Advice for flagstone floor renovation

[SiloSi]

Hi all, I’m currently renovating a mid-19th century cottage with flagstone flooring. This had several patches of damp and/or condensation/sweating appearing (esp. in summer).

Originally there was a hearth built up and protruding about a foot from the opening, with a false wall at the back and the outlet from the woodturner going straight back into the cavity and no flu in the chimney, with consequent very poor draw (picture 1).



I have cleared all this material to reveal the original much bigger opening and dug out the fire place, which revealed bedrock under the rear wall and coming in significantly into the fireplace (pic 2 - areas of damp marked in red)



My original intention was to lift all the flags and remove the blue DPM that I am sure isn’t helping the flags breath and maybe forcing damp into the walls, then relay on lime (3 sand : 1 3.5N lime mix). I’ve lifted several flags where you can see hints of the blue membrane, as well as white sacking that’s been used… nice! (Pic 3)



Although there is damp at the wall to the right of the fireplace and under the flags butting up to it, there was no membrane under them, though there are sacks being used as membrane in the next row of flags. However, there’s no sign of damp under the huge flag removed from in front of the fireplace that had no membrane, just gritty sand as the original substrate.

There is evidence of damp at the back of the fireplace where the bedrock is, so I’m wondering if the damp is coming in from the outside here and along the wall to the right of the fireplace? There is a narrow pathway outside behind this wall, but no French drain fro drainage.

My questions are as follows:

1. Should I re-bed the big flag straight back onto the apparently dry sand as is (if it ain’t broke, don’t fix it?!), or should I lay on lime mortar? It’s so heavy, I can’t see movement being an issue if on a flat bed of sand and I’ll of course mortar the joins.

2. I’m assuming re-bedding all the other flags onto lime is the most sensible option, and not to attempt to repair/replace the existing membrane. If so, what thickness of mortar should I use and do I need to remove all loose and/or damp sand/rubble under the flags first, and if so, what should I use to level up below the mortar?

3. What about the damp issues at the wall and back of the fireplace? Lime or membrane here?

4. For rebuilding the hearth, am I legally required to have a raised hearth above the level of the original floor and protruding into the room (as per original fireplace), or can I just lay flags into the fireplace at floor level and sit the wood-burner in there? After all, flags aren’t exactly combustible!

Any further advice greatfully received.

Thanks.

Simon

 

[SiloSi]

Well I guess I’m going to crack on, as a bit more research has answered most of my questions (re-bed on c 20mm lime mortar throughout, dig a French drain outside and fill with pebbles to combat penetrating damp from outside, hearth protruding into room defo required), but please feel free to throw in any comments!

Thanks

Simon

 

[a twig]

Hearth just has to be non-combustible and visually discernable from the rest of the floor, it doesn’t have to be raised - although if it is that then meets the definition of visually discernable

Agree laying on lime mortar - a search of the forum will find a few threads where people have done the same.

If high ground levels are an issue you could consider a Zebcolm trench as an alternative to a French drain

 

[SiloSi]

Much obliged!

 

[SiloSi]

Hello again,

Still in a bit of a quandary, as bedrock clearly extends under all the flagstones at quite a shallow level. Just pulled up a flag that had DPM under it, and as expected, all the sand/rubble underneath was very damp.


However, although the top surface of the sand under the big flag that had no DPM is dry, brushing back to the bedrock, the bedrock and sand/earth/rubble immediately above is damp. The rock is uneven so the sand is quite thick in places.

I don’t if the damp is rising from below or forced in from the sides from flags that do have DPM? Wondering if I should clear the sand completely to let the bedrock dry out as much as possible before laying lime mortar, or whether I can crack on, on the basis that the damp doesn’t seem to be rising through the sand that much, and re-bed onto that?

Here’s the state under DPM:


And here it is under the big flag without membrane with damp bedrock exposed in middle:



Thanks

Simon

 

[a twig]

Crack on - the lime you are laying the slabs on will allow excess moisture up through the joints, the important thing is to then ventilate the room so the moisture can be exchanged / expelled

 

[SiloSi]

Thanks again. Actually putting it on hold for a bit, as got advice from lovely lady at Mike Wye, who advised that I’m best off using nhl. 5 not the weaker 3.5 I have. Luckily I only got one bag in as a tester and can use it for pointing elsewhere, but she reckons I’ll need about 10 bags of lime and mix a 2:1 sand lime mix, so I need to get a load in. Can really recommend them for price too… even with a £65 delivery fee, still only works out at about £20 a bag, as opposed to the £35 odd at Travis Perkins I was paying!

Cheers

Simon

 

[a twig]

If you want to really be depressed NHL3.5 is about £9 a bag at cost to Travis Perkins

 

[a twig]

For me a 2:1 NHL5 is a bit much, that will be very hard and not a huge amount more breathable than a 6:1 cement mix…

How long would you be able to leave the floor before it starts getting regular traffic?

 

[SiloSi]

Fair point! And obvs a lot more expensive than the 3:1 mix I was planning… I’m planning on doing the floor bit by bit and not all at once, so can avoid areas that are curing/setting…and also avoid breaking my back!! I’d like to reinstate the stove in the fireplace asap, which will hopefully speed up drying out in an otherwise damp cold room, so doing that area first.

So what mix and strength would you recommend and how long should I leave it before regular walking on it?

Thanks again.

 

[SiloSi]

Just seen your previous post…£9 to TP?! What an outrageous mark-up! They actually advertise it at c.£45 trade price but I usually manage to knock them down…even so!

 

[a twig]

I’d be inclined to stick with NHL3.5 and sharp sand in a 3:1 ratio - research has come out over the last few years that shows as the hydraulic limes carbonate they get stronger / harder.

Maximum compressive strength should be reached in around ~28 days so it sounds like that should fit with you planned approach - definitely no harm in starting with the wood burner end and getting that up and running, will be a huge help with drying the room and drawing damp air out.

 

[SiloSi]

This is great info, thanks so much! Have a great weekend

 

[Itchinfilth]

I've yet to have a DPM under my flags in houses Ive had (always on top with a nice layers of concrete and other rubbish on top ) Where flags needed to be relayed previously I layed on lime as you are doing but in my current house I went minimum intervention and layed back on the earth as found with sand to level and pointed in lime. An option to consider.

 

[SiloSi]

Thanks for that. Where the flags have sat on membrane, there’s also a load of cement and not much in the way of sand/earth before bedrock. I’ve got to get in a load of sand anyway and with the great price Mike Wye is quoting, it makes more sense to me to bed them in firmly with lime. Cheers

 

[SiloSi]

Hello again,

Been doing a bit more research and spoke to someone at Ty-Maur, who has recommended clearing out to bedrock and levelling up with a layer of fine (up to 12mm) foam glass aggregate dry mixed with some lime. This apparently will have the twin benefits of providing some degree of thermal insulation (which would be difficult to achieve any other way without digging out the bedrock that’s only a few inches down) and also helping prevent rising damp from damp bedrock.

The only downside is the extra cost, not just of the foam glass, but needing a compactor, that’d be an extra £300 to buy in (don’t really want to do it in one go, which makes hiring one on multiple occasions expensive).

Anyone had any experience of using foam glass or have any thoughts about it?

Thanks

Simon

 

[a twig]

You’ll be able to sell the compactor on afterwards and get the majority of that £300 back. Others on here have more experience of this foam glass stuff so I’ll leave that to them

 

[SiloSi]

Yeah reselling did occur to me. Actually saw a handheld vibrating plate/compactor for about £40 new on eBay (someone also selling identical looking one at £400!), so wonder if that would do the job, or if big petrol one would be best?

 

[Stevers]

Providing insulation and damp resistance with blown glass in one step seems too good an opportunity to miss. I had no idea that electric whackers existed or could be found brand new on Ebay for less than £250, but if I ever needed one that's what I would look at. I did buy my Belle cement mixer in the knowledge that I would be at least the third person to sell it on still looking immaculate - I even got a day out to Minehead to collect it, calling at Dunster on the way back!

 

[SiloSi]

Cheers for that! I’m thinking the £40 handheld one, which looks fairly lightweight probably won’t be sufficient, but Evolution do an electric whacker plate on offer at £229, which looks promising with good reviews saying it’s fine for smaller indoor jobs and works as well as some more expensive petrol ones…