If you live in an older home that was built before the 1940′s, your chimney was probably built without a clay tile liner. Starting in the 1940′s clay tile liners were built into the chimney during construction, and in more recent times many homes are built with prefabricated metal chimneys rather than with traditional brick and mortar ones. Whatever type of chimney you have, it is critical that the flue, the opening running up the inside of the chimney that carries smoke and combustion gases out of the building, not have any damage that could allow your chimney to leak.
If an inspection by a qualified chimney professional reveals problems, you may need to have a new chimney liner installed or your old liner repaired or replaced. The inspector might make a visual inspection of your chimney system, or he might have the equipment to make a video recording of the inside of your flue.
In old unlined chimneys, missing or cracked bricks or mortar can make the chimney leak. In chimneys lined with clay tiles, the inspector will look for cracked or broken tiles and missing grout. If you have a metal chimney liner, the inspector will make sure there are no leaky joints or other damage. If your inspector finds damage that cannot be easily repaired, you will have to choose to either replace your existing liner or install a new one.
If you are handy with tools and have a good understanding of how fireplaces and chimneys work, you may be able to do most of the liner work yourself, but you should consider the relative costs involved in doing your own work compared to having a chimney professional do it for you.
The first thing you need to do is check your local building and fire safety codes. In some areas, you will be required to have a licensed chimney specialist make repairs or do new liner installations for you. If you find that it is permissible for you to do your own chimney work, you still might decide it is better to let an expert do it. Replacing or installing a chimney liner requires precision work under potentially dangerous conditions. Whoever does the work will have to spend a lot of time climbing around on your roof, mucking about inside your fireplace and on both ends of your chimney, and potentially doing extensive masonry work requiring removing and re-mortaring bricks and tiles. If a metal chimney liner is being installed, the worker will be cutting sharp metal and making precise fittings of joints, bends and caps. The work has to be completed correctly or the flue will not work well and could be more dangerous than the existing situation. So, unless you are confident you know what you are doing, it might be best to let an expert do your chimney repair and lining installation work for you.