Is this normal? Huge slot under exterior door sill.

[Andys]

This is our back door, on a '90s extension so not strictly a period question. The door sill and the bottoms of the side pillars have rotted because they're sitting at ground level.
Having removed the sill, the strip of damp course and the packing slices of brick, I can see the concrete foundations of the house and the base of the outside path. Between the two there is a great big slot about 9 inches deep - is this normal, some kind of isolation between the two, or is it a massive hole waiting to be filled with rainwater?
Do I fill it or leave it and cover it as before? I wouldn't want to lose the cat down it.
I'm inserting fixings through the door posts into the brickwork for stability and thinking of laying a brick sill which can take the dampness.

Edited to add: ah yes, no, the slot is actually between the block foundations for the outer brick skin and more block foundations for the inner blockwork so it's an extension of the cavity. As it's all well below the damp course in the outer brickwork I could fill it with some sort of pouring concrete, no? What troubles me is the way it looks so easy for water to run off the outside path, right down this gap.

 

[Cynan]

I’m no expert, but I’d be inclined to drop a waterproof membrane of some sort down against the house and then fill with concrete.

 

[Andys]

Ah yes, thanks for that bit of clear thinking Cynan. Sounds logical to me

 

[Lime]

The problem as I see it is that the outside ground level is too high in relation to the step.

 

[Cubist]

Just a thought on this one - but not I may be having a senior moment here.

I seem to recall that builders would sometimes leave gaps like this to prevent the spread of damp between two structures built at different times. Your idea to install and DP barrier and infill with concrete should work therefore but it may be an idea to talk to an 'old' builder.

 

[Andys]

Mm yes, the ground level is probably too high. I've laid the sill as a brick course ( on their sides) which brings the sill above ground level and closes the gap but with a damp course layer up against the inner green tiled bit. I'll add some door-edgy bits in oak and see how it holds up.

The gap turns out to be part of the wall cavity and is ventilated into the rest of the cavity from the sides of the door frame.

 

[Lime]

Mm yes, the ground level is probably too high. I've laid the sill as a brick course ( on their sides) which brings the sill above ground level and closes the gap but with a damp course layer up against the inner green tiled bit. I'll add some door-edgy bits in oak and see how it holds up.

The gap turns out to be part of the wall cavity and is ventilated into the rest of the cavity from the sides of the door frame.

Yes that's right, it would be.
I would not attempt to fill the cavity with anything.
When the alterations were originally carried out the door threshold should have been constructed as one piece of stone/cement/wood/metal/plastic extending from the inside of the building to the outside.
It should have had some sort of raised weather bar running from one side of the door frame to the other under the door to prevent rain being able to enter the house.
The outer part of the threshold should have had a slope on it from the raised bar to the outer edge of the sill and, if the sill overhung the outside ground level, a drip edge under the sill.

An example to give you an idea of what I'm talking about:-

http://isamaremag.com/adjustable-door-sill-futuristic/adjustable-door-sill-4-splendid-threshold-oak-cap-36-9-16/

Although there are many differing methods to construct a door sill (or threshold plate) they should all have much the same contour as in my example to prevent water ingress.

 

[Andys]

Thanks Lime!