Considerations and installation methods
Conventional masonry chimneys can be used in timber frame construction once consideration has been taken for the protection of fire spreading from the chimney out into the timber frame structure, and that differential movement has been accounted for in the design between the masonry chimney and the timber frame.
There are 2 principal methods for constructing masonry chimneys in external timber frame walls.
Method 1: The masonry fireplace and chimney are constructed outside of the external timber frame wall; the chimney is constructed in a preformed aperture on the exterior of the wall. By using this method, the structural integrity of the frame is not altered through the installation of the chimney and it reduces the quantity of openings that would be required if the chimney were located internally.
Method 2: The internal linings of the chimney are preinstalled, and the fireplace and chimney are built inside the room. Unlike method 1, this method requires trimming of both roof and floor members to accommodate the chimney. It is important also that consideration be taken at design stage to allow for movement of the timber frame around the chimney where it protrudes at roof level.
Flues from the chimney should be installed either by allowing the proprietary flue to exit through the external wall, which protects against the spread of fire and smoke internally, or, alternatively, if the flue is to be positioned within the wall, it should either act as a cavity barrier by its own construction or requires a cavity barrier to be installed. Proper sealant must be achieved to ensure flue gases cannot enter back through the timber frame and back into the building.
Where the blockwork around the flue is less than 200 mm, a minimum 40 mm gap should be left between the timber frame and the chimney. Any metal ties that are used should be as per the manufacturer’s specification; typically, the metal ties are supplied by the timber frame manufacturer.
For fire safety reasons, all timbers must be at least 40 mm clear of chimney or 200 mm from a flue. A 12 mm movement gap must be left between the top of the chimney and the floor joists where masonry chimneys are constructed after the plasterboard has been fixed to the timber frame and the plasterboard continues behind the chimney.
For fire safety reasons, all timbers must be at least 40 mm clear of chimney or 200 mm from a flue. 200 mm min. of blockwork to back of fireplace recess; also, provide a min. 40 mm gap between chimney and timber frame where blockwork around flue is less than 200 mm.
Construction and fixings
Where chimneys are allowed to be constructed in timber frame construction, the chimney should not breach the party wall unless specifically certified and approved. The construction of the fireplace must not impact on the effectiveness of the party wall's acoustic or fire resistance properties. Combustible materials (excluding floor boards) such as skirting, architrave, mantle shelf, etc. must be separated 40 mm from the outer surface of the chimney or be a minimum distance of 200 mm from the flue.
Fixings for installing chimneys should be fit for purpose and extend through the plasterboard into the timber studwork to give the correct support to the chimney. Any chimney system approved for installation into timber frame structures must be installed as per the manufactures instructions.
