Using 9" retaining wall to build garden room

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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. :unsure:

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

garden room.jpg
 
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?
 
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.
 

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