I am going to install a sheet metal roof (Clix) on my newly built roof, and the roofer said he wants the chimney penetrations to be prepared when he comes. Holes should therefore be made in the roof, and some kind of box should be built above the roof so that he can cover it with metal.
His suggestion was that the box should be built with minerit or equivalent, which also the chimney sweep thought sounded good.
Does anyone know how to build such a box? How high above the roof should it extend? Should it go through the entire roof (boarding, air gap, hardboard, insulation, plastic)? How far below the roof should it extend in that case?
Since the box should be heat/fire resistant, it becomes a bit tricky to build it - you cannot really use wood.
His suggestion was that the box should be built with minerit or equivalent, which also the chimney sweep thought sounded good.
Does anyone know how to build such a box? How high above the roof should it extend? Should it go through the entire roof (boarding, air gap, hardboard, insulation, plastic)? How far below the roof should it extend in that case?
Since the box should be heat/fire resistant, it becomes a bit tricky to build it - you cannot really use wood.
The crucial factor is which chimney you plan to have in the "box." You must start from there, dimensions, insulation requirements, distance to combustible materials, etc. It is possible to build a fireproof shaft from gypsum, insulation, and wood studs. The wood studs are, of course, on the outside of the shaft.
Regarding material choice, it is always safest to check with the chimney sweep, as he/she is the one who needs to approve everything. Generally, it is required that there is 100 mm to combustible material, and if you go below this, minerit boards are necessary.
How you build the box is somewhat up to how you want to do it, you can frame and lower an entire shaft, but expect it to take time. The easiest way is probably to open up around the chimney and let the minerit boards go down a bit. The framework then rests against the rough-sawn plank and is clad with, for example, plywood on the outside to make it possible to nail underlay felt at the transition to the roof. Don't forget the glue here, this must be tight!
The height is determined by the chimney; you probably want the box at the height of the crown.
/Kent
How you build the box is somewhat up to how you want to do it, you can frame and lower an entire shaft, but expect it to take time. The easiest way is probably to open up around the chimney and let the minerit boards go down a bit. The framework then rests against the rough-sawn plank and is clad with, for example, plywood on the outside to make it possible to nail underlay felt at the transition to the roof. Don't forget the glue here, this must be tight!
The height is determined by the chimney; you probably want the box at the height of the crown.
/Kent