External horse hair mortar?

[Mo1820]

New to the forum and appreciating all the advice as we live in a grade 2 Georgian end terrace which needs a lot of work. The joiner working on our french doors removed the crumbling render from the brickwork arch and door reveals - behind it are gaps on average 10-20mm wide down the sides, where the timber frame's inserted behind the brickwork. It almost looks like a cavity wall but is solid London stock. Lining the gaps is old horse hair (presumably lime) mortar that's crumbling away - is it meant to be there? Did they always get it right in the old days?

I was going to get some pre-mixed lime mortar from somewhere like Cornish Lime with fibres to fill the gaps but the joiner says horsehair render is only meant to be used on interiors not outside - is he right? This is inside a cavity on an external wall. I thought to use pre-mixed lime mortar or lime render to point in the gaps which are about 10-20mm wide but should I use something breathable to fill the cavity first - like the fibrous lime mortar - as a substrate to point some non-fibrous mortar onto, rather than use foam injected insulation?

Would like to leave the brickwork reveals bare and not re-render them as the render just seems to crack and debond after a while, so the mortar in the gaps needs to just be flush pointed in rather than re-render the reveals. Any advice welcome.

 

[fernicarry]

You could pack the gap with rolled up newspaper to reduce the amount of mortar needed. Another option is foam backer rods. Haired lime tends to have better gap bridging properties as the fibres "coagulate" and hold it together so perhaps that was the original intention.

You'll need a fillet of something flexible between the mortar and the door frame as the latter will move around seasonally. Traditionally this would have been burnt sand mastic. I use a modern mastic like LM250 first then go over with the burnt sand for a belt and braces solution.

 

[Cubist]

The best material to use to pack those cavities and provide an effective support for any kind of mortar or burnt sand fillet is Oakum. Typically used as a caulking material, alongside caulking cotton, for boats, it is also extensively used for similar purposes in timber frame structures.

 

[Mo1820]

Thanks a lot fernicarry and Cubist. I'd not heard of burnt sand before. Annoyed as I literally only just binned a load of really thick brown packing paper this morning that would've been a good newspaper alternative?! I looked up burnt sand and it's saying to use with boiled linseed oil - does anyone happen to know of a decent pre-mixed version or is it best to mix on site?

I'll look up Oakum too but not sure how to calculate how much I need for the above products as it's not as straightforward as calculating per m2 of brick pointing. Basically around the french doors there's about 6.5m length of fillet needed that's about 10-20mm wide - would the depth of filler/burnt sand need to be about 1.5x the width of the fillet - say 15-30mm to squeeze into the void, after fitting the caulking / packing material? The depth of the void is about 100mm (although in one place my ruler goes right in as if it's a cavity wall!) Do you know how to calculate how much material I need to buy - will I need a 25kg bag of burnt sand? Many thanks.

 

[Cubist]

With cavities that have such varying widths and depths a different approach is needed. Just pushing paper or oakum into the cavity before applying a fillet of mortar or mastic will only result in your using far more packing material than is necessary or worse, the mortar or mastic constantly falling into the void and never achieving a weather seal. Your best approach, in my view, would be to use a stainless steel mesh - chicken wire - which is cut into strips and rolled up before being pressed firmly into the cavity. The aim is to create an armature/skeletal structure that has a tight compression grip on the outer and inner faces of the gap. Once in place the armature is then packed with mortar to create a solid mass that adheres to the mesh and the inner and outer faces of the cavity that can then be shaped and 'pointed'.

 

[Kearn]

It’s a good job your joinery isn’t a lime plasterer! Haired plaster was , and is used externally. No reason not to.

 

[fernicarry]

does anyone happen to know of a decent pre-mixed version

Search for "trowel mastic".