Building Garden Retaining Walls - Questions

Maffy52

Hello, am building some two retaining walls for a patio next to the house (see very rough sketch attached!).  The garden slopes away from the back of the house, so a patio needs retaining walls to support it (the retaining walls are green in the sketch). The aim is for 4 courses of brick on top of a concrete footing. One wall goes from the back of the house down the garden (about 5m long) and then a second wall parallel to the house (also about 5m long).

I thought it was going to fairly straightforward but now I've started, I appreciate it is trickier than I thought!
I have two questions I am 'stuck' on;
- Should I leave a small gap between the footing/retaining wall and the brickwork of the house (see red arrow)? I was going to leave about 1 cm. However, I see videos of guys building extensions and they just pour concrete right up to the house. Should I do the same?
- I am doing this alone, by hand. Its tough to mix and pour the  entire footing in one go. I was going to pour half the footing (from the house to the purple arrow), then do the next half a few days later. Then do the footing for the other retaining wall which is parallel to the house also in two parts. Is there any problem with doing this?

Really hope this and my sketch makes sense. Anybody done something similar?

Thanks.

sharepicker

Others will have more professional instructions but if I was doing it : providing the footing from the house is BELOW the damp course of the house no need to leave a gap but maybe a vertical DC especially if the brick wall rises above the house DC. (plus a bit of weather expansion allowance) Brick 4 course is not very weighty so I dont see why it needs to be poured in one go but instead of the join being abrupt maybe stagger it by a foot or insert 2/3 reinforcing rods - sort of like joining wood a half-joint. My main concern would be the patio wall or top slabs being below damp course.

Maffy52

sharepicker said:

Others will have more professional instructions but if I was doing it : providing the footing from the house is BELOW the damp course of the house no need to leave a gap but maybe a vertical DC especially if the brick wall rises above the house DC. (plus a bit of weather expansion allowance) Brick 4 course is not very weighty so I dont see why it needs to be poured in one go but instead of the join being abrupt maybe stagger it by a foot or insert 2/3 reinforcing rods - sort of like joining wood a half-joint. My main concern would be the patio wall or top slabs being below damp course.

Thanks, yes everything is well below the DPC, the footing will be level with the top foundation block and then there are 7 courses of engineering bricks above that, so my wall will still have three courses of engineering bricks above it before the DPC.
So, sounds like I could run the footing and brickwork right up against the house.

Your suggestion of reinforcing is a good idea, didn't think of that. Maybe just two or three reinforcing bars (about 1m) across the 'joins in the footing? 

sharepicker

I can't see why you can't go up to the house wall in this case. Depending on how thick your footing the problem with not pouring in one is you may get movement/crack a few years on so it's a matter of strengthening the join area. You may be worrying about something that's unlikely to happen. Until a pro builder chimes in I would keep browsing you tube and the like. Try running a question through ChatGpt like "can you pour concrete in 2 stages along the length of wall foundation"

Section62

Maffy52 said:

Hello, am building some two retaining walls for a patio next to the house (see very rough sketch attached!).  The garden slopes away from the back of the house, so a patio needs retaining walls to support it (the retaining walls are green in the sketch). The aim is for 4 courses of brick on top of a concrete footing. One wall goes from the back of the house down the garden (about 5m long) and then a second wall parallel to the house (also about 5m long).

I thought it was going to fairly straightforward but now I've started, I appreciate it is trickier than I thought!
I have two questions I am 'stuck' on;
- Should I leave a small gap between the footing/retaining wall and the brickwork of the house (see red arrow)? I was going to leave about 1 cm. However, I see videos of guys building extensions and they just pour concrete right up to the house. Should I do the same?
- I am doing this alone, by hand. Its tough to mix and pour the  entire footing in one go. I was going to pour half the footing (from the house to the purple arrow), then do the next half a few days later. Then do the footing for the other retaining wall which is parallel to the house also in two parts. Is there any problem with doing this?
...

As this is a garden wall rather than an extension, I would leave a gap in both the footing and the brickwork.  10mm is as good a number as any.

The reason is mainly that the gap will make allowance for movement/expansion, but also for drainage and to avoid any risk of the patio supporting wall doing damage to the house wall.

The easiest way is to put a strip of sheet polystyrene/Celotex or similar vertically against the house wall before filling the footing with concrete.  If you do the same  where the bricks are then I'd set the polystyrene/Celotex about 15-20mm back from the front face of the wall so there's some space to put pointing mortar or flexible sealant if you feel the need.

Ideally footings would be poured in one go, but if it has to be done as two then a vertical join is best.  No harm in using some rebar across the joint, but for a wall which is only 4 courses high then it probably isn't essential.

Yorkie1

Have you allowed space for water to run off from the patio? 

Maffy52

Section62 said:

Maffy52 said:

Hello, am building some two retaining walls for a patio next to the house...?
...

As this is a garden wall rather than an extension, ....

Thanks, this helps me alot, have been 'stuck' on this for a few weeks (once I do the footing there is no going back!.).. 

The reason is mainly that the gap will make allowance for movement/expansion, but also for drainage and to avoid any risk of the patio supporting wall doing damage to the house wall. This was my thinking as well, just didn't want to risk any damage to the house wall.

The easiest way is to put a strip of sheet polystyrene/Celotex or similar vertically against the house wall before filling the footing with concrete. I have a thin piece of plywood, which I am going to stick against the house wall, makes a gap of about 10-12 mm.

Ideally footings would be poured in one go, but if it has to be done as two then a vertical join is best.  No harm in using some rebar across the joint, but for a wall which is only 4 courses high then it probably isn't essential. Thanks, was wondering about the joint. I thought to make a slight angle at the end of the first section (so it is slightly longer at the bottom, by 5-6 cm). Then the second section would be slightly 'on top' of the first section at the join, but never seen an example to follow for this (they always 'pour' in one go!). But will follow your advice and have a vertical joint (avoids having a thin edge of concrete)...

Was going to use a 'semi-dry' mix of concrete as much easier for me to handle, do you have any experience of this?

Maffy52

Yorkie1 said:

Have you allowed space for water to run off from the patio? 

thanks, yes, Have been thinking about drainage also. Was planning to put a few bits of plastic pipe in between the footing and the first course of bricks (like weep joints). Thinking that should be enough, but it is really just guess work on my part! Maybe need some gravel around the internal end of the pipe (?)

For surface water, haven't got that far in thinking yet, but I guess need to slightly angle the patio away from the house. just a couple of centimetres(?)

Section62

Maffy52 said:

The easiest way is to put a strip of sheet polystyrene/Celotex or similar vertically against the house wall before filling the footing with concrete. I have a thin piece of plywood, which I am going to stick against the house wall, makes a gap of about 10-12 mm.

Plywood isn't a good idea - it isn't sufficiently compressible, and may swell when it gets wet, so is the opposite of what you want.

Maffy52 said:

Ideally footings would be poured in one go, but if it has to be done as two then a vertical join is best.  No harm in using some rebar across the joint, but for a wall which is only 4 courses high then it probably isn't essential. Thanks, was wondering about the joint. I thought to make a slight angle at the end of the first section (so it is slightly longer at the bottom, by 5-6 cm). Then the second section would be slightly 'on top' of the first section at the join, but never seen an example to follow for this (they always 'pour' in one go!). But will follow your advice and have a vertical joint (avoids having a thin e

The 'overlap' section will be thinner than the rest of the footing, so will have less shear resistance - if there's an issue with movement then the thinner section of the footing is where it is more likely to crack.

Maffy52 said:

Was going to use a 'semi-dry' mix of concrete as much easier for me to handle, do you have any experience of this?

Dryer mixes are harder to 'work' - you need to make sure the concrete is well compacted, and if the mix is too dry it is harder to get it to move into voids.  One of the big DIY mistakes is to add too much water to make the mix easier to work, which results in weak concrete which is particularly prone to frost damage.

The exception is using 'dry mix' - with no added water.  This is easier to shift and compact than other types, but the lack of water means it takes a long time for the concrete to fully strengthen. (could be weeks or months)

stuart45

What thickness is the concrete going to be? Also how thick is the wall?

Maffy52

stuart45 said:

What thickness is the concrete going to be? Also how thick is the wall?

Yes, good question, as also not sure about this!
I thought a footing about 30 cm wide and 12-15 cm deep is sufficient for 4 course of brick. Seems reasonable but not really any science behind this!

The bottom two courses will be double (facing brick which is the nice looking brick, like 'fake' stone, and the brick behind being a simple concrete brick (like a small block!). For the top two courses was going to have facing bricks with one concrete brick but standing on its long edge. This will give a gap at the top to set the paving slab into.

This is a rough cross-section I what I planned to do. Do you think it seems reasonable?

Should the wall be in the middle of the footing or nearer to the front? also not sure about this now I think about it!..