Both, more than likely. The mason should replace the masonry products and the roofer should work with the mason to achieve a regletted counterflashing and flash the roof under counterflashings.

There are a very wide veriety of gas fireplace inserts.  

1. Check with the city to see what you can install.  Some cities are now requiring sealed units

2. Do not try runing a gas line by yourself.  Hire a licensed plumber to do it.

3. http://www.csia.org/?gclid=CL_Qk-SZ0dACFZBefgodOrUNRA Contact a CSIA certified chimney sweep to help you install. 

Thanks

Philip Anderson

HDR Remodeling Berkeley Ca

http://www.ncsg.org/  The National Chimney Sweep Quild is a great source for licenced and certified professionals in your area. If they don't do the work, they will know someone in your area who does.

Philip Anderson

HDR Remodeling

Berkeley Ca

No, but have you thought about contacting a manufacturer and asking for a local referral. Sometimes it is an unknown fact but home remodeling product manufacturers have authorized dealers and installation crews listed on their site! This guarantees the best match. We always explain to homeowners having a certified installer is more important than price! Residential Renovations works very hard to make sure all installers from roofing, siding and windows to decking, sunrooms and beyond is installed by professional crew members who are trained as the manufacturer warranties. Good luck with your project! You can visit our website at www.ResidentialRenovations.com to learn more about Residential Renovations - serving Ohio and Michigan homeowners!

For a great number of years, I was a mason and chimney sweep before rolling out to full bathroom and ktichen remodeling.  It still is my first love.  Please have a certified, licenced chimney sweep out to look at the chimney.  /http://www.ncsg.org/ can put you in contact with a certified chimney professional in your area. Please do not use it until it is checked. 

Philip Anderson

Berkeley Ca

Hard to say based on the limited description available. Is the fireplace constructed completely of masonry? or is there an insert in the firebox? Is it framed walls with stone/brick veneer? Is the chimney a metal pipe or clay flu?

If you haven't used it and it is cracking, I would suggest that the foundation was possibly inadequate and it is settling.

Or the materials used were inferior materials, or the mortar wasn't a quality mix.

There are many factors that could be the cause. Hard to say based on the limited information.

I will say that converting from a wood fireplace to a gas fireplace shouldn't be much of an issue. Converting from gas to wood on the other hand is virtually impossible without major remodeling and expense.