How should I prepare federation era fireplace for hearthstone install?

Question

I’ve been restoring my federation-era fireplace to install a free-standing hearthstone fireplace that will be flued through the chimney. After removing the cracked hearthstone and lime from the recessed hearth box I’ve decided I want to install a T-shaped hearthstone. I'm thinking of approaching it like his:

* removing a layer of brick in the fireplace to bring it closer to level with the recessed hearthbox; 
* creating a dam with builders wrap that covers the fireplace and hearthbox in a T-shape; 
* lining the bottom of the dam with cement sheeting; 
* pouring a perlite concrete mix with reinforced mesh to build up the base for the 20mm thick hearthstone to sit just proud of the floor.

I'm wondering the following:

* Will a perlite concrete mix (5:1) be strong enough to hold a 20mm piece of engineered stone plus a 90kg+ freestanding firebox?
* Is there another (better) way to proceed? 
* What am I missing?

Dave-1 · Answer

Good Evening @doberon

I am in full agreement with @JacobZ on this one. Looking at the state the bricks surroundingthe fireplace are in with their open cracks I would hea dtowards a bricklayer who has experience with fireplaces as a first stop. Mainly to make sure its stable and then you can find out if its possible to remove that bottom row of bricks. I do not know of the regulations for the hearthstone but think there will be some kind of fire rating involved.

Dave

JacobZ · Answer

Hi @doberon,

That sounds like an interesting project, and it's great to see you're taking the time to restore the original fireplace before installing the new firebox.

Unfortunately, this is getting into an area where I wouldn't feel comfortable providing specific structural advice. Not only does the hearth and the structure beneath it need to support the weight of the engineered stone and the 90kg+ firebox, but it also needs to comply with the installation requirements of both the fireplace manufacturer and any applicable building and fire safety regulations.

While a perlite concrete mix can provide insulation benefits, its structural properties can vary considerably depending on the mix ratio and application. Whether a 5:1 mix would be suitable for supporting the combined weight of the hearthstone and firebox, as well as whether it would be suitable with the structure that sits beneath it, is something that should be confirmed by a suitably qualified installer, builder or engineer. The same applies to the proposed build-up method, reinforcement requirements and the preparation of the existing fireplace recess.

Before proceeding, I'd strongly recommend obtaining the installation specifications for the particular fireplace you intend to install and discussing your hearth design with either the fireplace supplier, a licensed fireplace installer or a builder experienced with solid-fuel heating appliances. They should be able to advise on the required hearth construction, load-bearing requirements, clearances and insulation values needed for your specific installation.

It may also be worth checking whether the existing chimney and fireplace structure require any modifications or certification before the new flue system is installed.

Allow me to tag some of our helpful members to see if they have any advice or experiences they can share - @TedBear, @Nailbag, @Dave-1, @Noyade.

Apologies that I couldn't be more help.

Jacob