On 07/05/2018 23:28, Tricky Dicky wrote:
Expansion is minimal along the grain. However its going to be hard to
make any sensible estimate unless you know the water content when it was
installed.

Also kiln dried lumber tends to exhibit less seasonal change than air
dried wood.

What is currently supporting it?

The beam is a section of pressure treated decking joist which along with all the rest of their timber products was stored in open sheds at the supplier. It is supported on two gate posts one affixed to the garage the other about to be attached to the house wall held in place in rebates by coach bolts.

Richard

Can you leave an expansion gap at the ends?

Mike