Tiling in a damp fireplace

Don’t create a cavity using wooden battens and a backer bored and don’t tile over damp or unstable plaster.
In a 100-year-old house with damp in several areas, you should establish the cause of the moisture and choose a tile substrate that can tolerate the conditions before installing split-face slate

In a 100-year-old property, your walls are built to "breathe." Covering them with an unventilated cavity, a standard modern backer board, or thick split-face slate will trap moisture behind the tiles, eventually causing structural rot, failing adhesive, and blown plaster. Furthermore, using timber inside a fireplace recess poses a significant fire hazard and violates building regulations if it sits within the clearance zones of a working stove or open fire.The most reliable approach is to fix a specialized, breathable, or non-combustible substrate mechanically to the wall, or remove the old plaster altogether.Option 1: The Cavity Method (With Major Structural Changes)Building a cavity out to hide the damp can work, but you must not use wood battening.The Frame: You must use metal firrings or metal stud tracks instead of timber. This removes the risk of wood rotting behind the wall and complies with fire safety standards.The Board: Use a heavy-duty, cement-based, non-combustible backer board (such as HardieBacker or NoMorePly).The Cavity Trap: If you create a completely sealed cavity using a front border, the moisture "pooling" at the bottom will have nowhere to go. It will travel sideways into your room's main walls or under the floorboards. To prevent this, you would need to leave small, hidden ventilation gaps at the top and bottom of your border to keep the air moving.Option 2: Backer Board Straight Onto the PlasterIf you rawlplug and screw a cement backer board directly over the existing plaster, you eliminate the fire hazard, but you are still trapping moisture.The Risk: The damp plaster beneath the board will eventually degrade, turn to mush, and lose its grip on the brickwork.When to use it: Only choose this method if you first strip the problematic plaster away, seal the bare brick with a specialist tanking slurry or a damp-proof membrane (DPM), and then mechanically fix the board to the solid masonry.Option 3: Tiles Directly to the Plaster (Not Recommended)Split-face slate tiles are incredibly heavy and require a high-strength tile adhesive.The Weight Issue: Old gypsum or lime plaster cannot support the weight of heavy stone tiles.The Moisture Issue: Applying a non-porous layer of slate and adhesive acts like a plastic wrap over the damp wall. The moisture will push the plaster off the wall, causing your expensive tiling job to blow and fall forward.The Professional RecommendationFor an old, traditionally damp house, the safest, most permanent route involves removing the old plaster entirely:Hack off the plaster: Strip the inside of the recess down to the original bare brickwork.Apply a Render Coat: Apply a sand and cement render scratch coat mixed with a waterproofer/salt inhibitor. This creates a barrier against the pooling moisture without needing a cavity.Alternative Boarding: Alternatively, mechanically fix a cement backer board directly to the bare brick using masonry screws (bypassing adhesive entirely).Tile: Fix your split-face slate using a high-quality, flexible, heat-resistant rapid-set tile adhesive directly to the fresh render or cement board.

I sugest to use backer board straight on the wall

don’t batten it out with wood.
Because you already have damp:
Wood battens can trap moisture ? risk of rot and mould
Creating a cavity there can actually make damp worse, not better

Best option:
Fix a cement backer board directly to the wall (or use tile adhesive if the surface is sound)
Then tile onto that
Even simpler (if the plaster is solid and not crumbling):
You can tile directly onto the plaster
Key point:
Keep it breathable and avoid trapping moisture behind the tiles.

Personally, i would be stripping it back to brick, then give it a few days to air out and dry of. All being well I'd fit a concrete back board or something similar. This gives extra protection for the damp brick. Also it's a good level surface for the tiles to go on to

Do not use wood battens in a damp fireplace, as they can absorb moisture and rot. It’s better to fix a cement tile backer board directly to the wall and tile onto that after treating the damp area as much as possible.

Given the age of your home and the specific issue of "pooling" damp, your choice of installation method is critical. If you trap moisture behind a non-breathable surface like slate, it will eventually force its way out—either by blowing the tiles off the wall or by pushing the damp further into the masonry.

In this situation, I wouldn’t recommend tiling directly onto plaster, especially with existing damp, as it can lead to failure of adhesive over time.

Best practice would be to first address the source of moisture as much as possible. For the tiling, I would install a cement-based tile backer board (such as HardieBacker) rather than standard plasterboard.

If there is ongoing damp, creating a small cavity using treated timber battens and fixing a backer board on top can help with airflow and reduce moisture build-up behind the tiles. All materials used should be suitable for damp environments, including flexible adhesive and grout.

Overall, a solid, moisture-resistant backing with proper preparation will ensure a long-lasting and professional finish.

Screw cement backer board to the wall

Use tile adhesive

Add your split slate tiles