An insurance claim has come to myself to sort....

The claim is 'storm damage' but it is clear it is simply penetrating damp from poor external pointing being stone wall construction. The gable has now been pointed externally. We have hacked off all the internal plasterwork and render (1" thick) to the side of the chimney wall and found the mortar to have ~13% MC by Tony's favoured weighing method

Desiccant dehumidifier currently in-situ. Should be dry this week.

What is the best method of building up the internal render?

Dot and dab?
Lime render? If so, what mix?
Sand and cement render with water-proofer?

What is the best plaster? Standard thistle multi-finish?

This is insurance work and as much as I'd like to be green in principle with IWI or similar the job needs doing. My usual plasterer has said to just use standard sand/cement with a waterproofing agent then skim as normal.

3 sharp sand : 1 part lime putty for base coats,

Neat lime putty for the skim,

Ain’t easy but lime putty is available online

Various lime suppliers have been mentioned on here a lot. Many do courses that are worthwhile. Take advice from your chosen one.

I hope you've pointed outside with lime mortar? Otherwise the problem may recur.

The basic choice is between (a) lime putty/fat lime/air lime and (b) hydraulic lime.

Lime putty hardens by drying and by carbonation. It's quite a slow process - a week or three - and needs to be watched and re-compressed whilst it is drying. Until it's good and hard it can be washed off by torrential rain. The advantages are that the end result is more flexible and won't crack as easily, and that you can keep any spare pretty much indefinitely in a sealed container. Even when mixed with sand etc. You can buy pre-mixes as well as pure lime from the suppliers.

Hydraulic lime hardens by a chemical action, much like regular plaster. It will even set under water (hence the name). It's more rigid, and stronger and you have to discard any that's already mixed since it will just set. There are various 'strengths'. Setting takes a lot less time, but still longer than regular plaster.

You mix the lime with an aggregate. This is often sharp sand, but you can also use chalk, which allows for a much finer finish. You also mix everything except the top coat (and sometimes even that) with fibres like horsehair or polypropylene to increase the tensile strength. If you mix fibre in the top coat, along with chalk, then you can burn off any bits that stick out afterwards. And sometimes there are magic additives.

Hi VE, although you say 'render' you mean plaster -internal - don't you? In that case it's not vulnerable to weather, but still to low temps if they exist.

Posted By: VictorianecoThe policyholder has apparently re rendered the whole outside wall, it looks like it's been done in cement render.

I'd turn down the work. You'll only be called back when the problem recurs.

Are things happening v quickly? In your 1st post you said '' The gable has now been pointed externally''. Then in your penultimate post you say ''The policyholder has apparently re rendered the whole outside wall, it looks like it's been done in cement render.'' Same wall but things changing fast?

Tempted to agree with djh, but if he/she really has 'locked up' the outside wits/c render, maybe you can still help *the structure* with lime, but you might not *cure the problem*

I wouldn't fix it, so long as the 'reinstatement' part is done correctly then the cause of the damp isn't an insured issue?

Is lime render/ plaster easy enough for a plasterer who has never used it before?

Desiccant dehumidifier been in place for a week, mortar still showing as 12.5% moisture content

Indicative of existing ongoing q issue. My thoughts as mentioned by others are to walk away?

Sample taken and weighed

Sample put in oven at 120c for 1hr then weighed, then another hr and weighed until no further change

Calculation made as follows:

Weight before drying - Weight after Drying
---------------------------------------------------- x 100
Weight before drying

What calculation do you use?

I use the same as you there are variations on the theme

Posted By: VictorianecoIt's a ground floor wall in question. Either way we just get asked to remediate the damage ie. Hack off, redry and redo

As much as I'd like to investigate the problems etc and would actually like to help out I don't get paid for it and the insurable would never cover it

If you continue with this job IMO you should put a disclaimer in writing stating (in your opinion) the problems otherwise when/if the work fails 6 months down the line they can't claim poor workmanship