OK how do you lay a hearth stone?

[Simpo Two]

In a couple of weeks a large slab of pre-cut stone (Hartham Park limestone to be precise) 1300 x 760mm x 40mm will arrive and need to be fitted. The existing ground is the solid concrete floor of the room. The stove is a Stovax Stockton 5 multifuel - so under the grate is an ash pan and there's a 4-5" gap underneath between stove and hearth. Whatever the bed is, it will only need to be 5mm thick.

The local stonemason says use a solid bed to transfer the heat. The local builder says dot and dab. Chesney's website says dot and dab. Somebody else said flexible tile cement.

Who's right?

Edited by Simpo Two on Sunday 13th October 11:23

[Simpo Two]

Thanks guys. The idea behind 'flexible', which makes sense to me, is to cater for any expansion/contraction as it warms and cools. Then again a solid bed would conduct heat straight into the concrete floor... then again dot and dab would give a cavity for the hot air to esacpe by. I can debate it all ways!

The unknown quantity is how hot/warm it will get under the stove - I suspect not as much as feared due to the large gap beneath, and the fact the firebed is over an ashpan.

[Simpo Two]

Sound input, thanks all.

It seems there isn't much to worry about after all, apart from dropping it and breaking it during fitting!

I think, bearing in mind that mortar is not my thing and I note the comments about getting it to 5mm, that I might just buy a small tub of tile adhesive, and use my big 10mm-notched trowel, and treat it like a humungous tile - and just get a strong friend in to help.

If the base is level, and the adhesive is applied evenly, then the hearth will be level too. We'll have to place it an inch or two forwards while we get our hands out, then shove it back to the wall.

Level, thin layer, careful DIY job and no need to call in a builder. Sounds like a plan?

[Simpo Two]

Hmm.

So I went into B&Q to buy a tub of tile adhesive, enough for 1m2 whch is all I need.

Except that almost every product in the display was called 'grout', and of the two tubs that were called 'adhesive', both said they could only be used up to 3mm thick.

So do I pile the adhesive up ignoring the instructions, or use grout? At the end of the day the slab is flat, it's not going to move anywhere, it just needs to be properly supported, especially in the middle. If anyone has a link to something appropropriate (not a 20kg bag please!) it would be welcome. Instant grab is no good as it will need to be slid back after placing.

[Simpo Two]

B17NNS said:

Does it really matter what bed thickness you have so long as you have a solid bed all over?

Somewhat yes; it needs to be enough to run a 79 strand speaker cable underneath, but not so high that the fire surround reveals the lintel.

I had hoped to use a thinner slab so I could run the cable in a 10mm copper tube, but they said 40mm was as thin as was safe in that size. Hence there's not much leeway on the bed thickness.

[Simpo Two]

Belated update...

In the end I took advice from the stone mason, who has laid many hearths. He said there was no need to do it in sections, gave me some BAL tile adhesive, recommended dot and dab, and described the packing route as Binns suggested. He said the adhesive would suck the hearth down onto the packers.

This proved to be an excellent idea. To prevent rocking I used three sections of spare wood strip, and discovered that the existing concrete bed was actually a few mm low on one side. Hence some packing pieces were added until the triangle was exactly level in all directions.

Then to the hearth - a single slab about 75kg that two people could barely lift, and certainly not place carefully into an alcove without mishap. So I dug out a little set of little wheels that someone had given me for moving white goods and we set it all up 'ready to roll'.

I tied strings to the wheel bogies so we could get them out afterwards, and we did a dry run in and out to make sure the hearth would fit in the cavity. It did.

Then I splodged dabs of tile adhesive around the ground, taking care to leave two 'tracks' for the wheels to run in and out. Then after a deep breath we carefully rolled the hearth into place through the field of dabs. Lift up the front a little, pull the wheel bogies out by the strings, and lower the hearth onto the packing pieces. Newton did the rest.

It sounded rather hollow at first, and I thought I hadn't used enough adhesive, but as the adhesive and hearth dried out (it took a week!) it sounded nice and solid.

It now has a 75kg stove on it and about 100kg of marble surround, and is just fine. And contrary to popular belief, it doesn't get very hot underneath the stove so sections weren't necessary.

It shows that the mason was right in every respect and that careful planning is the way to fit a 75kg hearth that no-one can lift, to within a millimetre

[Simpo Two]