Re: Footings for sandstone block wall.

[Timberframe123]

Hi
I am in the process of re-erecting a sandstone block wall which has collapsed due to tree roots, ivy & poor past repairs.
What should I use for the footings ? Originally it looks like the blocks were laid on a thin layer of lime.
many thanks

 

[malcolm]

I went modern for my garden wall and went for a 450mm wide and 300mm deep concrete foundation. The top of the foundation is around 150mm below ground level. Concrete is fine to use on an old house so long as it is only used underground (or for chimney flaunching).

 

[Timberframe123]

What about the concrete in contact with the sandstone not allowing it to breath ?

 

[malcolm]

Not a problem - if everything were breathable the water would only move from the ground into the sandstone. It's the lime mortar that allows water to evaporate away into the air and keeps the stone dry.

 

[stuart45]

Concrete foundations were used in some Victorian properties, with lime mortar for the walls. Portland cement wasn't popular for mortar due to it's appalling workability, as some old bricklaying text books describe it.
Some peop!e who have only used lime mortar, have trouble with cement mortar.

 

[Timberframe123]

So it’s ok to lay the sandstone blocks directly onto the concrete & use Lime mortar between the sandstone blocks ?
thanks

 

[malcolm]

Concrete is really good for foundations. Once it's in you can build with lime mortar directly off the concrete foundation. Definitely use lime mortar for a sandstone wall otherwise the lifespan of the limestone would be limited.

 

[Cubist]

You only need to worry about 'breathability' when it concerns the fabric of the house - its about getting the moisture generated inside the house out of it as quickly and efficiently was possible. For external constructions like grand walls you can use whatever materials best please you.

 

[Timberframe123]

Ok, thanks all for the advice.

 

[a twig]

You only need to worry about 'breathability' when it concerns the fabric of the house - its about getting the moisture generated inside the house out of it as quickly and efficiently was possible. For external constructions like grand walls you can use whatever materials best please you.

Not entirely true, plenty of spalling rubble stone boundary walls around fields and roads in Somerset show the error of using cement based mortar with stone

 

[Cubist]

Erm, but was the spalling the fault of the cement or the stone?

 

[stuart45]

If certain types of stone are face bedded rather than on their natural bed it can help accelerate the spalling.

 

[malcolm]

Breathability is very important for garden walls. If non-breathable materials are used such as cement mortar or pointing the wall will become very wet and be unable to dry out. In the winter the bricks will spall especially if they are soft reds or sandstone.

Spalling tends to be more of an issue for garden walls than house walls because of the lack of gutters and overhangs. A drip edge in the coping is effective for keeping water off the wall in the first place. I rebuilt mine using 200 year old bricks and the bricks are of very poor quality and had spalled badly where they had been cement pointed. I was very careful with the drip edge design.

You can blame materials. A lot of natural materials and old bricks are much more prone to spalling than modern bricks. Though it would be more in the spirit of the forum to use mortar that allowed the poor materials to last for hundreds of years in the first place - Lime.

However concrete foundations are fine as walls do not dry through their foundations.

 

[Cubist]

Must admit I've found this thread, and in particular the differences of opinion, much more interesting than I had first thought to be likely.

What has me a little puzzled is whether a Portland cement mortar is more or less likely to contribute to spalling of the bricks or stones in a garden wall. I'm more than happy to change my opinion but I would have thought that a cement mortar, being less permeable to water than lime, would absorb less and therefore transfer less to the bricks or stones. However, I can also see that, at the interface between the mortar and the brick/stone there may be a tendency for water to collect near the underside of each stone/brick and that this may induce/assist any spalling. On balance though would it not be the porosity of the stone/brick that is the real determining factor?

 

[malcolm]

Have you never removed impervious materials and found a soaking wet wall in your house? Then found the wall dried out once it was able to breathe. Same thing. It might have taken years for the wall to become soaking wet.

The logic of keeping water out in the first place isn't sound. If that worked then tanking and waterproof paints would be fab on old houses. But they are not. Water always finds a way in. The trick is giving it a way out.

 

[Cubist]

I am more than happy to concede that a cement based render is entirely a bad thing and also that cement mortars used in the construction of a wall 'may' inhibit gravitic drainage of water accumulated in the faces of brick or stone and thus perhaps contributing to spalling but... I think that if the faces of the bricks are exposed to atmosphere, then the permeability differences between lime and cement would provide only a very small marginal difference to the likelihood of spalling.

 

[malcolm]

Bricks aren't very permeable. Lime is very breathable so most of the drying out of a brick wall is through the lime mortar joints and not the bricks themselves.

 

[a twig]

If I still lived in my old house I could nip out and provide numerous examples all within a few hundred yards where rubble stone boundary walls have been ribbon pointed in cement mortar and the stone is in horrendous shape. A few houses further on where the wall had been left to it’s own devices, no stone damage whatsoever

 

[stuart45]

I guess they were something like Blue lias or Hamstone, but definitely not Chert.

 

[Stevers]

The mortar also needs to be weaker than the stone or brick - for some stone work found in Somerset this needs to be very weak. If such walls are hard pointed in cement the stone very quickly crumbles. Faced with stone like that I would consider using putty lime, or at least let my favoured NHL3.5 down with Hydrated Lime.

My house has Inferior Oolite quoins and mouldings, but these are not 'inferior', just 'underneath' other Oolite beds. Which bed the stone came from within each quarry can make a huge difference, and I was told that the Oolite ashlars, random rubble (sides and back), and dressed White Lias (front) all came from different beds in a quarry just up the road from here. As that's since been completely filled with household rubbish - we shall never know!