NHL 3.5 for 1930 soft red/blacks?

[20sHouse]

Hi All,

Linked to another post but different enough to deserve it's own post.

We are in the midst of having this porch (it does have a cavity wall) and balcony wall raked out and repointed (note the upper balcony wall is going to be coming down/replaced as it's unstable). It's a 1930 house, the previous owners repointed with cement which is clearly not right but luckily did not blow the brick faces.

The proposal is NHL3.5 given the time of year and strength over the original type of Lime that was installed. Some of the bricks are soft reds and after 95 years are solid but not hard like a modern brick (well the bricks we are keeping). Would you use NHL3.5?

Also the balcony wall is either being rebuilt with reclaimed bricks or closest match new ones, the brick layer is suggesting using NHL3.5 with a bit of cement to give it strength where it failed before, then NHL3.5 lime at the front to match it all into the porch below it. Given it will be exposed front/top/back, it won't have any damp problems and flexibility should also be fine.

Any differing thoughts?

 

[stuart45]

You don't need to add cement to NHL 3.5.
How deep have the joints been raked out?

 

[20sHouse]

You don't need to add cement to NHL 3.5.
How deep have the joints been raked out?

Their thoughts on NHL 3.5 and cement (only in the balcony wall) was that, it will also be holding up the railing and will be a little fragile until it sets?

Joints will be 20mm deep (in the picture above, this was only the first cut to get past the previous cement pointing, before being tapped out) and then the proposal of flush pointing in NHL 3.5 (no cement for this bit)

 

[stuart45]

Tests on mortar have shown shown that adding small amounts of cement to a lime mortar can actually weaken the mix. This was one of the results from the Smeaton project.

https://www.jstor.org/stable/1504464

 

[HafodYBryn]

I wouldn't use any cement with NHL3.5 - it's not needed.
Also as we're coming up to winter, you may find it does'nt set before the frost comes in - usually they say NHL3.5 or lime takes a good few months to set... completely, hence why it's mostly done in spring.. damp conditions and all that. Ring up Ty Mawr Lime, they can give you some good, solid advice on lime.
If you are going to use cement - use cement. If you use lime - just use lime and any builder that says that it still will have the porous element that is needed needs sacking - sorry.

 

[20sHouse]

Okay thank you, quite clear, no cement with lime for the balcony wall.

 

[20sHouse]

Tests on mortar have shown shown that adding small amounts of cement to a lime mortar can actually weaken the mix. This was one of the results from the Smeaton project.

https://www.jstor.org/stable/1504464

Did you also have any thoughts in terms of using NHL3.5 with these bricks, raked out to 20mm?

 

[paulc]

Last bit of brickwork I did here (late 1920s semi, very soft red facing bricks), I used a lime putty mix.

 

[larry]

I need to do a similar job with soft red bricks. I haven't done this yet but lots of advice on here which is read has suggested NHL 3.5 is too hard, and to use a putty based mix or at least NHL 2. But I'm also planning to wait until next year to avoid frosts....

 

[a twig]

I used NHL2 on my soft reds

 

[stuart45]

Mortar should be weaker than the bricks. That way any movement in the wall fractures the mortar rather than the bricks. For repointing a wall it's better that the mix is slightly weaker than the original mix. If it's stronger, even if weaker than the bricks, the movement will be slightly different and more likely to crack.
The bricks in the photo don't look really soft, although not as hard as extruded bricks. If the whole wall was built in a mortar like NHL3.5, or a 1/1/6 mix it would probably be fine. Many modern bricks, apart from wire cuts, are quite soft, but are still fine with a 1/1/6 or equivalent mix.