What to plaster my walls with?

[tom.harrigan]

Mostly just checking - I tend to get quite nervous when embarking on the next job.

I have quite a bit of plastering to do onto bear rubble/mud built walls. They did have a layer of mud and straw covered with lime plaster, but the mud and straw daub has given up adhering to the walls. I'm left with very rough walls, which present about 1/3rd mud facing, which isn't particularly strong. Unless someone stops me, I'm going to apply a 1:2.5 mix of Castle hydraulic lime and "plastering sand".

It's at times like this I ask myself why I bother with the extra expense, effort and nerves when I know that a cement render will be easier, cheaper and do an excellent job...

T

 

[Nemesis]

Well you know of course that cement will be bad for the building. However, why hydraulic lime?

 

[tom.harrigan]

A couple of walls were "renovated" with cement render in the 50s. They are absolutely fine.

I use the cheapest suitable materials I can get hold of without too much effort. My local Jewsons delivers hydraulic lime to my doorstep. They also deliver cement - which is much cheaper! I am not going to risk hydrated lime!

T

 

[Nemesis]

Oh well, suit yourself. Bit sad for the building, but...

We have used hydrate extensively and had no problems at all.

But you can get non-hydraulic delivered to your door.

 

[tom.harrigan]

What's the problem with hydraulic lime?

What have you used hydrated lime extensively for? Believe me if I thought I could get away with rendering with hydrated lime I would - it's much cheaper! I don't want to risk it.

T

 

[MdB]

Not an expert here but I would guess that mud and rubble walls would cope the best with being covered in cement as there isn't the wood (probably) to rot nor cob to collapse. Having said that my damp mud and rubble cement covered walls are now completely dry now the cement has been removed.

Again, not an expert but isn't it a good idea to have a harling coat as the first layer on bare walls or is that just cob?

 

[Nemesis]

Are you talking external render or interior plaster?

Hydraulic - not always the best, depends on what strength and what it's used for.

(Mud and rubble can get very wet and very damaged by cement... just like any other walls. They need to breathe just like any other trad building.)

 

[tom.harrigan]

Seriously though, what is the problem with hydraulic lime, and what have you used hydrated lime extensively for?

The lime plaster over mud and straw daub has come off internal walls!

T

 

[Nemesis]

The lime may have come off - consider why. Is it ancient? It doesn't last forever, but it may have been in place for many decades before giving way with age and damp and movement of building.

We have used hydrate to build external walls, to plaster repair internally.

Andrew Gayton has used it one his award-winning work on his home I know.

Hydraulic - it's the right thing for some applications, depends on the strength, not really what I would use for others.

What are you using it for?

 

[Gervase]

Don't use what builder's merchants call 'plastering sand'. It's a very fine, soft sand designed to be used with modern cementitious plasters. You need to use sharp sand - sometimes sold as plain builder's sand, and sometimes as concreting sand - which has lumps in it up to the size of a grain of rice. If you use a soft, fine sand, the plaster will crack and not have any strength.
Ideally you'd use hydrated lime, bit in truth hydraulic will still give you a more sympathetic coat than anything cement-based. It's neither as breathable or flexible as hydrated lime, and can't be worked as long to give a really tight, dense finish, but if it's a choice between some bags of NHL3.5 and a some bags of Blue Circle, pick the lime.

 

[Pford75]

If this is external render, it's also worth pointing out that I wouldn't start any work until the Spring when risk of frost has passed.

FWIW, our flint wall was constructed with hydrated lime and we had no problems.