Poly drainage channel in concrete slab - before or after pour

[Dave Robinson]

About to pour a slab, which ultimately will have a polymer and galvanised steel grid drainage channel in it at the front of the workshop that's going on it. Is it ever customary to try and incorporate a channel for this prior to pouring the slab, or is it better to retrofit once the concrete has set?

 

[pilsbury]

Someone with real knowledge will answer soon, but I'd be inclined to set the drainage channel in position before the pour. It will then be all set in place. Having to get angry with your new slab with a Stihl saw and chisel sounds like a pain in the balls

 

[Dave Robinson]

Now, I see your point, but that's interesting, because I'd wondered that leaving it until later when the plan of the workshop had been fully marked out on the slab might be the preferred approach. I guess there's probably opinions out there, hopefully the voice of experience will appear!
Oh yeah, anyway, you're assuming I lack the desire to get angry with a power saw and concrete !

 

[bernardgreen]

I'd wondered that leaving it until later when the plan of the workshop had been fully marked out on the slab

Mark out the plan in virtual reality before pouring the slab. Plant posts outside the area of the slab in line with the sides of the workshop, stretch string between the posts to "mark" the outline of the workshop walls. Now you can plan the drainage.

Also put in place a duct for power cables and a second duct for other ELV cables that might be needed ( alarm, telephone etc ) before pouring concrete. Then the cable come up insode the workshop

 

[Ian H]

If the top of the slab is lower than the bottom of the channel then do it after. You can set the channels onto their own mix in exactly the right place.

 

[Doggit]

In an idell world, you'd have the drain in place before you pour, and it would then be a nice neat job, but unless you can fix it in place, then it's going to move as you pour the slab, and it'll very likely get the cement inside the channel as you pour. A 9" angle grinder with a diamond blade will go through the concrete easily, and if you do it before it's gone off too much, then it'll be an easy dig. You can use an sds drill with a chisel on it to clear the centre out.

But as bernard suggests, put in any power channels before you pour, but make them big enough to allow a draw string to be left in it.

 

[bernardgreen]

, but unless you can fix it in place, then it's going to move as you pour the slab,

Making a dummy channel out of scrap timber to form the slot when the concrete is poured might be worth the effort and avoid the nessy job of cutting concrete

 

[Doggit]

Actually Bernard, you've got a good idea, but it's possibly been overworked. As long as a beam of wood with release agent is dropped in while the slabs being poured, it can be pulled out before the concrete goes hard, and then tidied up later. If it's fractionally larger than the channel, then it can be filled with a very wet mix to settle the drain in place.

 

[r896neo]

I dont really get all this faffing. Drainage is always installed before a slab? Its not rocket science, just check and recheck your falls and levels and get it done. If you cant accomplish the task of accurately marking out the position and levels of your slab before hand your in for a shock trying to level and float finish a concrete slab.

I would only consider doing it afterwards if your drain fell at the edge of a bay otherwise you will end up with a weak different coloured mortar joint abutting the drain rather than a nice strong continuous concrete.

 

[Doggit]

Now that brings in an interesting point R896; I've never seen a fall in a drainage channel, as they tend to follow the level of the slab.

 

[Dave Robinson]

Thanks for the responses & I'm glad it's led to discussion as it suggests it's not as simple/daft a question as I thought it might be! Not at all surprised that people have their preferred methods all leading to the same eventual outcome.
As per Bernard's suggestion, it makes sense to create a slot slightly over-size with timber etc that I can set the poly gully in later.
Slab won't be level, it's between a C-shape plan block wall with a fall to the open side. The drain will follow the fall.
I've also apprehension about getting the amount of concrete required perfect when I'm trying to be millimetre perfect by setting the poly drain to a given height. I'm going to need 5m^3 concrete, so it's beyond a mix-on-site job and will be trucked & barrowed in (35m run from the road), and I don't really have anywhere to accept spare concrete, so having things like drains in place that require an accurate finished level of the slab bothers me.

 

[Doggit]

Then drop in a floating timber channel after the slabs been poured; the displacement won't affect the level of the slab much, if anything.