Gypsum plaster in old houses?

[Lacan07]

Hi all,

I had a tough day today at my house yesterday and Ive wandered on here for a pep talk and some advice. I just wanted to ask how many of you are wedded to the idea of only lime plaster in your old house, even on the interior walls on the upper floors?

I'm currently renovating the upper floor of my house and desperate to move in, what with paying rates and rent elsewhere. My electrician has finished first fix but wont power up the upper floor until I've plastered the walls.

Ever since I started my project I became very interested in plastering and lime products in particular, and as I've had a lot practice in the last year 'dubbing out' all the holes with plaster I decided to have a go a skimming one of the upstairs walls with a product called lime Green Solo. I prepared the wall knowing full well that skimming was a completely different kettle of fish and well, to cut a long story short, I got it badly wrong. Not unexpected on a first attempt I guess but I'm disappointed.

I really didn't want much if any gypsum in the house (early 1800’s, history of damp issue downstairs), even on the upper floor, interior walls. I could try another product like St.Astier but I suspect its a steep learning curve and I don’t really have the time. So now I’m looking for a plasterer, but was wondering if I ought to let go of my 100% allegiance to lime.

 

[paulc]

Gypsum plasters are quick and (relatively) easy to work with, and just about every town has tradesmen that can do a good job with the stuff. However, gypsum is water soluble and doesn't tolerate being damp for a prolonged period of time. On a damp wall, it will eventually turn to a friable, powdery mush. Lime plaster on the other hand, isn't water soluble unless it is acidic, and copes with with relatively higher levels of moisture. In addition, it is more resistant to mold so you are less likely to get large areas of black mold growing.
Gypsum plaster is difficult to recycle, more often that not, ending up in landfill where it reacts with organic waste to produce hydrogen sulphide (a toxic gas and a contributor to global warming). Lime plaster can be recycled either by burning to produce fresh calcium hydroxide, or as a hardcore (it compacts down real nice).

As to using gypsum in a period property over lime, that is only something you can decide - If the walls are likely to suffer from damp, then I'd say go for lime every time. On ceilings and stud walls where lath & lime would have been used, plasterboard & gypsum skim is quick and easy. Around a fireplace and chimney breast, use lime - Gypsum plasters break down when exposed to 50°C or more.

 

[a twig]

I had a hybrid approach in my place, if rooms were perfectly dry etc then left the gypsum. If moisture control etc required then lime, to the extent that one room I only lime plastered the external walls but left interior gypsum.

Worked fine for me, I tended to use NHL based plasters on the walls as I found those easier and quicker - and only used “fat” lime where the flexibility was more important - i.e. old bouncy ceilings

 

[MatthewC]

If you've had a go at plastering with gypsum, I guess it finished badly because the gypsum set before you had it smooth? If so, you ought to try using lime as it stays workable for a lot longer, and is most satisying to do. The scratch and float coats in lime mortar are relatively easy (so long as you are not doing corners!) although I will admit the top coat in fine plaster is a bit more difficult but still, in my view, not as hard as gypsum.

I will also admit my skill and speed would never compare with a professional, but then I'm not in a competition.

Matthew

 

[malcolm]

My place was gypsum skimmed over lime in the 1920s. It seems to work. The gypsum is breathable enough for an upper floor and I was fortunate to have wallpaper on top of mine so I could get rid of the modern emulsion which was not so breathable.

Modern plasterers do seem to prefer to use what they know. I had some plasterers do some skimming in St Austier. They found it similar to gypsum to apply. I did a room in it myself - it has a chemical set so only a couple hours working time and even the ultrafine is a bit gritty. For me I found lime with silver sand much easier. Throw the trowel at the wall and have another go the next morning.

 

[Lacan07]

Thanks all for the replies.

If you've had a go at plastering with gypsum, I guess it finished badly because the gypsum set before you had it smooth? If so, you ought to try using lime as it stays workable for a lot longer, and is most satisying to do. The scratch and float coats in lime mortar are relatively easy (so long as you are not doing corners!) although I will admit the top coat in fine plaster is a bit more difficult but still, in my view, not as hard as gypsum.

I will also admit my skill and speed would never compare with a professional, but then I'm not in a competition.

Matthew

Thanks Matthew, I was using Lime but got it badly wrong, although I think the mixing instructions are incorrect. This seems to be confirmed by comments i found online. The mix was to thick and therefore difficult to apply and also scuppered my timings as it dried to quick. By the time I went in to sponge float the surface flat it was too late!

 

[MatthewC]

I took that variable out of the equation when I did my house ten years ago, by buying ready mixed lime mortar (in 20kg tubs) which just needed knocking up in a mixer. The only issue is that if you buy it haired then you must use it fairly soon (I forget exactly how long). The mortar and fine plaster all have an indefinite life if kept wet.