The maintenance for most outdoor fireplaces is fairly similar. In order for the fireplace to function effectively, it needs to be kept dry and ash free, and the chimney needs to be cleared of debris. Along with these basics, it is important to consider the placement of the fireplace and the materials from which it is made.
Step 1 - Outside Materials
The outer material of the fireplace is what receives most of the damage, so it is important to consider how these materials will hold up to the elements. Cast iron will benefit from a nice coat of paint. Clay should not be moved, as it will easily break. Copper will quickly develop a patina when exposed to rain. The best way to keep an outdoor fireplace looking nice is to seal the outside with either paint or a heavy-duty outdoor sealant.
Step 2 - Remove the Ashes
In order to build good fires that aren't dangerous, you should regularly sweep out the ashes from the central pit. It is not as important to get the interior clean as it is to get it empty of debris. Do not use paper bags or any kind of flammable materials when dealing with fireplace ash. Using a metal bucket and a metal scoop, remove the ashes and allow them to sit overnight before you dispose of them.
Even if there seem to be no coals or open flames, the heat from the ashes is a fire hazard in and of itself. Why leave them in the fireplace when they can be composted and used to improve the soil in garden beds? Also, if the ashes are removed after each use, it will result in fewer sooty puddles and stains after it rains.
Step 3 - The Chimney
Avoiding getting water and debris down the chimney is an important part of maintaining outdoor fireplaces. Since the interior of the chimney is not sealed to the same degree as the outer edges, it is much more susceptible to damages. Covers can be purchased and then placed over the hole when not in use to greatly diminish any problems. Sweeping the chimney out with a wire brush once a year will also help remove any build-up.
Step 4 - Placement
The best way to minimize the amount of fireplace maintenance is to consider where your fireplace's location. Try to place the fireplace out of areas that receive high winds. This will keep ashes and embers from getting caught in the wind and winding up in unwanted places.
If the fireplace is directly exposed to the rain, a splash guard or protective mat directly outside the opening will keep the patio from being stained with puddles of ashy water.