What are expansion joints?

Hardibacker reccommends expansion joints for certain situations. What are they and how are they done?

As temperature and humidity changes, different materials expand and contract at different rates. The expansion joint is a small gap that allows the material (Hardibacker in this case) to expand without putting excessive stress on the adjoining piece or wall. Too much expansion will cause buckling.

You put the gap in when you are laying the board.

[This message has been edited by R. L. Boatright (edited September 20, 2000).]

Adding two cents to a fine response.

A case scenario may help: an interior floor installation. Foam backing rod (FB) is placed in the lower section of joint (below the tile and above the subfloor) between the backerboard (BB) or mortar bed (MB) and a flexible sealant (silicone caulk is one product) (SC) fills the gap at the tile level. W = width of joint. <- >M< -> movement directions.

The joint accounts for linear movement (ie: temperature changes, swelling with moisture).


<BLOCKQUOTE><font size="1" face="Verdana, Arial">code:</font><HR><pre>
___________ ________
tile |SC|
-----------|SC|--------
BB or MB |FB|
===========| |=========
SubFloor W = 4 * &lt;- &gt;M&lt; -&gt;
_________________________

[/code]

Typical Interior spacing 24'- 36' each direction.
Exposed to sunlight or moisture 12'- 16'
Also used with abutments to other surfaces (walls); intersections of dissimilar surfaces (concrete, wood), and where linear movement is likely to occur.
Typically "W" is not less than 1/4"
An accepted minimum standard is the width equals four times the anticipated movement.