Extensions to Listed Buildings

[MatthewC]

We are planning to apply for permission to demolish an old brick extension to our listed 1830s house (in a conservation area), and erect a slightly larger, better built single storey extension. Our intention is to use Cotswold stone with a slate roof, and wooden sash windows, to match the original house, although I assume that we will have to have a deeper foundation, a DPC and a cavity wall.

I don't want cement pointing, so my original intention was to use a mixture of mortars, with cement below the DPC and lime above, but am now having doubts. If we have a solid cement floor with a damp-proof membrane, will that force moisture towards the old stone wall where the two structures will join? (And there will be a solid limecrete floor the other side of that...) That makes me think I need a limecrete floor in new extension.

My question is: do we build the extension with cement mortar, or lime mortar, or both in different places? If both, where do I use which?

Matthew
http://houseintheenchantedforest.blogspot.com/

 

[MdB]

If not listed then the new build will have to meet building control regs. This means it will have either be built using modern materials and techniques (as given in the approved docs) or you will have to prove that whatever method of construction you choose will meet those requirements. As lime mortar is well known to be a lot weaker than cement, you will have to get a lot of calculations done to prove its strength for wind loading and the like. This will probably end up with a very thick (ie expensive) double skin wall with insulation in the middle. Finding someone to do calcs for lime might be difficult too as it is unusual.

As an example, the rammed earth building I sometimes work in had to do a lot of experimental tests of the building method for building control to be happy. Lime is more common than rammed earth but I doubt BC comes across it often enough for it to be something they are immediately happy with. I could be wrong though.

However, I can't see any good advantage for using lime apart from its possibly better environmental credentials in its manufacture. As it is a new build, there is really no reason not to use modern techniques to make it watertight and warm and hence I would probably go for cement. Not that I particularly like saying that but the alternatives for you construction technique (structural stone) mean that the alternatives would be difficult and expensive. If it was a wooden structure for example and the stone was just infill then I think there would be more alternatives available.

 

[Gareth Hughes]

I've never come across a situation where a new-build in lime mortar has caused a problem with Building Regs. That level of detail (lime or cement) really doesn't come into consideration.
There's no good reason why the extension shouldn't be built entirely breathably, with no DPC, and with a limecrete floor, etc.

 

[MdB]

Gareth knows a lot more than me on this so listen to him (fortunately for you). I am surprised though.

Does lime mortar meet BS EN 771-6 or BS 5628-3:2001 then? I've no idea as I don't have access to these documents. Approved doc A has these as requirements for stone construction to meet normal regs.

A chat with building control will sort this out though.