Expansion Gap ... Urban legend or Fact

[ABN]

When laying floors be they floating, glued or nailed we are always told to leave an expansion gap, dependant on the size of the room, of between 10-20mm on all edges.

Ok for floating floors I can understand this. However I can’t when it comes to glued or nailed.
Lets take the case of a floor nailed to joists. Are we to believe that the joist nearest to the wall is actually going to move 10+mm closer to the wall?
Lengthwise however I can see that the joist could grow by that amount.
In the case of a wood floor glued to a concrete sub floor are we to believe that the glues will allow the floor to slide back and forth 10+mm.

Whilst I understand that wood will expand and contract it does seem in simple terms that the expansion gap is more of an Urban legend than actually needed.

That said I’m still leaving a substantial gab in the floor I’m laying at the moment.

Anyone have any thoughts on the matter

 

[big-all]

wood will expand along its length by no more than a mm or two
where as expansion in its width is several times as much
a 3m length of pine by 44mm square will expand more in its width than its length

engineered flooring will expand in all directions[reconstituted wood mdf chipboard ect] but less over a large area than pine accross the grain

plywood blockboard ect will only expand slightly [a mm or two at all edges]as its solidly glued and all the expansion and contraction is held within the wood except the very edges

even real floor board that are firmly fixed will expand[or try to]
the tension contained by the next board so each board pushes the next
so theres a tendancy for the edges to push out a bit to relieve the preasure even on glued or nailed floor

and dont forget the nessesity for clearence so the wood dosnt touch the wall so you dont get rotting wood incase off dampness

 

[WoodYouLike]

Big-all is right, definitely no urban legend.
Solid Oak can expand 3mm per meter width, 4 meter wide room needs 12 - 15 mm expansion gap etc
Beech can expand 7mm per meter width, that's why most beech floors come 'steamed' to reduce this nervous characteristic of this wood.

 

[andy_aj]

Along these lines, the wood for my floor is acclimatising in the room. Laying it out earlier, without any cutting down the combined widths of the planks allow the maximum gap on either side of the room with the edging just covering the gap. The question is, is the wood as likely to contract when it cools further and leave a gap?

Any thoughts?

 

[chappers]

its expansion and contraction due to moisture content not thermal expansion

 

[WoodYouLike]

Any thoughts?

Well, you must also remember that Winter is coming = central heating = lower humidity = shrinking.
But that's the natural way of wood, it will expand again in Spring, closing most of the (tiny) gaps between the boards again.