which mortar for brick sills to 1897 house?

[hobbaabboh]

Newbie, so hi to all!
I am currently repointing my red brick, late Victorian [1897] end terrace, on the rear elevations/ gable wall, with a 3:1 sharp sand/ hyd.lime mix with a rubbed finish to mortar to expose the stone aggregate - which is looking rather nice!
However the original mortar mix between the brick window sills [in a solid engineering blue brick] seems to differ from the rest of the house - a much harder and smoother mix than the main house [more of a cement based consistancy]. Definately original, although unsure which mix/ ratio to use to match the existing mortar...and want to get it right. Also what mix ratio should i use for chimneys?
Anyone know? Thanks in advance.

 

[Andrew Bodenham]

Welcome to the forum.

You won't usually have to wait so long for a reply - maybe all our lime mortar experts are suddenly on holiday.

Sounds like you're doing a fine job of your repointing, and i can't really why you couldn't use the same mix on the sills as elsewhere. If the sills are 'brick on edge' I suppose you could argue that the mortar joints will get more weathering than in vertical brickwork, and so need a more durable mortar. However, it's my experience that the freshness of the lime, a good choice of sand and proper mixing are more critical factors in the performance of the mortar.
Ditto for chimneys.
Watch out for frost, working at this time of year.

 

[biffvernon]

Ordinary Portland Cement was available in 1897 and although the trarditional lime mortar would have been used for the walls, in special situations such as the joints between engineering bricks in a window sill it does seem likely that OPC would be used, water-tightness being the first requirement.

Your lime mix should be fine for the chimneys, though in very exposed situations some prefer to use a hydraulic lime.

 

[michael]

Not wishing to state the obvious but from previous discussions on this forum, I have learnt that the mortar should always be softer than the brickwork. Hydrated lime mortar (a very soft mortar) is therefore obviously ideal for very old soft porous bricks and its breathability allows any water to be drawn from the brick, acting like a wick and then allowing the water to evaporate away. Consequently if cement mortars are used on old porous brickwork, the water escapes through the brick instead of the mortar causing the brick to deteriorate particularly in frosty conditions if the water in the brick turns to ice.

All you lime heads out there (no offence intended), correct me if I’m wrong, but I understand that by the time Portland cement came along in the late nineteenth century, the bricks were being fired at much higher temperatures, they were becoming increasingly less porous and much harder and so the breathability issue was becoming less critical to the extent that practically all brick houses built today with very hard high temperature fired bricks use cement mortars almost exclusively without any detrimental consequences.

My point is therefore, if the bricks you describe are indeed engineering bricks then I guess they will be very tough very hard impervious little so and so’s where breathability is not so much the issue but strength is. So maybe OPC mortar will be fine in this instance? But see what the lime heads say first.

 

[Ian Bond]

If you don't want to use OPC, there's always hydraulic lime.

Available from PPUK shop and elsewhere...

http://www.periodpropertyshop.co.uk/acatalog/Hydraulic_Lime_Mixes__Dry_.html

Should be up to the job ...

 

[hobbaabboh]

Thanks for the advice guys and clarifying situation - decided to keep with the NHL 3.5 mix for main brickwork/ arches/ chimney's etc with a rubbed joint finish and use the opc for the engineering brick sills

 

[michael]

I think the lime experts will be advising you to use a ‘Hydrated’ (non hydraulic lime) on the main brickwork either in powder and sand mix or a lime putty and sand mix and to only use the ‘Hydraulic’ (Natural Hydraulic Lime e.g. NHL 3.5) on the engineering bricks and on the chimney if exposed.

 

[Gervase]

Holiday? What's one of those when it's at home? :roll:
Engineering bricks are very hard and dense, but OPC mortars are even harder - I'd recommend using an NHL5 hydraulic lime with the engineering bricks if you want a hard mortar. It will eventually cure to around 7 Newtons (about the same as the bricks) rather than the 70 Newtons that OPC mortars can reach.
That said, if the bricks are not critically load-bearing (and on a sill rather than as an arch I'd assume not) then an NHL3.5 mortar would be more than adequate (and easier to source from your local builder's merchant, as many don't seem to stock NHL5). Just mix it 1:3 with sharp sand and use that. If you use the same sand you're using for repointing it will have the advantage of matching the rest of the house, whereas an OPC mortar would stick out like a sore thumb.
As for the right lime to use for repointing, hydrated (non-hydraulic) is usually the way, but a lot of late Victorian houses were built with Blue Lias hydraulic lime - so an NHL2 would be a reasonable match (though the Singleton Birch NHL1 would be a perfect solution, almost exacly equivalent to the mortar which was used in most of the major urban expansions of the late 19th century).

 

[Nemesis]

You can, of course, have mortar analysed.

I also add - if it doesn't require repointing, don't do it. So many places are being repointed when the original mortar is sound.