Rendering outdoor aerated blocks

[deedee9]

Hopefully a simple one for those of you in the know.
I have a few small raised beds in the garden made of thermalite aerated breeze blocks (max of 2 high). I am hoping to render them. My question is how eg type of mix - presuming 1 cement to 6 sharp sand with PVA or rendamix in it. Will apply PVA to block 1st too. Presuming I don't need lime as some suggest. Number of coats is another question. It seems easier if I could just skim over the blocks to fill in holes / roughness. Would this be okay or would it just fall off. Otherwise presume 2 coats needed ????
If it matters all walls are curved and to be painted after drying.

Many many thanks, D

 

[TexMex]

This is a tricky one. Personally, I don't think thermalite blocks are suited to garden walls. They are not usually used below the DPC, and your run of the mill garden walls don't have a DPC (as it creates a week point in the wall). Furthermore a DPC in a raised flower bed would be a total waste of time.

The trouble is, thermalite blocks soak up water like it's going out of fasion. Once it gets into the block work, (from the inside), a bit of frost exposure is going to blow the render or even break up the blocks.

I recon about your only chance of success, is to use a really strong mix (such as 3:1, and perhaps add a waterproofing agent. This will slow down the ingress of moisture into the structure of the wall. Even then, I wouldn't give it a long term garauntee, as the use of such a hard render on such a soft substrate seems wrong.

I wouldn't bother with the PVA, instead, I'd just give the walls a good soaking first, and then keep the surface moist by spraying occaisionally as it sets. The extra suction will give you a better bond with the blockwork.

I'd be interested to hear if anyone else has experience of using thermalite blocks in this type of setting. Personally I haven't, but my gut feeling tells me this is going to end in tears.

 

[JerryD]

I would tend to agree, Thermalite would not be my first choice for flower bed retaining walls. You should have used a dense block (solid concrete) or a dense brick.

However, it's too late for that now so how to render? Exactly as Tex says but I would fix expanded metal lathing first (stainless steel, not galv). This needs to be extremely well fixed to the blocks and will act as a layer of reinforcement to hold the render together.

S/steel mesh lathing is quite expensive though

 

[deedee9]

Thanks ever so much for this information.
Hope I can save some of the work. I'm actually a bit behind schedule with the work so all the beds did not get built this week. As such will try to go for the concrete type of blocks recommended above or maybe even brickwork. I already have the thermalite in though. Hence 2 questions :
1) If use thermalite blocks, would a rough waterproofed render on inside help or even polysheeting on inner soil surface. Or would you guess the render would still be weak.
2). If use high density concrete block / bricks as recommended above. What is recommended render mix.

Thanks ever so much for this advice. I really do appreciate your comments. deedee9.

 

[JerryD]

1) If use thermalite blocks, would a rough waterproofed render on inside help or even polysheeting on inner soil surface. Or would you guess the render would still be weak.

Waterproofing the inside will definately help but watch you don't create a pond rather than a flower bed! Rainwater needs to be able to drain out, either through the bottom (preferred method) or through weep holes (not so clever visually) I'd render internally to seal all gaps and poorly filled joints then paint the inside with a waterproof bituminous paint (you must get one suitable for planters - some of these waterproof bituminous paints will kill plants)

2). If use high density concrete block / bricks as recommended above. What is recommended render mix.

You can use a harder render here, about 3:1 with some sharp sand added, two coats (scratch coat then top coat)

 

[BiGDave]

They are not usually used below the DPC

http://www.thermalite.co.uk/content/224.cnt

 

[TexMex]

http://www.thermalite.co.uk/content/224.cnt[/QUOTE]
That link applies to TrenchBlocks. Although made by Thermalite, it is not thier run of the mill product. I'm sure if DeeDee9 had these, he would have specifically mentioned them. Especially when prompted by my suggestion that they are not usually used below DPC.

 

[kevplumb]

i remember when they introduced them (my old man was a brickie )
the first question he shot at the rep was would use them as the footings in your house ?




answer came back



no


says it all

 

[deedee9]

Wish I hadn't start this query off with words to the effect - "Here's an easy one ..."
Thanks for all the above. Thermalite blocks bought were the basis type - Shield sounds familiar (but i will check and advise). Following advice given, I have simply used up the blocks to form a firm (ish) base and used recycled bricks for rest of build. Intention is to leave unpainted.
Had to render a small section of blocks used to raise levels which I attached the reinforced mesh to and have applied one coat of render so far. Will keep you posted how long it holds on for. My fingers & **** are doubly crossed but not holding my breath.

Many many thanks to all, D.