repairing concrete door threshold

whatsthenews · 19 Nov 2020

Recently had our UPVC back door replaced and when they took out the old door frame they broke the concrete threshold/step that it sits on.
The top of the threshold where the door frame sits is fine, as is the whole of the outside face. The damage is all on the inside.
Its entire at one end for a small distance (so we could fix shuttering to that) but then broken off at an angle down to the floor at the other.
We're fitting 50 ml PIR insulation with a floating 18mm chipboard T&G (2.4 x .6) floor over the top, so we need to sort it. The 68mm will still be slightly below the top of the threshold, and the inside edge sits slightly back from the internal wall perimeter since we've insulated the walls so we could put some ply shuttering in line with the wall at one end and the existing entire front of the threshold at the other end.
There's no heating in the room ATM but we do have a portable electric heater. What's the best compound /mix to use? It's probably 100 mm x 600 mm max.
Thanks

^woody^ · 19 Nov 2020

This might where a picture is better than words

whatsthenews · 19 Nov 2020

Thabks Woody. I managed to avoid the temptation of online poker. This is with some ply in place.
We're looking at blue circle quick repair ready mix concrete. You put it in dry then add water? Goes off in 30 minutes. I understand that we'd have to remove the ply to smooth/ feather it . After how long if we used this?
Will it bond with the broken face Ok?

whatsthenews · 19 Nov 2020

sorry you can't really see the broken face. I'll move the ply and post a pic.

whatsthenews · 19 Nov 2020

whatsthenews · 19 Nov 2020

Anyone else help with this please?

^woody^ · 19 Nov 2020

Yes, shutter it and use a very strong fine mix 2:1 (small stones like grano or just screed sand) with SBR for bonding. Mix it wet but not soaking, and tamp it and vibrate the shutter to toughen it up.

There will be issues with damp and thermal bridging but you seem to have covered that in your OP?

whatsthenews · 19 Nov 2020

^woody^ said:
Yes, shutter it and use a very strong fine mix 2:1 (small stones like grano or just screed sand) with SBR for bonding. Mix it wet but not soaking, and tamp it and vibrate the shutter to toughen it up.

There will be issues with damp and thermal bridging but you seem to have covered that in your OP?

Cheers Woody
Is screed sand the same as sharp sand?

There's a DPM under the concrete floor, but the floor is slightly (about 10mm) below the external DPC level so after heavy rain we used to sometimes get a bit of damp in a couple of places at the very bottom on one external wall. Not helped by bock paving sloping towards the house and water not draining away, but that's been dealt with now.
We put DPM 300 mm up all the external walls and the same distance across the floor before we fixed the battens (and also DPM on the back of the battens)
Guess we need to tape/glue the edge of the DPM onto the concrete floor, or is that not required?

Hadn't quite worked out how to finish the DPM on top/face of of the threshold. The top of threshold is about 150 mm higher than the block paving, and there's guttering running along that end of the garage.

We've put 25mm PIR up stands around the perimeter of the room (stopped the 37.5 mm insulated plaster board 100 mm short of the orig' floor to allow for the flooring and skirting) walls have total of 75mm insulation in top to bottom, so thought we would put 75mm up stand against the threshold. Does that sound OK?

^woody^ · 19 Nov 2020

Screed sand has larger bits of stone flakes and grit, whilst sharp sand is just sand. Some people confuse them and use the same name for each, but you do need screed sand not just sharp sand.

You need a barrier to damp on the back and top of that cill as it will bring damp in from outside. This could be a liquid DPM such as bitumen paint once the concrete has dried sufficiently, or a sheet of polythene and some silicone at the edges.

You will have your insulation up the side of the threshold, and presumably you are taking the timber flooring right up he the frame, so at the least a 3mm layer of insulation will do on the top, but the more the merrier to prevent any cold spot for condensation to form on the top of the the concrete

whatsthenews · 19 Nov 2020

Ok.
So paint the room side and top of the metal cill with the bitumen paint?
The finished floor level will actually be about 20 mm below the top of the threshold. Bit of a pain, but we had no choice because we needed to get the floor to the same level as the kitchen floor in the doorway where there was previously another big concrete threshold which is now gone.
Maybe we can cover the remaining threshold with some insulation and then chipboard, glued and all the joints siliconed?

^woody^ · 19 Nov 2020

Why don't you just cut the threshold down with a disc saw?

20mm is mad and a trip hazard.

whatsthenews · 21 Nov 2020

How long do we need to leave the shuttering in place please? Max 90mm x 20mm ?

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repairing concrete door threshold

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