Large Expansion Joints in Hardwood Flooring

**m_ouwerkerk** · 11th July 2010, 10:41 PM

Hi, I've got some thoughts on expansion joints, and to be honest I reckon they're a bit overrated. I know the general rule is that if the hardwood flooring is going to be over 6m or so, they you should have a 10mm expansion joint in the middle, but I really can't see this doing stuff all (I do agree with putting in a small gap every 6-8 boards, but to me that depends on when you lay the timber).

I've just put down 110mm boards. I aclimatized them very carefully (in the garage for a couple of years, in the house for 2 months), laid them when the relative humidity was high, secret nailed them to death, and used a heap of glue. I had a very big span of nearly 10m in one point, but I'm not worried, because the humidity is only going to drop from here, and the humidity where I live does not change much between the seasons. Now I know you're not supposed to secret nail over 80mm, but I used some pretty good nails (porta-nails), and heaps of glue, and I just can't imagine the boards going anywhere, because the glue (ultraset / sika) is unbelievably strong - it's way stronger than any nail - please let me know if you think different, but when I test it out, the wood breaks before the glue lets go! The job has been sitting there for about a month (I'm just finishing it up now), and there's only the slightest (almost undetectable) cupping near my bifold doors because the heat from the sun has caused the top to dry out a bit fast. The humidity still hasn't dropped yet, so no gaps have opened up.

Anyway, back to that 10mm expansion joint, here's my point: If the wood moves even 3 mm, the job's pretty much ruined - the glue will give 1 or 2 mm max, and the nails will start lifting., and the whole job will buckle. Therefore, what is the expansion joint going to be doing, and why would you need a big gap around the edge of the job? The main thing is that the job a) must be held down properly, and b) not expand too much, so therefore I personally reckon the only way to be safe is

1) make sure your job will only shrink after you have laid it (or at worst only expand a small amount) by laying when the humidity is higher (assuming everything is aclimatised) and if you can't do this:
2) put in regular gaps (every 6-8 boards).

Does anyone have any thoughts on the validity of the 10mm expansion joint in the middle of the job, based on the above?

**johnc** · 11th July 2010, 11:38 PM

I thought you had a 10mm gap on the outside edges only, for overlay flooring which is covered by the skirting anyway.

**jimbur** · 12th July 2010, 09:57 AM

I'm probably way out but it sounds like the type of expansion joint you'd have to provide with concrete not with wood. Gap round the wall covered in skirting as said earlier but T&G are expansion joints in themselves.
Cheers,
Jim

**r3nov8or** · 12th July 2010, 10:25 AM

It's obviously unsightly to leave a 10mm gap in the middle of the room, e.g. in an L shaped area. So the expansion gap can be catered for by 'firm but loose' (i.e. not extremely tight) clamping over long distances where there is no wall/skiting to conceal the gap. I'm sure the BCA or relevant AS mentions this. In general I think its best to allow for the expansion in the immediate area where it may be needed, rather than only several metres across the room.

**m_ouwerkerk** · 12th July 2010, 02:38 PM

Just to clarify, I'm talking about a 10mm gap filled with cork.

**r3nov8or** · 12th July 2010, 03:50 PM

Well that would be less unsightly, but, I think, still unneccesary. I'll dig out the AS that mentions it. I'm sure its 'to standard' to cater for expansion along the whole room width rather than in one place with 10mm.

**GraemeCook** · 12th July 2010, 04:17 PM

Originally Posted by m_ouwerkerk

Hi, I've got some thoughts on expansion joints, and to be honest I reckon they're a bit overrated. I know the general rule is that if the hardwood flooring is going to be over 6m or so, they you should have a 10mm expansion joint in the middle, but I really can't see this doing stuff all (I do agree with putting in a small gap every 6-8 boards, but to me that depends on when you lay the timber).

My Tas oak floors, mainly messmate, are up to 10 metres and do not have any visible expansion joint, cork filled or otherwise, apart from the usual gap behind the skirting. After 130 years they are still in A1 condition.

Cheers

Graeme

**r3nov8or** · 12th July 2010, 06:55 PM

AS 1684.4-1999 Section 5 Flooring, says
5.3 Expansion Joints
For continuous floor widths over 6 m measured at right angles to the flooring, intermediate expansion joints shall be provided in addition to the perimeter gaps. This joint shall be either a single 10 mm wide gap (under a wall or across a hallway), or smaller gaps with closer spacings to an equivalent space (for example 1 mm at 1 m spacings or loose clamping).

**m_ouwerkerk** · 13th July 2010, 01:19 PM

yeah, and it's that 10mm gap in the middle that I reckon is a waste of time - I just can't see it doing much (as well as the gaps around the edge).

**jimbur** · 13th July 2010, 03:23 PM

Originally Posted by GraemeCook

My Tas oak floors, mainly messmate, are up to 10 metres and do not have any visible expansion joint, cork filled or otherwise, apart from the usual gap behind the skirting. After 130 years they are still in A1 condition.

Cheers

Graeme

I find that shrinkage rather than swelling is the usual problem.
Jim

**smoothop** · 23rd July 2010, 05:23 AM

I used this sealant that my mate recommended to me recently that I thought worked well- it's from Dow Corning. I found that it can handle severe horizontal and vertical expansion joint movements. Maybe this will be useful.

Dow Corning Pavement Sealants for highways, bridges and runways - Dow Corning

**GraemeCook** · 23rd July 2010, 04:37 PM

Originally Posted by jimbur

I find that shrinkage rather than swelling is the usual problem.
Jim

True enough, Jim. Joints are now open by about 1mm. Suppose I'll start worrying in another 200 years.

Cheers

Graeme

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