This article is from the Pool & Billiards FAQ, by Bob Jewett with numerous contributions by others.
Some have suggested that a home solution like installing roof
de-icing cable, may lead to concerns about fire. In addition,
concerns have been raised that if the slate is not heated
uniformly, then the heating element is really not doing the job
properly, and then the table is probably going to have a
different roll in different sections of the table.
Commercial table heating systems have peak powers of over 500
watts, and well insulated and solidly constructed. Be careful.
A typical commercial heating system has sections of plywood
mounted a few inches under the slates forming closed chambers.
Heating wire (special resistance wire) is mounted on insulators
in these spaces. A thermostat controls the power to keep the
slate only a few degrees above room temperature.
 
Continue to: