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Posted: Mon Oct 03, 2011 7:53 pm
by glenne_n
Hi There,
I'm building a 9" hollow block retaining wall/raised planter and planned to install a polymer DPC on the first course of blocks as advised as good practice on this website. I'm a bit concerned as to how much the DPC will decrease the strength of the joint [if at all] inasmuch as you will now not have a solid mortar bond bewtween the two brick courses. Using engineering bricks is not an option at this stage. Would mortar mixed with waterproofer suffice or is there another alternative.
The wall is three 9" block tall topped with 600x300 copings and I plan to render and/or paint the front face. The rear of the wall will have the recommended, impermeable membrane, gravel backfill, weepholes and geo-textile.
I also planned to fill in the blocks with concrete and sink a few re-bars.
Posted: Mon Oct 03, 2011 8:03 pm
by DNgroundworks
I cant see the point of a dpc in a retaining wall?! I maybe wrong though. My thoughts are that the wall will be damp from the earth behind it anyway so a dpc therefore would be pointless and on a low wall a dpc would/could decrease strength.
Just my thoughts.
Posted: Mon Oct 03, 2011 8:10 pm
by DNgroundworks
LLL will be along shortly, hes our resident bricky/wall builder 
Posted: Mon Oct 03, 2011 8:13 pm
by glenne_n
It was a recommendation from a written article on this site. The rear of the wall will be protected[hopefully] by a membrane to PIFA 1200 standard. I'm planning to render and/or paint the wall.
Posted: Mon Oct 03, 2011 8:58 pm
by Carberry
DNgroundworks wrote:I cant see the point of a dpc in a retaining wall?! I maybe wrong though. My thoughts are that the wall will be damp from the earth behind it anyway so a dpc therefore would be pointless and on a low wall a dpc would/could decrease strength.
Just my thoughts.
Im a shite bricky but my understanding is that you stick some visquqeen or something behind the wall, some gravel behind that and a membrane behind that with some weep holes so clean water filters out without wall staying damp.
Posted: Mon Oct 03, 2011 9:05 pm
by DNgroundworks
I am to a shite bricky! Ive done two retaining walls of a decent size in my time, the first was 250mm shuttered concrete jobby, no visqueen just gravel back fill plus land drain. The second was 150mm block laid flat making the wall 450mm thick plus the stone facing, again no visqueen just the drainage? I constructed them as instructed, they are still there now, no probs?
hmm, im thinking now that the visqueen is definatley worthwhile, never really thought about it! Ive never heard of a dpc being used in an external free standing wall though.
Posted: Mon Oct 03, 2011 9:15 pm
by lutonlagerlout
shite slab layer here now :;):
I have never put a dpc in a retaining wall and have no intention of starting
there is no point
what is important is the drainage and the weepholes
retaining walls fail because of water building up at the back of them
also its better if your rebar is set into the concrete foundation while it is still plastic(wet)
cheers LLL
Posted: Mon Oct 03, 2011 10:28 pm
by Carberry
I know the theory, I'm just really slow. Got 10m worth of wall to build this week. A wee girl walked past, pointed excitedly and asked about the castle I was building 
Only 3 courses high of dense concrete block, 5 courses for the pillars.
Posted: Tue Oct 04, 2011 1:03 am
by glenne_n
O.K guys, got the picture, I only thought about the DPC as I planned to render and paint the wall. The original question, however, is, would the polymer DPC be a weak point and would a layer of mortar with waterproofer suffice.
Another question is when I install the weepholes, what is best to use as a seal where the rear membrane is punctured.
The link on this site about putting a DPC in a retaining wall is at:
//www.pavingexpert.com/featur03.htm
Posted: Tue Oct 04, 2011 1:22 am
by local patios and driveway
The reason a dpc is pointless is that water can enter the wall from the top and could the sit on top of the damp course, making a trap for dampness. No dpc means water can capillarate downwards as well as up, giving it a better chance of drying out as it naturally would.
Posted: Tue Oct 04, 2011 7:07 am
by mike builder/landscaper
when i build raised planters, it is normally brick but my method is this. assuming your coping stone is 300mm wide. Face brickwork and blocks or enginering bricks will be 250mm wide fill the void on the up,tie wires every 3 course 18inch apart. 40mm plastic pipe in first course for drainage 18inch apart.visqueen to go in behind the wall and over the course of eng bricks or block and coping stone holds the visqueen in place. you need the visqueen in to stop damp coming through to the face brick or render to stop it blowing in the frosts. one point to note if your useing thick coping i would put some screws in and wire between the screws so the mortar has more to grab to.
Posted: Tue Oct 04, 2011 7:16 am
by seanandruby
i'd never put visquin behind, if anything i'd just black jack it. water just sits between visquin and wall keeping it damp.
Posted: Tue Oct 04, 2011 7:51 am
by glenne_n
Thanks chaps, all interesting stuff and plenty to go on, I'm just an occasional humble DIYer [I normally let the pro's get on with it as long as the price is right] so can somebody explain what is "black jack"
Posted: Tue Oct 04, 2011 3:31 pm
by DNgroundworks
I thinks its a black bituminous coating..........i think.
Posted: Tue Oct 04, 2011 4:17 pm
by glenne_n
O.K the "black jacking" sounds easy enough is this an acceptable way of doing it, a work colleague is muttering something about it not allowing the wall to breathe. If black jacking is acceptable any harm in the 1200 PIFA membrane as well. I already have it and won't be using at again as I never want to see another wall for the rest of my days.