Pointing internal slate flag floor

[Geraint]

I have a Victorian slate flag kitchen floor suspended on timber joist floor. The flags are laid on the joists, and between them a loose-ish lime plaster resting on timber planks. The slates are at least 1m long and an inch thick and are from the Blaenau Ffestiniog area.

1. There are a few joints that need filling. Some of the gaps are at least an inch deep. Can anyone recommend a suitable material for filling/grouting? What about a colour that suits the slate?
2. Could there be slight movement in such a floor that would affect a hard pointing?
3. One slate has a slight wobble. Could filling with mortar stop the wobble, or will I need to lift it with suction pads?
4. Sealing: any views on sealing internal slate flag floor?

I'm asking here because the advice on the site is mostly for external paving, am wondering to what extent it could apply to internal floor.


Any advice would be great, thanks.

[lutonlagerlout]

Geraint
the whole thing sounds wrong
flags and timber do not mix as you will always get movement with timber
are the flags laid directly onto the joists or is there a layer of marine ply or similar?

we have a similar job to this coming up soon and are removing all the timber and infilling with concrete

if its on timber any grout would need to be highly flexible

cheers LLL

[Geraint]

The floor has been in place since maybe the Victorian era and is in good condition. It lies on several 9x3 joists. The floor lies above the cellar so you cant concrete. The building is listed so I can't rip up the floor. It looks fantastic and I want to keep the original character.Most flags fit perfetly with no gaps. They are huge, about 1.8 x 0.8m and 25mm thick.

[Tony McC]

The flexing worries me. None of the usualy pointing products will provide suffient flkexion to cope with what you have, so I'd be looking at internal grouting products with a high flex modulus. Possibly summat that is recommended for bathrooms where the constant extreme changes in humidty often cause a significant level of movement.

Colour: with the Blaenau slate, a light coloured jointing will give good contrast to highlight and frame each flagstone. Something buffy-sandy coloured perhaps.

Wobble: never, ever rely on pointing to rectify movement in the bed. It rarely works, and on the few occasions that it does, it tends to be a very short-term fix. Lift and re-lay is the only real fix.

Sealing: all depends on what effect you're looking for. Companies such as Extensive with the Lithofin range offer a huge selection of very good products of which one is more or less guaranteed to do exactly what you want.

[Geraint]

Thanks for the advice, will be trying some flexible grout.

With sealing slate: a German neighbour has used a sealer caller 'Rember fluat', which is used in Germany and Spain. I can't find much info on this, anyone know anything about it?

http://www.remberproductos.com/de....cha-217

[Tony McC]

Slate is practically impermeable, so sealants tend not to work too well.

I'm not familiar with the Rember product, but looking at the list of materials for which it is claimed to be suitable, it seems to be a jack-of-all-trades. My German isn't very good, but I see no mention of slate.