Building off a retaining wall

[DonkeyApple]

Just looking for the reason why one wouldn't in this particular instance:

Footings just poured for a 10m single garage down the side of a house that is just going to be single skin. Very simple build.

Due to the angle of the ground the property is on the rear has been dug out and there will be a retaining wall built on top of this concrete pour with the single skin wall then being built inside of it.

What I can't fully get my head around is the engineering reasoning for not just building the garage wall off the retaining wall? I can't see why they need to be two parallel walls as they will both sit in the same footing and if the retaining wall were to move then it's obviously going to screw the garage wall next to it anyway. Plus, you have the hassle of any moisture getting between the two walls.

[PhilboSE]

Well the retaining wall will be permanently damp and won’t have a DPC. Probably not attributes you want for an enclosed building, especially a single skin.

[hidetheelephants]

How high is the retaining wall? To use the garage wall as a retaining wall would require tanking or some other waterproofing and more fastidious attention to drainage.

[DonkeyApple]

2' going up to 4'. I guess any water that finds its way into the cavity will go below top of damp of the pad anyway. It's just more about wondering what all the reasons are.

[sfella]

Don't create a small cavity, will be full of rubbish in no time and causing issues.

There's no structural reason you can't build off a new retaining wall but you'll need to damp proof it, this will be by way of tanking, you could externally tank it prior to back filling.

As an aside, single skin garages are pretty rubbish, if you can stretch to it get an internal skin of thermal blocks inside,will be much nicer space in the long run

[DonkeyApple]

sfella said:

Don't create a small cavity, will be full of rubbish in no time and causing issues.

There's no structural reason you can't build off a new retaining wall but you'll need to damp proof it, this will be by way of tanking, you could externally tank it prior to back filling.

As an aside, single skin garages are pretty rubbish, if you can stretch to it get an internal skin of thermal blocks inside,will be much nicer space in the long run

Thanks. It's only a cavity in the sense that the two walls aren't tied in so there will be some form of gap. There will be DPM in this gap so that any moisture that comes through the retaining wall just goes down below the damp of the garage floor. The inside is having a 100mm warm roof and 100mm along the walls as I agree that the sweating from a single skin is annoying in a garage/workshop.

[PhilboSE]

DonkeyApple said:

Thanks. It's only a cavity in the sense that the two walls aren't tied in so there will be some form of gap. There will be DPM in this gap so that any moisture that comes through the retaining wall just goes down below the damp of the garage floor. The inside is having a 100mm warm roof and 100mm along the walls as I agree that the sweating from a single skin is annoying in a garage/workshop.

Hmmm. The retaining wall will be damp across its entire height/length (assuming it’s retaining earth/souk). What makes you think the damp will go “down” below the garage floor? It will evaporate from the face of the wall into the inside of the garage. If you’ve insulated this and added a warm roof (with the usual membranes) then the damp will have nowhere to go and it’ll create a nice humid atmosphere resulting in mould and rust.

Single skin is bad enough for damp issues without adding to them.

[DonkeyApple]

PhilboSE said:

Hmmm. The retaining wall will be damp across its entire height/length (assuming it’s retaining earth/souk). What makes you think the damp will go “down” below the garage floor? It will evaporate from the face of the wall into the inside of the garage. If you’ve insulated this and added a warm roof (with the usual membranes) then the damp will have nowhere to go and it’ll create a nice humid atmosphere resulting in mould and rust.

Single skin is bad enough for damp issues without adding to them.

It won't be able to go across due to the dpm between the two structures so only up or down. Any damp will have plenty of options such as buggering off through the large gaps around the garage door or the air bricks. The biggest source of moisture will be a wet car going in.

[PhilboSE]

Ah ok sorry I missed you we’re back to 2 structures. Thought you were still using the retaining wall in the garage.

In that case I’d try to use the dpm of the garage as a continuous sheet to lap up the back of the garage between the 2. Avoid any gaps in the membrane if you can.