Applying lime to cement and gypsum and management of damp?

[DevenishEstates]

Hi all,

I'm restoring a C16th farmhouse which has a mixture of lime plastered and cement/gypsum plastered walls - fortunately the majority being lime - although in an appalling state.

The floor of the interior of the property (at the rear) is about 1m below the outside ground level (which can't be lowered) and these walls are where the majority of the cement/gypsum has been used - I presume in a forlorn attempt to manage the damp in these walls (wall construction is stone rubble between 600-900mm thick).

I've dug down at the rear of property (outside) to install a dwarf wall (to allow a breathing space between the rear wall and the ground) as well as installing a French drain. Limecrete floors have also been laid throughout.

I propose removing the cement/gypsum to about 1.5m internally and replacing with lime. This will leave me with a lime/gypsum convergence about 1.5m up the wall.

Any ideas how I should approach plastering a finish lime coat to the whole wall - i.e. getting the lime to stick to the gypsum/cement?

I also have a small blockwork wall which I need to lime plaster - will lime readily stick to blockwork - assuming I manage the suction correctly?

In others experience will the planned works (French drain, lime plaster, breathing space, Limecrete floors) successfully allow these walls to dry out?

Any comments/advice gratefully received.

Daron Devenish
(Developer with a conscience and love of old buildings!)

 

[middi]

There appears to be a conflict in mixtures of materials you are proposing to use, if I understand correctly.

The use of lime mortars is obviously the render to be used in restoring old buildings like you describe, it defeats the objective if used over existing "hard" cement based renders, ideally any existing renders of this type should be removed first.

As to blockwork, if these are of the cement type, lime render will only become a cosmetic addition and it will definetely stick to it.

As long as the masonary type substrate is not dry on application, lime renders will stick to any of these.

 

[Gervase]

Lime will stick admirably to blockwork, so no worries there.
But why keep the modren stuff at all? As Middi says, using lime over the top of hard cemientitious coatiungs defeats the whole purpose of using lime - you might as well use gypsum plaster, wait 10 years for the cracks and damp to appear. and then do the whole thing again and so on in perpetuity.
For that's what you'll have consigned yourself to doing if you leave the gypsum in place. The building will move once you sort the ongoing cause of damp, and you will get cracks which will continue for around five years as it settles into an equilibrium.
So you may as well get the imprevious stuff off now and sort the whole problem at once, rather than try to stitch lime to gypsum in the knowledge that the gypsum is likely to crack ere long.

 

[Nathan]

I too am in a similar situation and posted earlier without a response. I may be mistaken but the two previous posters have missed your point about removing the gypsum. As myself you wish to remove to 1.5m and replace with lime, attaching/stitching to the gypsum higher up - only going over with lime wash.

I have just started a v short plastering course and asked how best to attach the two types of plaster. He came back saying that I should use Unibond. Having never used this before and it being the first lesson I shall quiz him again and let you know.

Meanwhile, SPAB may help(they have a help line) and posted at http://www.spab.org.uk/publications_Q&A_renders.html is the below article which seems to answer our query:

Q. Does this mean all the cement render on my pre-19th century house should be removed?

A. Not necessarily. Although remedial action should ideally involve removal, this may cause further damage to the fabric. A small area should therefore be removed as a trial.

Where the render adheres well, it is probably best to leave it to age naturally. Sometimes a compromise may be possible, perhaps just removing the render at the base of walls or hollow-sounding patches where localised deterioration has occurred.

 

[MDE]

Try Mike Wye's DG27 on the gypsum / sand & cement bit and then lime plaster over it. I've done this a couple of times - it seemed to go on OK and you can't see the join.