Mortar for the Roof

[Geoffrey]

Just wondered if anyone has an opinion on this. My roof is currently having a bit of maintenance done. The spec from our builders (specialist in period etc. etc.) was that any mortar repairs to the ridge, new flashings etc be done in lime/sand/ cement mix. When I questioned the inclusion of cement in this and my concerns that this may cause problems in the long run they said it was standard pracrise where roofing was concerned to give extra strength against the elements. Is that right?
Fortunately the guys who turned up to do the preliminary overhaul yesterday hadn't brought mortar ingredients with them - but I noticed on their instructions only sand and cement was specified and no mention of lime.

 

[Nemesis]

Don't let them near it with that mix.

We had roofers who bedded some ridge tiles on an outbuilding with just that mix, and rendered a chimney. Yes, they said they would use lime, and we were away on holiday when they completed the job... grrr. It's going to be redone next summer when other roofing is being done.

Anyhow - it's failing spectacularly, just as the Smeaton Project found when similar mixes were tried.

Either sand and cement if you must (but I wouldn't I know it's often gauged with a bit of hydrated lime) or preferably lime and sand... in this case I'd suggest you consider moderately hydraulic, although I'm sure others will have views on this.

Specialists in period I think would recommend the latter...

 

[Geoffrey]

Sorry Evelyn - which is the truly awful mix?

 

[Nemesis]

Sand cement and lime... apart possibly from a tiny bit of hydrate the purposes of which I think are related to back magic.

Sticking some cement in a lime mix weakens it ... it's complex and you would need to read the research! Smeaton Project.

When we get our roof done next year (hopefully) we will be using moderately hydraulic and sand - no cement! Comes in a bag from Jewson's. Gervase is the person to really bore you sorry I mean enlighten you - about appropriate mixes though!

Sorry if a bit incoherent as I'm supposed to be doing something else...


If in doubt also give SPAB a ring and ask.

 

[Geoffrey]

Many Thanks

 

[Gervase]

*tsk* Rude woman!
To bore you on the subject - if you're flaunching a chimney or bedding ridge tiles, you'd be best off with a strongly hydraulic lime - NHL5 - as this has the most resistance to the sort of conditions you'll find on the most exposed part of your house. I'd get a bag from Jewsons and use it with some sharp sand in a ratio of 2.5:1
If you're repointing the windward side of the chimney or doing any other mortar work on the roof, I'd use a moderately hydraulic lime - NHL3.5. For any other work and repointing generally, use non-hydraulic lime in a 3:1 mix.
But don't, on any account, be tempted to mix cement with any of the mixes.
English Heritage used to do that, and recommended that everyone else did too, until it was noticed that a lot of the mortar was failing. As Evelyn - sorry Nemesis - has said, lime and Portland cement don't work.
The Smeaton project - which as far as I know is still ongoing - is of course named after the fellow who built the Eddystone lighthouse and who was one of the great British pioneers of naturally hydraulic lime, made from Blue Lias limestone.
Ironically, even Smeaton had doubts about this wonder material, and keyed the stonework of the lighthouse together with extraordinary carved dovetails (which must have made his masons love him!). He needn't have worried. As the Pantheon in Rome shows, buildings made with hydraulic lime do last.

PS: if you're ever doing blockwork with modern mortar, a bit of bag lime in the mix (try 5:1:1 soft sand, cement and lime) will give you a lovely sticky mortar which is almost a good to work with as real lime mortar. These days, however, most brickies use FEB or a similar wetting agent to give a 'fluffy' mix which is horrible to work with.

 

[Nemesis]

Well it's all fascinating!

The moderately or strongly debate was one I recall GK having for Knedlington... different advice from different sources!

But - lime looks far, far better on a period building too. And is better for the environment.

Evelyn/Nemesis - same thing really ask the other half!!!

 

[Geoffrey]

Gervase - a big thankyou - will talk to the builders with this suggestion.

 

[biffvernon]

Smeaton had doubts about this wonder material, and keyed the stonework of the lighthouse together with extraordinary carved dovetails (which must have made his masons love him!). He needn't have worried. As the Pantheon in Rome shows, buildings made with hydraulic lime do last.

To my recollection, the weather around Eddystone is not always the same as that typical of Rome.

 

[NT]

Lime and cement mixes: I read some research on it and IIRC 1:1:6 was not prone to premature failure, but all the other mixtures were.

NT

 

[BMcD]

Is this the paper you read NT

http://www.buildingconservation.com/articles/cement/cement.htm

If not it's still an interesting read

 

[Lime]

From BMcD's paper:-

the use of cement tends to lead to the user treating the gauged lime mortar as if it were a fully hydraulic lime or cement. Too much reliance on the initial chemical set leads to neglect of the importance of the longer term carbonation of the non hydraulic component present

I think that must be one of the greatest dangers and when coupled with the consequences of segregation we may have trouble in store with many buildings previously repaired with the lime/cement mix.

 

[biffvernon]

Anyway, cement manufacture gives of more greenhouse gas than lime so just don't use it. At all. Ever.

 

[NT]

Anyway, cement manufacture gives of more greenhouse gas than lime so just don't use it. At all. Ever.

All in all it is lower energy, but to be fair there are other factors.

Lime isnt appropriate in every case, eg steps and lintels.


NT

 

[biffvernon]

Lime isnt appropriate in every case, eg steps and lintels.

That's where you use stone. As I said, don't use cement. At all. Ever.

 

[Gervase]

As I said, don't use cement. At all. Ever.

Seconded. At all. Full stop. Amen.

 

[NT]

Lime isnt appropriate in every case, eg steps and lintels.

That's where you use stone. As I said, don't use cement. At all. Ever.

I thought stone goods were even higher in energy use than cement. If not, why do they cost more?


NT

 

[Gervase]

Depends on how they're got and what sort of stone they are. Limestone and bluestone, which split nicely along the grain and can be sawn without too much trouble would, I imagine, have less embodied energy than a concrete lintel, given that each takes the same amount of fuel to transport it.
They cost more because it takes more skill and time to realise them. The same reason lime plastering costs more than gypsum skimming...