Breached DPC / Ground levels / Damaged Brickwork - Solution required!

[Victoriandad]

Hi Everyone,

First time posting here and I sincerely hope I can get some advice!

We moved into our Victorian terrace last year, there were issues with damp and I have completely replaced the sub-floor (sits independent from external walls), added numerous new air bricks and cleared 3 skips of crap from the sub-floor area.

I recently discovered a damaged waste pipe which was leaking into the ground below the patio. This had loosened 2 bricks at the bottom of the footing (spreader course), the bricks are at the junction between the house and kitchen (side-return). I have repaired the brickwork and it doesn't look like any other footings are in any way compromised.

During my investigation of the above problem, I noticed that the original kitchen DPC actually sits below the existing patio and only 3 courses above the footings. The DPC on the adjacent wall (dining room, french doors) is 1 course lower than the kitchen. There is musty smell in some of the kitchen cupboards and the floor is poured concrete. None of this was picked up on the Survey.

The course of bricks immediately above the kitchen damp course are badly damaged in some areas.

So, I am removing the patio and want to lay either yorkstone flags or gravel, but, I only have 265mm from the DPC to the top of the first spreader course footing. Ideally the patio should have a sub-base or at least 75mm with 30mm bedding layer and 50mm flags, that would only leave 110mm below the kitchen DPC and even less below the dining room DPC.

1. My understanding was footings are only disturbed if I dig below a 45 degree angle? Could I safely excavate to the top of the final spreader course, giving me an extra 85 mm?

2. Being so close to the footings, is it safe to use a wacker plate to compress the sub-grade & sub-base?

3. How serious a problem are the damaged bricks and should I remove the 'render' which covers the 2 course of bricks below the bell-cast?

 

[MatthewC]

1. My understanding was footings are only disturbed if I dig below a 45 degree angle? Could I safely excavate to the top of the final spreader course, giving me an extra 85 mm?

2. Being so close to the footings, is it safe to use a wacker plate to compress the sub-grade & sub-base?

3. How serious a problem are the damaged bricks and should I remove the 'render' which covers the 2 course of bricks below the bell-cast?

Hi there and welcome.

I have a small amount of experience in this sort of thing - if you click the contact thing on the right hand side of this, you will then see a blue spot which is a link to my blog. (I can't believe that I moved in five years ago!) In answer to your questions:

1. Think of railway embankments. I used to work for Network Rail and the national geo-structural engineer once told me that in fact the force is all transferred something like 15 or 20 degrees from the vertical. Hence the 45 degree angle is regarded as safe as a working figure for us amateurs - just don't do it when its really wet! I personally would go down 45 degrees from your second step.

2. Nevertheless, I personally wouldn't use a wacker too close to the wall, but that's not a job I had to do anyway. I think my groundworker did in one place, but he was pretty skilled and was gentle with it.

3. I would remove the render under the bell cast (is it cement?) and repoint the bricks with NHL. When it's all dried out I would expect them to be fine, but if any are really crumbling then they might need replacing. I would also suggest that, if you lay flags, you still should have an 8-12 inch width of gravel up to the wall; that reduces the splashback on the wall; if you have the level of the gravel 50mm or more below the level of your flags, that increases the clearance considerably and promotes drying out.

I presume you are laying the right sort of membrance beneath?

Matthew

 

[Roger H]

repoint the bricks with NHL

Maybe use a low strength one - NHL 2. I'm hearing a lot about the eventual setting strengths being way in excess of the strength at 28 days.

 

[Victoriandad]

Hi Matthew,

Thanks for the advice, very much appreciated.

I've decided to install a stabilised gravel patio. This will require me to go down to a depth roughly in line with the top of the bottom spreader course. 75mm MOT / 15mm sharp sand / 50mm gravel. Yes, I will be using a suitable geotextile below the sub-base.

I'm happy you have told me to steer away from using the Wacker near the wall, its the same advice I've received from a well respected source on another site. He suggested I don't lay MOT for a 500mm strip around the wall, ensuring I stay wall clear of the spreader courses. I'll simply dig the 500mm strip to a level and lay a little MOT, tamped down manually. How's that sound?

I'll be sure to remove the render and repoint the bricks, unless they do need replacing then I'll get an expert in to complete the work! There is plenty of repointing to be done on the house as a whole. Concrete mortar repairs are aplenty as is shoddy brickwork which I paid for last year. I intend to get quotes for repointing the lot, so any advice with regards to repointing is appreciated.

Now I'm going to grab a brew and read your blog!

Any idea on a traditional looking decorative gravel for patios and driveways?

Thanks again,

Gavin

 

[plasticpigeon]

I may live in a house similar to yours with similarly shallow footings. It is my personal opinion that digging down isn't really needed. I'm guessing this is a rear yard area outside your back door or kitchen window so it will only get foot traffic. I really think 75mm MOT is overkill for an area that will get light foot traffic. Why not take up the slabs level it off, put down breathable membrane and gravel and be done with it. We have blue pavours laid on earth in our yard. This was most likely the original floor covering. It is hardwearing and allows drainage between pavours if they are laid on earth or sand rather than cement or an impermeable membrane. There is a tendency for weeds and the like to grow in gravel even with a membrane so pavours may be an idea worth considering!

 

[MatthewC]

Any idea on a traditional looking decorative gravel for patios and driveways?

My groundworker told me about some stuff called "self-compacting gravel". It's fairly fine (0-10 or 0-15mm), looks good (see blog) and does what it says whilst remaining porous. Unlike pea gravel or anything else smooth, it doesn't really have anything loose on the surface and so nothing washes outside my property. I have no idea what traditional gravel would look like but I'm very happy with mine. If you use it, make sure that you keep a 100kg or 200kg for future repairs as you can't buy it in small quantities.

 

[Flyfisher]

We considered 'self compacting' gravel for some paths. It's the sort of thing widely used in National Trust properties (and Buckingham Palace apparently) and looks lovely. It's basically a 10-ish mm gravel plus fines, i.e. everything from 10mm down to dust. Breedon Golden Amber is the 'classic' stuff, but it needs careful laying and is pretty expensive. I'm also not entirely convinced about how permeable it is, because it's the fines that create the self-binding effect and the laying process requires firm compaction for the binding action to work and they stress the importance of adequate camber to avoid pooling. http://www.breedon-special-aggregates.co.uk/breedon-golden-amber-gravel/laying-instructions

I was quoted £106 per tonne for 10 tons, delivered loose. Coverage is about 8m2 per tonne when laid to the recommended 50mm finished thickness.

In the end, we went for 8mm pea shingle on 50-75mm compacted railway ballast, blinded with sharp sand. As mentioned, weeds will grow in gravel, regardless of membranes, as the gravel collects dust etc, but they're fairly easy to pull out plus there is always the chemical warfare option :shock:

 

[Victoriandad]

Thanks for the information fellas, really useful.

I've now removed the old patio flags and am just digging to the correct level to lay down the MOT later today. I've decided to keep 500mm clear of the footings where I will leave the existing ground as a base for the gravel stabiliser, this will prevent the need to dig lower than the footings. Also, I'm going to use a hand tamper to level and not a wacker plate, a pal in the trade has advised me to steer well clear of using one with such shallow footings.

I have discovered a potential issue. When digging next to the original drain I discovered there was a void beneath with and around the sides of the drain pipe. A single brick footing appears to have come loose and the ground is soft. I have no idea if the void was left by the cowboys who laid the patio or is due to water erosion. How best I repair the brickwork there? Do I have to use a lime mortar below DPC? I repaired another area recently, using a cement sand mix, will this damage the existing brick work?