OK, I have an external basement stairwell planned, about 7x2m, outside our basement. The step into the door is designed currently 350mm from the stairwell floor to allow some space for insulation etc.

 

I'm thinking I'll use the bottom of the stairwell as a sump with sump pumps to remove the rain. I need, say, 100mm min XPS insulation on the floor, near the door into the basement, to partly counteract the thermal bridges.

 

So, I think I need a sort of big 'shower tray'-like shape to funnel the water into one corner, maybe 100mm high at the high edges going down to say 30mm for a 1 in 100 slope.

 

How might I make this? Sculpt some big bits of XPS with a hot wire to get the slope and screed or cement board over? Freehand somehow in screed, maybe with a thick non-flowing screed and some long wedges as guides for some kind of simple tamp? Something else?

Make the shape in XPS and lay up GRP over it, with whatever 'non-slipperiness' you want to incorporate? It works for boats (and indeed shower trays).

3 hours ago, Redbeard said:

Make the shape in XPS and lay up GRP over it

I don't think so.

The styrene in the resin will dissolve polystyrene.

You can use a polyurethane foam though.

 

Thanks ST, I knew you'd know. I stand corrected. I somehow had in mind that XPS was OK. The rest stands, though!

Is the area you want to cover 7m by 2m, or is that the total area and you want to make this tray in a smaller part of it?

 

Making anything that is thin, and of a large surface area will flex, it is why insulated raft foundations are thick, steel reinforced, concrete and not thin paper mache.

>>> Is the area you want to cover 7m by 2m

 

Ah yes, I was ambiguous - that's the internal area of the staitwell which needs to drain i.e. the area of the 'shower tray'.

 

It's the floor of an RC box that is the stairwell.

We laid tile over screed for our patio that had some clever falls built into the screed so that rainwater wouldn’t pool in the patio. My builder used steel profiles about 50cm apart to help him outline the progressive changes in height and then the sand and cement screed was poured over the area by the screed company and my builder used a very long edge to shape the screed so that it was flat but with a fall. He was very skilled though… in his 60s and been doing this kind of thing for decades.

@Adsibob

 

Thanks v interesting. Presumably not a ‘self levelling’ screed but a thicker ‘stays where it’s put’ kind.

OK I think I'm looking at something like this then: