Using 9" retaining wall to build garden room

AFT4B · 2026-01-17T09:22:50+0000

Hi,

We are having a retaining wall rebuilt after reducing the height of our garden. The tree beds are varying heights around the edge of the garden and therefore the 9” wall steps up in one place from 0.8m to 1.5m and then back down again. See my crude artwork below.

As part of reducing the garden and building the wall, we have laid a concrete base for a summer house. The builder has proposed that given a fair bit of the retaining wall is going up to 1.5m due to the outside level of the ground, it would be less work to use those walls and continue the single skin to the front and remaining sides to create the walls for the garden room.

It makes sense and would mean we don’t lose any space as our plan previously was to build a timber framed building inside of those walls. The single skin would be studded and insulated on the inside.

Given the walls are against a flower bed, my concern is penetrating damp on those walls. The builder has explained something about DPC running as a tray between about 6 courses of brick/ block work but I’m not entirely sure I understand it.

Can anyone suggest if this is a good or bad idea and do I have valid dampness concerns?

stevie888 · 2026-01-17T09:32:33+0000

Perfectly sensible provided it's all properly waterproofed.

AFT4B · 2026-01-17T09:41:04+0000

stevie888 said:
Perfectly sensible provided it's all properly waterproofed.

Thanks for the reply. So the waterproofing would be by way of this vertical DPC or are you suggesting other measures are required?

If other measures, what would they be? Tanking the walls from the inside?

freddiemercurystwin · 2026-01-17T10:23:34+0000

Waterproofing structures like this is perfectly possible but meticulous planning, detailing and attention during the build is essential to avoid mistakes. Suggest you do a section detail through the wall/floor to show us how you plan on building/waterproofing it and post it on here for comments.

BTW a single 9" block width wall is probably inadequate for 1.5m.

stuart45 · 2026-01-17T10:37:27+0000

freddiemercurystwin said:
BTW a single 9" block width wall is probably inadequate for 1.5m.

Even less can be. Here's one that's about 3ft and failed I priced for.

freddiemercurystwin · 2026-01-17T10:43:36+0000

I was trying to let them down gently!

AFT4B · 2026-01-17T17:49:03+0000

freddiemercurystwin said:
Waterproofing structures like this is perfectly possible but meticulous planning, detailing and attention during the build is essential to avoid mistakes. Suggest you do a section detail through the wall/floor to show us how you plan on building/waterproofing it and post it on here for comments.

BTW a single 9" block width wall is probably inadequate for 1.5m.

hmmm... every builder who came to quote said a brick faced 9" solid wall with block on the back would be sufficient.

What do you suggest?

Re the water proofing, this is my understanding of how the vertical DPC would be placed. The image shows a gap but in reality I think these would be tied together using wall ties.

noseall · 2026-01-17T18:02:06+0000

AFT4B said:
hmmm... every builder who came to quote said a brick faced 9" solid wall with block on the back would be sufficient.

What do you suggest?

Re the water proofing, this is my understanding of how the vertical DPC would be placed. The image shows a gap but in reality I think these would be tied together using wall ties.

View attachment 405027

We did solid dense blocks laid flat faced with bricks for a 1m high retainer - i.e. 330mm thick.

1.5m tall retainer will need some serious oof.

freddiemercurystwin · 2026-01-17T20:52:33+0000

Rule of thumb for mass retaining walls is the width at the base of the wall should be about 1/3 of the height.

Dereekoo · 2026-01-17T22:19:10+0000

As well as the thickness, height,density of the wall there is also soil properties of soil be retained and bearing capacity and size of foundation to be considered which I should imagine rule of thumb gives scant regard as does your builder by the sounds of it. "We have always done it like this and have never had a problem sir" (note number5??)

stevie888 · 2026-01-18T10:44:47+0000

AFT4B said:
hmmm... every builder who came to quote said a brick faced 9" solid wall with block on the back would be sufficient.

What do you suggest?

Re the water proofing, this is my understanding of how the vertical DPC would be placed. The image shows a gap but in reality I think these would be tied together using wall ties.

View attachment 405027

To ensure full water tightness you need a continous membrane: in your sketch water will track down the back of the dpc and under it, Ideally you need a fin drain to divert as much water as possible: you only have fill on 2 sides so this should be easy

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Using 9" retaining wall to build garden room

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