so, wouldn’t that pretty much rip the staples/cleats out of the boards at the perimeter and leave the perimeter boards loose. Also, I assume that once the boards expand to their full limit contraction will result in small gaps between each board and the perimeter board will thereafter not move again. Looking at the situation another way, why wouldn’t the staples/cleats pretty much restrain the boards with the result that expansion will result in board cupping as opposed to general expansion of the floor. Can someone tell me what really happens? If it’s a combination deal, how much cupping and expansion can I expect – rule of thumb speaking? BTW I recognize that the answer depends on climate extremes (within the house) so would be interested in how extreme the climate variation has to be to gobble up the ¾ allowance and, conversely, for a house with very good climate control (minimum reasonably achievable variation in temperature and humidity year round), what the nominal expansion coefficient would be (in the width direction). For reference I plan to install a floor using 2 ¼ inch wide Bruce.

Many thanks in advance for all replys.

You have rows of boards across the floor. They are installed tight, during the winter, heat running drying the interior of the home out tremendously.

Now come spring and summer, the windows are left open and the humidity raises in the home significantly.

Now there are 128 rows of flooring across just one room of the home.

Each board in the 128 boards, gain moisture from the humidity and swell slowly, as it gains moisture content. If the humidity is left high and the boards gain 1/32 of an inch EACH, in the width from gaining moisture content and swelling, How much has that total 128 boards of flooring going to swell in dimensions, from its installed dimension in the width.

Cupping happens when the bottom of the board has more moisture content then the top of the board.

Hmmm. Graphic illustration of the expansion process - 4 inches under extreme conditions (128/32) or severe buckling! Any thoughts on which is the predominant process?

Still chuckling.

The worst I've ever seen here in Nor Cal, is about 3/32 of compression set. Guess we are pretty dry, and stable. That is on a wood sub floor with 3/4 solid, 2 1/4 inch stuff.

Thanks SK

Just to be clear, are you saying that the whole floor expanded 3/32 or was this the movement of each board. If the latter, how did the whole floor behave. Did each board deform in place or the whole floor move?

I'm about to do my first install of about 1,500 sq ft and the wood flows from room to room. I have an open plan house design with a large 35 ft x 35 ft area that covers the Great Room, Dining Room and Kitchen. There are very wide archways between each area. Placing an expansion gap and transition strip (hump) would look silly in these areas. Can I get away with continuously running the wood throughout or am I bucking for problems. Also, if I change direction in each area is this going to aggravate the expansion issue to an unacceptable level?

I'm assuming that when I run the wood into the bedrooms I must create a threshold gap and cover with "T" molding and there really is no other sensible choice.

BTW, we usually work hard at keeping the indoor climate consistant year round - not much open window stuff with the elements pouring in, and we use humidifiers in the winter.

Since I live in on the west coast, I'm not really qualified to speak as the authority on Ohio conditions, but you can read thru all the previous posts about expansion here, and check the NOFMA site for more info. Things I think are important; Make sure that the moisture content of the wood flooring and subfloor are within spec, (3-4%), The crawl space is covered with plastic as explained at the NOFMA site, Use 15lb felt under the flooring,
I've never had to leave additonal space between rows, but I know this works here in central calif. But I've heard of guys back east who use washers between the rows every ten feet or so as temporary spacers, and other ways to incorporate spacing into the floor.
As far as the 3/32 gapping goes, there was about 1100 feet of red oak in the house, it appeared that the floor was installed before the subfloor was within range, (difference of more then 3-4%) and slight gapping occured, looking like normal seasonal changes, but never closing up. It was the worst in the kitchen, where the biggest gaps occured, and could of been caused by a dishwasher leak or leaking pipes etc.
All wood floors will move, but with the 2 1/4" it is far less apparent the it would be with a wider plank like 5".

Flyer,

The cleats restrain the boards for the most part. In other words, if the bord would normally expand 1/32" unrestrained, then it might expand 1/64" or so with cleats. The cleats are specially designed to let the wood expand just enough. For example, if you used screws instead of cleats, the boards would probably crack. This is also why the air flooring nailers cost so much - the nail was designed before the nailer.

The second part of this is your subfloor. Your subfloor expands and contracts with humidity also. Plywood still expands, just much less than solid wood. And also, the flooring will be pulling it also. That is why the nails don't pull out on the last row - because the subfloor is part of the expansion equation also.

Can the floor expand 3/4"? Probably only under extreme conditions. If your floor was flooded for example. Also, I would imagine that as the floor gets older the nails begin to give a little more and expansion may increase.

Another related issue is wood crush. If you put a piece of wood in a vise and snugged it, then increased the humidity the board fibers would crush and the board would never return to it's original dimensions. This is why you want the floor to able to move somewhat, so you don't get the crush effect. Slight expansion even when restrained won't lead to crush since the wood fibers are elastic in this limited range. Of course over time the boards will crush slightly when they endure thousands of expansion cycles.

Finally, putting a good finish on the boards delays the response to humidity. If it normally takes 2 weeks for unfinished wood to acclimitize, it might take 4 weeks (a guess) for finished wood. Usually periods of extreme humidty changes are fairly short in duration. This is why furniture like tables are always finished on the underside.

IMO floating floors expand and contract more on a short term basis than solid even though they are engineered.

Great addition Alex! Thanks for the contributions

Alexh

Thanks very much for your thorough response. That was the detail I was looking for. Your explanation of the dynamics of the system - wood, subfloor, cleats, environment - makes great sense. I feel much better about the project I have ahead of me.

Thanks again

Flyer

Great explaination, alex.


At this point compression setting starts.

Still, 1/64 in a 12 foot wide room is 1 inch of movement, and you would see compression of the boards.

The force wood exerts went it swells is greater then one would think.

Well stated,Alex.

Your opinion regarding floating material mirrors mine. The reason reminds me of the joke about why a dog licks his privates. Same reason floating floors expand so quickly:

Because they can.

CHU

Boy, I just got me a good lesson on how amazingly quick and big they can expand. I'm putting down a little patch of 3/8" engineered. The wood has been in the house for a few months. We struggle to keep the RH above 30% when we run heat because we heat with wood.

So, it's very warm with high humidity and foggy condensing moisture in the air for a few days. I dry fit the wood in the house and take it outside to trim it as necessary and edge glue pairs of prepared planks together. The wood stays outside a few hours before I bring it in. Yesterday I note the humidity in the house is over 70% and turn on the a/c.

I have the bright idea, since the plank pairs are assembling tightly and laying absolutely flat to the slab with no clicking, clacking or deflection of any sort, I'll just leave it floating on the slab rather than glue it down. Then I realized the design I accomodated for no longer works because the wood expanded so much. What fit perfectly before bringing it outside is grossly off now. Being so dry, this stuff soaked up water like a sponge. It easily blew out by close to a 32nd per board.

The good news is it's shrinking back to size fairly quickly. The dry slab is sucking the excess moisture back out of it. I suppose I'll go ahead and trim and slot my ends for the header board but otherwise just leave it lay on the floor until it re-stabiizes before doing a proper glue down installation. I'll also ensure that the long edges that border the tile field are isolated with a line of sanded caulk.

Cheap lesson, learned quickly. The best kind

Don

I might be wrong but I dont think engineered wood expands as much as solids due to the manufacturing process of engineered. Drew

Yes, that's my understanding too.

Don