Lime pointing and rendering outside of old house - a few questions

[Black Timber]

Hi all

Great forum and I look forward to going through old posts and tapping into the wealth of info. In the meantime, I wondered if anyone could help please?

We've got some external walls that were covered in concrete render. This has now been hacked off and we're thinking to have it repointed using hot lime. There are sections of modern block and brick work around windows, which will be rendered in NHL2. My questions are...

1. Hot lime in mid-September. Seems from what I've read to be on the cusp of the season end, so any further views as to whether mid-September is ok?
2. Is covering the work in hessian for a week long enough or would longer be better?
3. The existing pointing seems to me to be more earth based, with small amounts of lime. I've removed all the loose and anything that succumbed to light prodding, which has left quite deep gaps on the whole. The difficulty is knowing when to stop, as in most places I think it would just keep coming away if I kept at it. But then the depth would be huge. Any tips on when enough is enough, and what is acceptable to point onto?
4. When having to point or render deeper areas, should it be done in layers? What are the potential problems if it is all filled in in one go? That's both for hot lime and NHL2.
5. I've pulled back the gravel to expose the stonework below ground level. Can this be pointed in hot lime as per the rest of the wall, or should something else be done when below ground level?
6. How do we go about selecting the appropriate aggregate coarseness for our situation / type of stone?

Sorry to ask so much. Any help on any of the questions would be massively appreciated.

Many thanks

 

[malcolm]

The season for lime is when it's not so hot that the lime will dry too fast and it's not so cold that there is a chance the lime will freeze over the following month. I'm planning to use lime from September to maybe mid November this year.

I use hession for about a week. It's useful for slowing drying and insulating against frosts. Couple of layers are better than one.

If the existing mortar has been very wet for a long time it will tend to be quite loose and will come out just by sliding a screwdriver along the joint. That's a useful way to remove the mortar. Mine was OK after between 1/2 inch and 1 inch. Point the whole depth in one go. The reason for the minimum half inch is so the mortar doesn't dry too quickly. Do the same below ground level.

Chose aggregate to match the existing mortar. It's nice to have a repair based on history rather than modern trends. I was lucky that a 50:50 mix of sharp sand and builders sand matched my aggregate and colour quite well.

 

[stuart45]

The aggregate size depends on the size of the joints. With random rubble stone you might need a coarse sand, whereas with some brickwork that has very tight joints a really fine sand would be needed. What type of stone is it?

 

[paulc]

Been doing some repointing over the last few months on some internal walls. The mortar has been drying out pretty quick, even after giving the bricks a darned good soaking. I've been mixing kiln dried sand in to lime putty (2.5:1) wand adding just enough water to moisten the sand. I've found that if it is too wet, the mortar cracks as it dries.
Some of the joints I've been patching have been 4" deep - A thin strip of wood was handy to poke mortar right in to the depths...

 

[Black Timber]

Many thanks for the responses, that's all very helpful.

@malcolm - sounds like mid September will be fine for us then, which is great. Would you mind just confirming whether it is hot lime that you're using?

As for the existing mortar, both that and the stone were both surprisingly very dry. The mortar is very dusty/sandy and just crumbles away, but not because it's been wet, as far as I can tell. The minimum depth is no problem at all. But some of the depth and voids is substantial. Quite a fair few inches in some places. I can measure when I'm at home if needed....

@stuart45 - it's a red sandstone. Thanks for explaining re aggregate size. Out of interest, if you used a fine sand in wide joints what would the problem be? Would it fail?

@paulc - thanks for the tip that's really useful.

Thanks again everyone.

 

[stuart45]

Many thanks for the responses, that's all very helpful.



@stuart45 - it's a red sandstone. Thanks for explaining re aggregate size. Out of interest, if you used a fine sand in wide joints what would the problem be? Would it fail?

@paulc - thanks for the tip that's really useful.

Thanks again everyone.

You will find that using a fine aggregate in wide joints the mortar will start to sag down a bit. The final strength will be lower as well as more shrinkage. Best to use a mortar with a bit of body in it for larger joints. If they are really large you can put some small stones into the joints.
My local supplier has building sand from 6mm down. The building sand he keeps in stock is different to the building sand you get from the normal merch. Standard building sand for use with cement mortar has rounded particles, compared to sharp sand which has angular ones. Rounded ones make a much more workable mortar. With lime, sharp sand isn't as much of an issue, but with cement it makes bricklaying really difficult. Even with a soft, rounded sand a plasticizer is required with cement mortar.

 

[88v8]

As for the existing mortar, both that and the stone were both surprisingly very dry. The mortar is very dusty/sandy and just crumbles away, but not because it's been wet, as far as I can tell. The minimum depth is no problem at all. But some of the depth and voids is substantial. Quite a fair few inches in some places. I can measure when I'm at home if needed....

It may be clay, not mortar. Clay + chopped straw, originally. Then lime pointed.
If you keep poking at clay with a knife and fork , you can eat your way right through the wall looking for 'sound mortar' because there never was any.

You can point any depth with NHL and sharp sand - always sharp, never soft, soft makes a weak mortar - just make sure the joint is damp so the lime will calcify.
As said, use a piece of wood to poke and tamp the lime into a deep joint.

Incidentally, if you find small pebbles wedged between the stones, leave them in place. They are known as pinning stones, and they were there to support the walling stone and prevent it from completely squashing the clay fill.

Ivor

 

[Zebra]

Might be an idea to scrape out as you fill in rather than all at once if it's very crumbly and bad. When I did my chimney it virtually all came out, so I only dared to do three courses on a single face at a time, not practical obvs if you're digging out yourself but having someone else out back.
Other thing I'd add is that it doesn't have to be hessian - old bedspread, cotton sheets, dustsheets, towels, anything to keep it moist and medium temperature underneath...

 

[Black Timber]

Many thanks for the additional replies. All really helpful stuff.

We'll go with a course aggregate in that case, and will get it from a lime specialist to be on the safe side.

@88v8 - yes, I think all of the original pointing is earth with maybe just a little bit of lime. We're on old red sandstone here. Certainly explains why it is so dry and crumbly. And thanks for the tip RE leaving the pinning stones.

You can point any depth with NHL and sharp sand

The pointing will be with hot lime. Is depth an issue with that?

Thanks @Zebra - but yes, have to get it all out in one go to then have it all re-pointed. No option to go bit by bit unfortunately. There have been a few hairy moments, with stones breaking, and a few large stones coming loose or even falling out. But the house is still standing so far, although I must admit to be relieved each morning when I do my daily check and find everything is still ok!

Cheers

 

[malcolm]

The pointing will be with hot lime. Is depth an issue with that?

I had to look it up. https://www.buildingconservation.com/articles/hot-mixed-mortars/hot-mixed-mortars.htm

Looks like lime is lime. It's important to keep it compacted. Make sure the gap is properly damp (not wet) by keeping on spraying water at it until it stops sucking water away. Bung the lime in then after a few hours knock it with a brush to compact it to reduce the initial shrinkage cracking.

I tend to fill really deep gaps to 1/2 inch below the surface and repoint later. That's because deep gaps take ages to dry and are too soft to knock after a couple of hours. They might take a day to get to that stage. I compact them with a pointing finger so I have everything starting from the same depth so I can go around the whole wall and finish it in one go.