Mortar for brick footings

[jsmac]

Hello PPUK,

Sadly, we're having to undertake a full demolition and rebuild of a timber framed single storey extension. It's striking that it has stood as long as it has, let alone not collapsed when a roofer was clambering on top fixing the slate roof.

The extended cold spell that we had a couple of months ago was a blessing in at least one instance. It caused the freezing and blowing of a small section of the external render off of the laths. This revealed that the timber plate was completely rotten (the ground levels had been above it for quite likely decades) and further examination showed that the rest of the timber frame was in extremely poor condition. Further excavation showed that the brick "footings" were only a couple of courses of bricks so dropping the outside ground levels to below the inside floor level was going to be problematic. Since this single course of bricks was barely holding together anyway we decided to start again.

A trench has now been dug and reinforced concrete footings have been poured. The plan is to now build brickwork up to the level where the older brickwork was. My question is what to use as the mortar. In one section about 4 or 5 courses of bricks will be visible above the new ground levels and in another section about 12 courses will be visible. Our bricklayer will likely suggest a sand and cement mortar. Is this recommended or would a slaked lime and sand mortar be more appropriate? Is there a reason not to use lime mortar in brickwork that will be underground?

Many thanks.

Scott

 

[MikeG]

Controversial area, this.

My own preference is for cement below DPC and lime above. You can achieve quite a close aesthetic match with lime mortar by using white cement and a pale sand. Grey cement just looks awful on a period property. Having seen lime mortar below ground umpteen times, I'm just happier with cement in this location. Others, quite rightly, will see things differently, and I don't claim anything other than personal preference (and a few years experience) to back up my case.

Assuming it was an ancient one, it's a pity to lose your timber frame. They really do have to be an awful lot worse than people think to be beyond redemption. Generally you can replace the sole plate and scarf in new post feet, and save most of the frame. If you were talking about a modern timber frame, then it doesn't matter.

 

[jsmac]

Thanks, Mike.

Unfortunately, the rot on the timber frame was from pretty much top to bottom. There were of course some timbers that were sound but they were the exception. The initial plan was to do exactly as you describe but when the render was removed and the timber frame uncovered the damage and rot was much worse than we had expected. We probably would have had to replace about 60 - 70% of the timber frame. We were able to save the roof and the timber cladded ceiling, however.

When we bought the house a number of the roofing tiles were broken so I suspect that there was a lot of water leaking through the structure for a number of years. We were able to save the internal timber cladding that that was on the walls. We'll reinstall it when the replacement timber frame is built.

On the upside we did find a window that had been covered in cladding for probably 120 years. We're going to use it as a template to replace some more modern windows in the dining room that were probably added about 40 years ago.

 

[MikeG]

That all looks like softwood. Is the house 19th century?

 

[jsmac]

Yes. We think that the original part of the house was built in 1830 - 1840 but it has had sections added over the decades and various repairs on top of earlier repairs.

This room was a small WC that we think was the original outside loo that was brought inside when the house was extended and turned into a lodge in around 1870.

 

[jsmac]

The bricklayer was surprisingly amenable to using lime mortar. So the plan is to use lime above ground and sand and cement below ground as recommended by Mike.

Mike Wye and AngliaLime do a premix mortar and since there is not a lot of brickwork it seems like this is perhaps the most straightforward way to proceed (the bricklayer was more amenable once I told him he didn't have to worry about ratios of lime and sand).

We have discussed using red engineering bricks as the inside layer of the double skinned wall as they're cheap. The wall will be covered in timber cladding inside so aesthetics are not a concern but I am wondering whether there is something else that I should be aware of. Similarly, should we use lime mortar on the engineering bricks or is this completely unnecessary?

We will probably only use about 300 bricks in total so we can use the same reclaimed yellow metrics for the inside skin if this is more advisable but if it does not matter then using engineering bricks will save us some money.

Many thanks.