The manufacturors recommendation is 1 from the fp and 2 from the chimney to combustable materials.
Fireplace attic chase.
Most of the units inspected by chimney professionals are improperly installed and a trip to the attic to check clearances is definitely time well spent.
Smaller dimensions for a rough frame of a chimney chase are about two square feet.
If this height exceeds 15 you are okay to install a attic insulation shield.
You can also taper the chase above the fireplace.
The base of the chase must accommodate the framing dimensions of your fireplace.
After you know where the penetration point will be for both attic and roof add 2 to all sides.
Doing this leaves room to encircle the chimney pipe.
Fire stops are self describing.
They are installed to stop a fire in the chase area where the vent pipe runs through to get to the roof.
My builder is saying that there is no requirement for a firestop since the chase and pipes are on the outside of the house with no floor or attic floor penetration.
The interior of the chase has plenty of clearance to the fp and chimney.
Climb into your attic and locate where your chimney system will enter this space.
However a home inspector may tell you that there is no fire stop or that it is installed wrong.
The rsf manual in the section about chases though they mean outer wall chases says.
Seldom do we think about the fireplace metal vent pipe running up through the attic and roof.
Inspecting the firebox clearances is more challenging.
Measure the height of the furthest point of the installation.
If the climate in your area is very cold insulate the chase to the top to keep the chimney warmer increase the draft and reduce creosote buildup.
As currently designed the distance inside the chase to the chimney or firebox will be more like 24 on average with the closest spot about 6.
We find leaks here especially where the top is framed and enclosed flat and the top flashing lacks adequate pitch and stiffness to avoid collapsing even becoming concave where there are snow loads.
Any wood that is too close to the chimney will undergo a process called pyrolysis and even catch fire at temperatures as low as 200 degrees.