Do you put polythene in your perimeter footing trenches?

The contractor is giving you and your client a vapour barrier for free. Why are you complaining? (Sure it might have been costed in, but the contract has presumably already been signed.)

Here down in AS land where I also presume you are from vapour barriers seem to be almost standard. We recently started pouring ~2000m2 of half meter deep concrete slab. The consulting engineers designed it without a vapour barrier and provided increased cover. But the concreter still used a vapour barrier.

We are the head contractor. By the time I realised that we were getting a vapour barrier for 'free' it was already rolling. It didn't seem worth it to cause confusion for us or the contractor for ~$300 of materials and maybe $1000 of labour.

All concrete slabs will have DPM under them so this discussion is only for perimeter foundations

Our drawings don't specify DPM in the perimeter trench and the contractor was actually a subbie (classic) to the foundation contractor
So he has no idea what the foundation contractor has priced - he was just there to dig the trench
I doubt the contractor has priced in DPM (as it wasn't specified) so if I changed my tune now it would be a variation charged to the client
As I said, I was 99% sure he was just looking for an excuse to not do his job properly
But I like to be open-minded and get other opinions to make sure I'm not missing something

No polythene in foundations here. Won't do any harm but I also don't think it'd do much good. Polythene against organic top soil is not going to last all that long.

We show the poly all the way across the bottom of slabs and the concrete beams.

I like the poly there because the contractors often let the slabs sit for a time period between setting them up and actually placing the concrete. It might take them a week to get inspections passed before they can pour.

If it rains, the poly keeps the foundation setup in better shape and also keeps a lot of mud off the rebar.

I think it's easier for the concrete to dewater the forms with the poly in place too.

Who does these inspections? Why does it take so long? That is asking for trouble, and the contractor should not be subject to that amount of risk and delay.

Disclaimer: I have limited direct design experience in concrete design both on ground or suspended. So please feel free to disagree or correct my statements. I'm in my early days of concrete design, while considering myself quite experienced in structural steel.

Greenalleycat said:

All concrete slabs will have DPM under them so this discussion is only for perimeter foundations

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I don't see how that makes a difference. Whether it is a trench-beam or slab we are still talking about the same thing. As long as it is structurally adequate then what is the difference?

Greenalleycat said:

Our drawings don't specify DPM in the perimeter trench and the contractor was actually a subbie (classic) to the foundation contractor
So he has no idea what the foundation contractor has priced - he was just there to dig the trench
I doubt the contractor has priced in DPM (as it wasn't specified) so if I changed my tune now it would be a variation charged to the client

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How is that your or the client's problem? If somebody starts putting in extra items not mentioned in the drawing then then can't exactly charge variations.

GeorgeTheCivilEngineer ( said:

No polythene in foundations here. Won't do any harm but I also don't think it'd do much good. Polythene against organic top soil is not going to last all that long.

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Pretty much the norm in most areas here down under. But from my reading far less common in North America. (I sometimes wonder how effective this is over the long term 50+ years vs improved cover on concrete.)

hokie66 said:

Who does these inspections? Why does it take so long? That is asking for trouble, and the contractor should not be subject to that amount of risk and delay.

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Where do you do your work? Are the inspection obligations on the engineer or the contractor. Here in AS land inspections do seem far less common particularly for residential. Engineers put the burden on the client/contractor to notify them and request them for inspections. Essentially washing their hands of obligations if such notifications and inspections don't occur.**

**I could out of line here, as my experiences might differ from others, and I'm still not a veteran engineer. I'd welcome comments from other people practicing under AS codes.

@human909, there is a huge difference. DPM is a moisture barrier that prevents damp rising through the slab and into the house
It is absolutely required for slab-on-grade foundations and you'd be non-compliant/creating a health hazard not to use it for slab construction

For subfloor construction it is not strictly required as there is an air gap between the ground and the floor (minimum 450mm to underside of joist in new construction)
Typical practice is to put DPM down under subfloors anyway so that moisture can't even get into the subfloor space

The question specifically relates to perimeter footing trenches though
There is a slim potential structural benefit as we could reduce side covers from 75mm to 50mm if we used it, but this really isn't important as you just design around it
There is a potential structural downside as you create a slip layer between your soil and foundation that will change that interaction (maybe that's a positive though?)

Really though, my question is: is there any benefit from an amenity/health perspective to putting DPM in your perimeter trench

Thanks Greenalley cat. Your explanation makes sense and adds to gaps in my full understanding.** I have nothing further to add. Though I'll readily listen to the wisdom from yourself or others.

**As I said in my edited post above there are significant gaps in my knowledge and experience on this topic.

Hokie -

This is not related to Green Alley Cat's post...but unfortunately, the slabs regularly sit for a week prior to pour.

The contractor's usually have to get two separate representatives from the City to inspect (Plumbing Inspector & a General Building Inspector). The left hand doesn't talk to the right hand at the city, so good luck coordinating both inspectors being there on the same day.

Also, if it's a small quantity residential project, then the contractors are at the mercy of the concrete suppliers. The suppliers always give priority to the large commercial and highway projects. The residential contractors are given the 'leftovers' in the supplier's schedule, which means they get their concrete late in the week. It's not uncommon for the suppliers to cancel concrete the day before a pour because all their drivers are maxed out on their DOT driving hours for the week.

Throw in a rain day or two, and the schedule really get's messed up. It's a crazy business...

@ Greenalleycat - I would still install DPC around the trench. Similar to why a DPC layer is installed on masonry wall. Without DPC moisture from footing will travel into your bearer up to timber studs. It might not be alot compared to a slab but it may cause moisture issues around the perimeter (i.e. mold, degrading of untreated timber).

EDIT: DPC - is damp proof course. Added youtube video:

All timber has to be separated from concrete with DPC so there will be a malthoid or similar separation under the bottom plate etc