Reusing old concrete slab (not entirely flat)

[Blevins]

Hello all

I want to build a lean-to flat roof shed following the Mike G plan of a 3 course brick plinth), attaching it to the rear side of my house (along a brick wall with two external chimney breasts).

I've taken down a much smaller rickety old shed in the same place and can see a complete concrete slab that extends from the side and the back of the house. It looks like the one the ground floor of my house uses so I'm assuming it was laid when the house was built in the '30s.

A quick look at the edge of the slab showed that it is at least 1ft deep (couldn't yet get to the bottom), but that it is not level, being sloped so that rain-water can flow off (I presume). Along the length (about 6.5 metres) we are talking maybe an 80-90 mm difference, and along the width (2.6m) maybe 30mm.

What is the solution here? Could I just pack the brick plinth with more mortar while I build it and then do a suspended floor given that the existing slab is a bit too sloped?

Or should I try and level the slab with more concrete?

I've no bricklaying (or shed-building) experience but I don't mind trying to muddle through...

Thanks

Simon

 

[9fingers]

Hi Simon
Welcome to the forum.
I suggest that building a level plinth is the way to go but NOT packing out with mortar but instead to cut bricks lengthwise so you have four courses in places where the slab is lower.

HTH
Bob

 

[Blevins]

Thanks, that's what I'll do then.

The other immediate building problem that comes to mind is that there is a downpipe from the roof guttering that goes down the side of the house I want to attach the workshop to.

I was thinking about just leaving it as the pipe seems well-sealed into some kind of drain/soakaway but someone suggested to me that there could be a problem with dodgy odours if enclosed by a structure.

Any thoughts?

 

[9fingers]

Ideally divert the gutter downpipe keeping it outdoors rather than bring it in through your building as it would be a source of leaks etc. However sometimes easier said than done and you have to compromise.

Bob

 

[Mike G]

Yes and yes to Bob's posts.

 

[Blevins]

Thanks for your help fellas.

Simon

 

[Blevins]

Hello again

As an update, the downpipe looks as if it can be re-routed without much expense, but I've also found out after removing some flag stones that the old concrete slab does not extend flush with the back wall of the house (which is what I'd hoped), but falls about 1.5 metres short (apart from a deep concrete edge strip which the whole patio seems to have). This would leave me nearly 5 square metres less than what I thought I was going to get for the shed base.

My plan is to mix up a little extension to the old concrete base and tie into the old with rebar. I'd follow the level of the old slab for continuity (even though it isn't level) and then take the suggested plan of levelling the brick plinth for the shed when I get to building it.

Does that sound feasible?

Thanks

Simon