Mortar ratio for pointing

[PaulZ]

Hello, so I've got an early 20th century (I believe ~1916) end terrace house located in Barrow-in-Furness. The property is built with fairly typical red brick, with a cavity wall. I've been getting penetrating damp in a few places and I'm pretty sure it's down to ageing pointing. I can provide photos but let's just say there are chunks missing here and there, and it's just a mess. Regardless of whether this is indeed the main cause of the damp it really needs doing.

Anyway long story short, I am going to repoint it myself since I can't afford to get someone in to do it (I mean I could get a loan but let's just say my job situation is a bit uncertain due to current events).

What's really throwing me off though is the ratio for the mortar. A mate has suggested 5:1:1 of builder's sand, lime and cement. My own research has thrown up just how much disagreement there is on this, with some people saying you should be using lime mortar with no cement at all for a building of this age and construction, and other people agreeing with what my mate has suggested.

The practical part of it all, mixing the mortar and doing the job, I'm absolutely fine with; I just don't want to mess it up with the wrong materials before I've even started. Thank you very much for any advice!

 

[Bonni]

A number of points.

A cavity prevents damp crossing from the outer leaf to the inner leaf, unless the cavity is breached with debris.

The outside brickwork does not keep the water out, hence the invention of a cavity. Build a wall, full mortar joints, pointing amazingly and then get a hose pipe on it. Tiny beads of water will still run down the back of the bricks.

5:1:1 is the correct ratio, but rather than add the lime, I add Optimix instead. Cement SHOULD NOT be used on solid walls, it's ok for cavity walls.

The end of a brick is the header. If you see alternative headers (Flemish Bond), alternate courses of headers (English Bond) or a header course between 3 and 5 courses of stretchers, it's a solid wall.

Solid walls, the damp soaks in the brick/stone and breathes out the mortar. Cement in the mortar prevents the moisture breathing out. Always use lime mortar. You cannot add Febtone to lime mortar.

Rake out to at least an inch deep before repointing. Years ago, I was told never to rake lime mortar out where you don't need to. When you repoint, it looks patchy unless you can work out the exact colour match.

Damp on the inside is due to a cold wall (lack of heat) and certainly with a bit of black mould, lack of ventilation, hence why modern windows have vents and old houses had airbricks.

If your mortar smudges the brickwork, the mortar is too wet. Nice to have soft fluffy mortar to lay bricks but much firmer to repoint.

If you use a mortar gun, you must add lime or it won't come out of the nozzle.

 

[^woody^]

A cavity wall would not normally have lime mortar.

In which case 4:1 with plasticiser - which could be proprietary or lime to make it about 5:1:1.

The 5:1:1 idea is not a lime mortar, but is purely for the workability, so is not necessary.

Lime will lighten the colour too.

And remember it's building sand, not anything else.

 

[PaulZ]

Thanks both for your replies, it's definitely a cavity wall as it was insulated by the previous owner. I had a borescope in there a while back to verify it was actually done. With regards the damp it is definitely penetrating damp in some places e.g. one patch of wall upstairs where the pointing is extremely patchy has some bubbling paint pretty much dead on so I'm guessing rain is coming through and enough is soaking through to get into the plaster.

The main damp issue is on the ground floor though. The living room floor is a suspended timber floor, and I'd noticed soggy carpet and wobbly floor around the edges. I got the boards up and sure enough, rotten timber. I had been operating under the assumption that there wasn't enough ventilation under there which was causing a build up of moisture until I noticed one morning after a particularly bad bit of rain there was a splash of water on top of the floorboards around the chimney breast. A roofer mate already checked out the chimney and said it looked fine so the only thing I could think was that the brickwork up the side of the house (effectively the back of the chimney breast) was taking in rain that was then running down and spilling out the side when it got to the bottom. Hence wanting to fix the pointing, because as I say even if it's not the actual cause it's still needing doing.

With regards to the method, I was planning on using a mortar gun just because I've got one lying about and it seemed like a better approach for a newbie. So yeah sounds like the right ratio for the house, and method, and yeah definitely won't be using any old sand!