Advice on concrete blocks

[Kurtville]

I am doing a little project to replace an existing greenhouse with diy wooden one and am planning to build it on top of a low (900mm) dwarf wall in concrete blocks. Currently thinking of facing the blocks with either charred timber or cement render. Thought it would be straight forward to choose the blocks but looking on line there are 3.6n and 7.3n and low density/ dense / medium dense/ lightweight and they give very little information on usage and no indication of weight of blocks, even on the light weight ones!

Can anyone shed any light on what would be a suitable option?

[pghstochaj]

Kurtville said:

I am doing a little project to replace an existing greenhouse with diy wooden one and am planning to build it on top of a low (900mm) dwarf wall in concrete blocks. Currently thinking of facing the blocks with either charred timber or cement render. Thought it would be straight forward to choose the blocks but looking on line there are 3.6n and 7.3n and low density/ dense / medium dense/ lightweight and they give very little information on usage and no indication of weight of blocks, even on the light weight ones!

Can anyone shed any light on what would be a suitable option?

For that, it doesn't really matter. I would just use these if they are being covered:

https://www.tradesuperstoreonline.co.uk/products/8...

I wouldn't use lightweight/aircrete blocks for this/outside as they have some specific downsides, although they are easier to handle and cut.

[Lotobear]

a 3.5N block will be ideal - another reasons to avoid lightweight/aerated blocks is they don't like render

[wolfracesonic]

Try and make a decision between render or timber clad beforehand. If you’re going to to clad it and will be needing to fix battens to the wall, go for 7n concretes, they’re denser and you’ll get a more secure fixing; or if you’re rendering, choose the 3.6n breeze blocks, they make a better substrate for rendering than 7n concretes in my experience. In either case try and incorporate a dpc, either a polythene one or a couple of course of blue engineering bricks. Avoid thermalite type blocks, rubbish for rendering as Loto said and they’re really soft of you’re leaving them exposed internally.

[Kurtville]

Thanks guys. This is all new to me so was hoping to use part of the wall facing the fence to have a go at the rendering to see how I faired before going to timber, if necessary, but take your point. I had thought of probably going for the 7.3n as there doesn’t generally seem to be much difference in price. Given the comments re one being better for rendering I will have to have a think on that.

As it is only for a greenhouse I wasn’t planning on putting a dpc in, just putting some in under the cill. Though I suppose if I might use timber facing then a dpc is probably a good idea.

[emicen]

On the weight front 7N blocks are about 18-20kg

Typically merchants will sell you packs or half packs, we’ve been getting 88 to a pack to give you an idea of numbers.

[RATATTAK]

Just FYI 7N refers to the strength of the block rather than the weight. A 7N block has a crushing strength of 7 newtons per square mm (N/mm2). For the use you're proposing it doesn't really matter though, just go for the one's that are cheapest/easiest to get ... as already said, a typical 100mm concrete block will weigh between 16 and 19 kg.

I'd go for timber cladding ... it's not easy to render blockwork that close to the ground. Include a DPC (plastic) about six inches above ground level, and paint the blockwork below the DPC. Fix the timber cladding on treated timber grounds.

[LooneyTunes]

emicen said:

On the weight front 7N blocks are about 18-20kg

Typically merchants will sell you packs or half packs, we’ve been getting 88 to a pack to give you an idea of numbers.

Are you not buying in full loads (direct from the plant) for your build? The discounts can be quite sizeable vs the same quantity via the merchants yard. Some merchants will sell singles as well as larger quantities.

[emicen]

LooneyTunes said:

emicen said:

On the weight front 7N blocks are about 18-20kg

Typically merchants will sell you packs or half packs, we’ve been getting 88 to a pack to give you an idea of numbers.

Are you not buying in full loads (direct from the plant) for your build? The discounts can be quite sizeable vs the same quantity via the merchants yard. Some merchants will sell singles as well as larger quantities.

We talked about it but artic access really isn’t great and because the stone masons were dragging their feet we didn’t have as much lay down area as we would have needed to take an artic at a time.

Think we wound up at 54 pallets in the end and I’m confident there’s more needed to finish

[Kurtville]

RATATTAK said:

Just FYI 7N refers to the strength of the block rather than the weight. A 7N block has a crushing strength of 7 newtons per square mm (N/mm2). .

Yes. I understood that but still thought that the stronger blocks would be more expensive.

[Pheo]

Only other factor is the 7n blocks weigh a tonne in comparison, very noticable!