I have a green-house which is seated on a concrete base. Between the
greenhouse frame and the base I put some wooden supports approx 40x20
mm. The aluminium from overhangs the wooden support so there is no
access to it once installed. The supports have just about rotted away
totally - been there for quite a few years.

I want to reseat greenhouse - replace the wooden lats. What would be
best preservative to apply to the wood - it will get very wet as rain
runs down glass and sits around the base.

Use tanalised timber,(pressure treated preservative)and retreat every 3
years all available from your local timber merchant