Retrofitting a fireplace,builder's opening too large? SOLVED

9 May 2012
Reaction score
United Kingdom
Hi guys,

When renovating our lounge (house is vc.1900, red brick), we opened the chimney breast up intending on putting in a wood burner. The cost of the flue liner etc precludes us doing that so we initially settled on sourcing a cast iron fireplace and wooden surround off ebay - which we've done (for £70 for the lot!!). However, we are considering moving abroad for a few years and are concerned that if we rent the house, someone may light a fire in it and, secondly, we'd love an open fire anyway!

We had a few HETAS people round to look (the only three within 25 miles of us in the HETAS website). The first two kinda shrugged and said "I only do woodburners", whilst the third guy wanted £1,500 as he wanted to erect scaffolding to "check the chimney stack". The chimney stack was rebuilt from the roof up 12 months ago, so I know is perfect - he also ran a smoke test and visually inspected the chimney both inside and out and commented it was in very good condition, unobstructed and with good updraught. I rang him back to see if he'd come down on price but he said he'd do it for £1000 but that's his bottom price as he's so busy - but said it would only take him two days!

I'm certain I can do this myself. I've done as much research as I can and feel I'm 90% there, but I'm stumped on the last 10%.

As this is a cast iron fire, I recognise I in effect create my own fire-back by filling behind the fire with rubble, then closer to the fire vermiculite and mortar. I know to slop the vermiculite mix up to form a throat to the chimney and cover this with a thin layer of mortar to provide a smooth surface.

The issues I have is with the top of the fireplace;

1. On all the example I've seen on the internet, there appears to be a 'gathering' or throat forming lintel that sits on top of the fabricated fire-back to guide smoke backwards into the throat of the chimney. As no fabricated fire-back will be used here, what am I supposed to do.. nothing?

2. Secondly, the builders opening is big - 36" wide and 44" tall at the top of the arch. The biggest fireplace we could find without appearing wrongly proportioned is only about 40", leaving a gap of about 1" at the sdei / 4" at the centre between the top of the fireplace and the bottom of the chimney breast. The surround we've bought covers this gap which is fine if we're doing this for aesthetic purposes - but what am I supposed to do if we want to return this to use? Is the fireplace too small (I don't see how though), is it the wrong design of fire, do I fill the void with fire board or similar, or do I just use fire ropes to seal it? Any advice or thoughts would be appreciated.

I realise this is a bit hard to visualise so I've taken photo and tried drawing the challenge I face...

Thanks for getting this far - would love to hear your thoughts!

Sponsored Links
Hi fogster74,

I just stumbled across your post and wondered how you got on. We have a similar problem in that the builders opening is a bit too high in comparison to the height of our fire surround opening. I'm wondering whether to fit a lintel and then brick up to the builders opening arch. What did you do?

Why dont you post a picof your fire opening with highlighted dimension lines for what width and height you require? The OP is a year old.
Sponsored Links
Hi! It's been a while; since my post my wife and I have emigrated to the Middle East and never got around to posting the final outcome! Anyway, here goes, hope this is useful (it was some time ago, but I think I've covered most things)!!!

After some superb advice from a guy who posts videos on You Tube, I successfully managed to install the fire. Here's what I did:

1. I was concerned regarding the state of the chimney so I got it swept and smoke tested. Our house is very 'airy' as it's old Victorian but I was also concerned about ensuring the fireplace could breathe, so I ensured the air bricks under our suspended wooden floor were clear and also installed an antique brass vent in the floor. I put this under the window in the photos so that any drawn air would some into the room and have an unobstructed course to the fire without passing people eg sitting on the sofas etc, which would have been unpleasant for them.

I also bought an additional two Carbon Monoxide alarms; one for the room with the fireplace in it, and one for the room above. Better safe than sorry! All was given the OK by the chap who came to look at the chimney.

2. I decided on a rustic slate hearth that met with Building Regs, rather than trying to use the old Victorian tiles that I discovered during renovation (as they'd been obliterated by carpet fitting nails, and didn't constitute a raised hearth nor meet building regs). The slate was bloody heavy; I got a stone mason to supply and cut to my dimensions. On advice, it's three cut into three separate pieces; this is to help prevent it cracking due to thermal shock during expansion etc. We levelled the rear (inside) of the fireplace using cement coloured with black cement dye; the effect was exactly what we were looking for!

The old hearth damage can be seen here:

3. To start installing the fireplace, I 'dropped' the arch to below the height of the top of the fireplace by installing a concrete lintel, and then bricked up above that:

4. I purchased a large bag of 'vermiculite' (an inulator which is incredibly light but looks like shards of a quartzite rock) and also 'rockwool' from Selco; although I thought they were trade only, they are quite happy to sell to the public. I ended up needing only a fraction of the Rockwool roll but it was much much cheaper buying a roll from Selco than anywhere else.

5. On the advice of the chap who I spoke to, I covered the back of the actual iron fireplace with corrugated cardboard; will explain more in a bit!

6. I then positioned the fireplace in-situ and used the Rockwool as a seal around the outside (top and sides). I screwed in place by using metal straps across the top corners of the fireplace and into the wall (I didn't try to drill the fireplace as I was afraid of breaking it in some way.

You can see the metal strapping in the corners here (this was taken before I fitted the hearth and lintel):

7. Once the fireplace was secured in position, I mixed the Vermiculite with cement and just a little water to make a rather dry, light mix, which I then used to fill the void behind the fireplace.Once level with the bottom of the throat at the back of the fireplace, I sloped the vermiculite up inside the chimney breast on the rear and sides so that the smoke was directed towards the chimney.

8. Remember I mentioned covering the back of the fireplace in cardboard? This is so that, when you first light the fire, it gets hot and eventually catches fire - this is the plan! This then leaves just a few millimetres of air gap between the cast iron fireplace and the vermiculite backfill. This means the cast iron fireplace can expand slightly, but you still have a 'fireback' to absorb the heat. If you didn't so this (leave a gap), the vermiculite would set hard against the fireplace and there's a good chance the fireplace would crack in use due to thermal expansion (and nowhere to go). Equally, if you don't backfill the fireplace, the air gap behind the fireplace can work as an insulator (the vermiculite actually absorbs heat from the fire and releases it after it's finished burning) and again, the fireplace could crack.

9. Once all was done, I attached the fire surround - job done!

(I sometime later plastered the arch to finish the job)

We were thrilled with the result. Things we learnt ...

- You need a good fireguard. We only had a temporary carpet but we got a number of scorch marks on it in the first few days. We purchased a clear glass one from a local fireplace centre

- The quality of the wood you burn is EVERYTHING. We initially bought wood from our local petrol station forecourt / DIY store, and it was a nightmare to get burning and quite damp. Then we were recommended and purchased a half-crate of kiln-dried Ash. It was absolutely amazing - got delivered plastic wrapped (our neighbour joked had we purchased the hermetically sealed firewood from Harrods!), was beautifully dry and caught instantly; we often found we needed virtually nothing more than some paper to start a fire. Superb people to deal with too - highly recommended!

- Finally, nothing beats an open fire and we used it continually; it was incredible to see the life it gave back to the house. Everyone loved it and no one could believe we'd done it ourselves!

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local