Patching up plaster (old cottage)

[andyandy2]

I'm slowly renovating and altering my cottage, I need to patch up some plaster around a window and door frame, the door and window have been moved and new lintels fitted so the walls are back to brick.

I've searched the forum and there seems to be differing opinions on which backing plaster to use, the exsisting is a lime based with horse hair by the look of it A couple of years ago I patched up some walls in another room and used browning, but have one patch which seems to be slightly damp and paint keeps blistering off

With this in mind i'm wondering if i should use sand and cement with some waterproofer in ( although i've never used it before) (walls are solid brick 14" thick)

Any advice much appreciated

 

[joe-90]

It's not a good practice to use undercoat plaster on external walls. I'd use a weak mortar mix myself - say 6 to 1.

 

[The-brickie]

I'd try and get something thats more in keeping with what you have, theres now pently of companies doing lime based motars, they also sell horse hair, these materials will help your house remain dry unlike the new light weight gypsum plasters which attract damp, not disrespect to joe-90 but I would avoid sand and cement too.

 

[Richard C]

not disrespect to joe-90 but I would avoid sand and cement too.

You need a lime based render which IS sand/cement/lime Wheather or not you mix an SBR water proofer in there is debatable; although I have experience with internal rendering base coats, the kind of specialist internal rendering your after is not my forte & you really need one of the render guys to respond in detail (Marsh!); failing that, I will see what I can find out!

 

[joe-90]

It's only a tiny little patch-up job where the window has been replaced - it really matters little what he uses.

 

[andyandy2]

Thanks for replies, What sort of mix would i need if i went for sand cement lime render, as joe says they are quite small areas but might be good to "experiment" on

 

[marshman]

Its always a hard decision to make on what to do for patching old lime mortar, the old mortar is very breathable so any damp
that comes through the brick is soon lost to the air in the room as any damp in the air soon taken out by the mortar and bricks under it,
if you use a waterproof sand & cement mix then that is a hard mix which on solid out side walls then become a cold spot which can form
condensation damp to the surface,
now if you suspect penetrating damp then scratch coat your patches with 4&1 + a waterproofer, then top coat with bonding and skim finish
probably your best choice and safest.
If you want to use sand, cement & Hydraulic lime and keep it soft then try 9, 2½ lime, 1 cement and you'll have to skim this next day
as it will suck like mad, you could go 10,3,1 to 12, 3, 1 they would all be soft and very breathable and let any damp pass both ways.
You could put it back like it was using lime putty mortar and skim your finish in lime.
You have a big choice on which way to go but are you just going to do the patches or are you intending to resurface the whole wall,
if you are going to skim the whole wall then 4&1 + w + bonding is the way to go and if they are in down stairs rooms.
You could use Hardwall backing plaster ( up stairs),
you could use Thistle dri-coat backing plaster, down or up stairs but you risk a cold spot and condensation damp.
Their are many way's and not all of them are right for the job in hand,
which ever way you choose give the old bricks a good dampen down with water just before you start.
One or more of the other old boys will probably pop in soon and give you more choices so wait a day or two.

 

[Richard C]

 

[dextrous]

How does he know that stuff?

 

[The-brickie]

not disrespect to joe-90 but I would avoid sand and cement too.

You need a lime based render which IS sand/cement/lime Wheather or not you mix an SBR water proofer in there is debatable; although I have experience with internal rendering base coats, the kind of specialist internal rendering your after is not my forte & you really need one of the render guys to respond in detail (Marsh!); failing that, I will see what I can find out!

No Richard, its not, lime motars should not (in my book) have any cement in them, a cement render can make use of lime to improve the mix.
http://www.buildingconservation.com/articles/cement/cement.htm

Plenty of suppliers online..

http://www.lime-mortars.co.uk/lime-mortar/hydraulic-lime-mortar/NHL2

http://www.mikewye.co.uk/mikeprices.htm

 

[Richard C]

Hi

No Richard, its not, lime motars should not (in my book) have any cement in them, a cement render can make use of lime to improve the mix.
http://www.buildingconservation.com/articles/cement/cement.htm

That’s really what I was referring to & is what I use as a render base coat, Marsh also seems to be recommending something similar. Presumably the original would have been lime putty which, I believe, doesn’t use cement; I did say specialist renders are not in my filed of experience.

 

[The-brickie]

Marshman made a lot of suggestions!

I'd still go with a more natural product that is actually in harmony with whats there and as Marshman says will breathe with the rest of the building materials used.

Marshman wrote:
You could put it back like it was using lime putty mortar and skim your finish in lime.

So it must be the way to go!

 

[joe-90]

FFS it's only a few chips around a window frame. Use anything that's handy.

 

[The-brickie]

 

[andyandy2]

Thanks for all the replies guys, still not sure what to use though, it is an outside wall and the most exposed part of the house, but not sure if its worth going the whole hog with lime putty.
In Marshman's post bonding is mentioned, is this just the lightweight i.e carlite bonding.

Joe it is more than a few chips around the window, i've had to build up brickwork under lintel etc, and also just want to experiment ready for other areas of the house, i intend to stay here so don't want future problems if i can avoid them.

Thanks again guys