Closing off chimney with mortar

[thierry]

Hi,

We have just finished exposing a large old kitchen fireplace from behind a fair quantity of plaster, bricks and various other material, and we now want to close off the old chimney flue so that the fireplace and hearth are open and cleaned.

Our neighbours upstairs has the same chimney in her kitchen and people who lived there before her seem to have closed it off with cement/mortar. They seem to have used a mould as the mortar is perfectly applied. Does anyone know how this is done? I was thinking of using MDF but eventually the dampness would go through -it's also very dirty in there.

If anyone knows what is the best way to block it off / cement it, an answer would be much appreciated.

Thank you

Rgards

Thierry

[Kalico]

You could use a wooden board though I would steer well clear of MDF. Good exterior ply would be fine.

It really should be ventilated and how you achieve that is up to you. Small holes drilled in the board is one way but will allow muck to fall through.

The correct way is to board over completely (or cement up above a plate for example) then install a grille or some other access just above in the flue/chimney breast. Usually, this is considered unsightly so is often avoided.

Be sure that the top of the flue is correctly capped to allow ventiation without water ingress. Do not allow any roofer to just close it off completely or you will certainly get moisture coming through the wall of the flue elsewhere.

Hope that helps but feel free to ask further.

Rob

[Andrew Gabriel]

I used a piece of plasterboard stuck in place with bonding coat plaster. I put some fibreglass loft insulation on top of it, rather more to act as a shock absorber for any bits of debris falling down the chimney than as insulation.

I installed an air-brick into the flue above the plasterboard. In my case it's an outside wall, so I vented to the outside (and into the lft at the top). Venting top and bottom is essential, or chimney breast gets wet (which is what had happened, and caused me to have to sort it out).

I also dropped a lighting cable down the flue from the loft, and have installed a small light on the plasterboard to provide accent lighting in the fireplace. Plasterboard is fitted some 6" above the fireplace opening to make the light concealled.