Sloping site problem

[Tim2021]

Hi,

 

I am hoping that someone can suggest construction details for the rebuild of this lean-to extension on the slightly sloped site.

 

The concrete farm track shown below, is slightly angled between 400mm and 1200mm from the rear wall of the finished project on the plans.

 

Unfortunately, at the highest end, the farm track is 200mm higher than my internal floor level.

 

I am demolishing and rebuilding the existing lean-to structure (currently the walls are mud/granite, very damp, no foundations and nearly 700mm thick).

 

So I need a wall design as thin as possible, 200mm higher outside than inside, suitably damp coursed and insulated.

 

My planning approval is to rebuild the lean-to as timber-framed, but there can surely be a concrete/brick dolly wall under that. 

 

I have made an initial proposal to BC but didn't include this detail and they haven't done a first inspection yet. I want to have it clear in my mind when they visit.

 

Thanks,

 

Tim

[markc]

If possible I would be looking to keep the ground away from the new structure by a deal with the track owner or making the new build slightly smaller to allow for a drain.

failing that you will need a watertight slab and wall construction at least until out of the ground plus a couple of hundred millimetres. Above that you could use timber or SIPs etc.

[Tim2021]

Thanks very much for the reply.

 

It is my farm track, I put it in myself about 15 years ago. I had never been inside the old cottage at the time and had no idea of the awkward steps down inside it. In theory I can do anything with it... I could even dig it out deeper but would prefer not to obviously...it's C35 with rebar and a hard bed below.

 

I'm also trying not to reduce the size of the extension of course, so am over-constrained in every way!

 

Can you suggest what a "watertight slab and wall construction" would look like please? How thick?  I'm imagining a cavity wall with the outer being concrete/rebar strip maybe 150mm thick?, 100mm cavity, inner wall 100mm block? DPC between..something like that?

 

Would BC want a whole load of structural calculations?? ?

 

At the lower end of the track, (coming out of the picture towards you)  the level continues dropping to be 100mm below the internal floor level within a few metres.

 

Tim

 

 

 

 

 

 

 

[joe90]

Re the height issue/damp, I would dig a French drain (trench filled with 50mm stone, no fines) below cottage floor level filled to track level extending beyond cottage to stop any ground water/rain from soaking the wall. Trench to end somewhere it will flow down hill. 

[Tim2021]

Thanks for the comments, I guess it confirms what I thought and there are no easy solutions.

 

I'm going to carry on assuming I can put the small ditch along the back.

 

Worst case, I'll have to dig up the track and re-lay it a bit lower if the inspector insists.

 

 

[ETC]

Simple block work cavity wall with the outer leaf possibly thickened to take any additional load. Speak to Visqueen about a site specific tanking/DPM/DPC detail.