Blockwork Movement Joints & Bed Joint Reinforcement

[Matt60]

In another threat I mentioned dot and dab and some members pointed out that this can cause drafts because (in basic terms) it will not seal any gaps/cracks in the mortar. Suggestions were to use a slurry coat to seal the walls first or simply render/plaster straight to the block work which is my preferred option. I have told the architect & structural engineer that's what I want and the latter has come back with the bold quote below. It seems reasonable what he's saying but obviously his tone makes it clear this could be a hassle. Can anyone with experience tell me if adding these movement joints would be much of an issue? I have copy/pasted his comments below and need to tell them both to continue specifying this or not. I dare say this is just another example of people doing the same thing they have always done because they've always done it that way. I just need a bit of confidence to continue with this wall finish.

 

• We usually recommend dot and dab finishes, especially with houses of this size, due to the tendency for the inner leaf blockwork to suffer from minor thermal shrinkage cracks, which are usually hidden with dot and dab finishes but will show through with wet plaster finishes. If you still wish to use wet finishes, then we may need to introduce additional movement joints or bed joint reinforcement to the inner leaf.  Technical literature recommends movement joints in blockwork at every 6 metres and at 3 metres from corners. Practically though this is often difficult to achieve without having joints everywhere!

Edited March 25, 2021 by Matt60
Spelling

[markc]

Or ... We only employ apprentice plasters and they can make an ok job of dot and dab but unlikely they can get an acceptable finish on a full wet wall.

[Matt60]

16 minutes ago, markc said:

Or ... We only employ apprentice plasters and they can make an ok job of dot and dab but unlikely they can get an acceptable finish on a full wet wall.

 

Probably often right, but in this case that quote is the structural engineer, it will be me who sources all of the trades. 

[joe90]

The only reason I can see cracks appearing is if airated blocks are used (I dislike them a lot) my longest walls are 10m long, no movement joints or bed reinforcement and in the whole house only two minuscule cracks have appeared after 2 years. The only movement joint I have come across was an extension I did years ago and the architect asked for it in case of differential settlement to the new foundations. If your house cracks it’s because the foundations are moving or not built properly. Small cracks are expected as a house dries out, timber shrinks etc. A good plasterer worth his salt will do a good job but so many nowadays can only skim plasterboard. 

[Matt60]

2 minutes ago, joe90 said:

The only reason I can see cracks appearing is if airated blocks are used (I dislike them a lot) my longest walls are 10m long, no movement joints or bed reinforcement and in the whole house only two minuscule cracks have appeared after 2 years. The only movement joint I have come across was an extension I did years ago and the architect asked for it in case of differential settlement to the new foundations. If your house cracks it’s because the foundations are moving or not built properly. Small cracks are expected as a house dries out, timber shrinks etc. A good plasterer worth his salt will do a good job but so many nowadays can only skim plasterboard. 

 

Many thanks.

 

What blocks would you recommend? It might be useful to go back with this as the Structural engineer and architect have to happy in order for me to get my detailed drawings which are being done right now.

[joe90]

Just to add, I employed a decorator as I hate painting ?, he commented that my house had less snagging than any other he had done recently and a lot of those were dot and dab. I am sure if dot and dab is done correctly it’s ok, but I hate it, I like solid walls. If you are sourcing the trades find a plasterer that also does render which is basically what the first coat is ?

[joe90]

1 minute ago, Matt60 said:

 

Many thanks.

 

What blocks would you recommend? It might be useful to go back with this as the Structural engineer and architect have to happy in order for me to get my detailed drawings which are being done right now.

I used 7n concrete, great fir hanging plasma TVs, kitchen cupboards etc (which you will find difficult with dot and dab). I do understand the argument of using aerated blocks fir added insulation but I had a 200mm cavity full filled with rockwall insulation.

[markc]

Fitting kitchens in DOT and Dab rooms is a pain especially in old places with uneven walls behind. Ive ended up cutting a strip out of the D&D plasterboard and installing a batten to fix the top cabinets to.

[Matt60]

2 minutes ago, joe90 said:

I used 7n concrete, great fir hanging plasma TVs, kitchen cupboards etc (which you will find difficult with dot and dab). I do understand the argument of using aerated blocks fir added insulation but I had a 200mm cavity full filled with rockwall insulation.

 

Thanks. I don't want to reduce thermal efficiency as I've already been steered away from my 200mm cavity down to 150mm and so I'm now trying to not lose elsewhere. On that basis, is the joints he's talking about going to give the bricklayer much extra hassle and therefore cost? 

[joe90]

As you have not even got your founds dug yet, have been given duff info about lintels, and your architect is (IMO) giving you duff info why not go back to 200mm cavity and have what YOU want. I hope others here give their opinion but don’t give up on your dreams!!!

[Matt60]

38 minutes ago, joe90 said:

As you have not even got your founds dug yet, have been given duff info about lintels, and your architect is (IMO) giving you duff info why not go back to 200mm cavity and have what YOU want. I hope others here give their opinion but don’t give up on your dreams!!!

 

Cheers Joe, I called the structural engineer, he did come up with a few other reasons why 200mm will be a pain all of which did make sense. Long story short, I will stick at 150mm as we are very late in the day and I don't want to deal with the issues that the extra 50mm brings. I think he and the architect are more or less finished and therefore I had to make instant decision now without paying them twice. 

 

He has changed the lintels though to thermally broken ones that are bonded to polystyrene in the middle. I have also told him I'm sticking with the render and plaster coat and asked him to explore reducing the movement joints and bed reinforcement. My last house was finished on all walls with render/plaster and presumably no movement or bed joint reinforcement. It did suffer with some cracks which I heavily chased out, bonded and patch plastered and they never returned.

 

Thanks again.

[PeterW]

1 hour ago, Matt60 said:

 

Many thanks.

 

What blocks would you recommend? It might be useful to go back with this as the Structural engineer and architect have to happy in order for me to get my detailed drawings which are being done right now.

What is the wall construction ..? Outside to inside ..??

 

Expansion and movement joints are different things - what is your maximum single span wall length ..??

[Mr Punter]

I would recommend 7.3 N medium density recycled aggregate blocks rendered and set finish.

 

The aerated type blocks are horrible to fix to.  If you have plasterboard on dabs the blocks will not be airtight and there will be outside air blowing around just under the plasterboard.  If you visit a house like this on a windy day you can feel the cold air through the electric sockets.

[Matt60]

6 minutes ago, PeterW said:

What is the wall construction ..? Outside to inside ..??

 

Expansion and movement joints are different things - what is your maximum single span wall length ..??

 

Its a mixture of block and brick. The main expanses have a brick bib at the bottom with k-rendered block above, a full fill Dritherm 32 and block on the inside.

 

Apologies if I said expansion, I meant movement joints - I have lots of unrelated issues right now and my brain is fried tbh. The maximum single span wall lenth is 3 meters.

 

Thank you for your input.

[Matt60]

9 minutes ago, Mr Punter said:

I would recommend 7.3 N medium density recycled aggregate blocks rendered and set finish.

 

The aerated type blocks are horrible to fix to.  If you have plasterboard on dabs the blocks will not be airtight and there will be outside air blowing around just under the plasterboard.  If you visit a house like this on a windy day you can feel the cold air through the electric sockets.

 

Thank you, I will suggest that and see what they say.

[Mr Punter]

I am about to start on a project with brick, 150 cavity, med agg blocks and render / set finish.  Using Hi-therm lintels mostly.  Not sure whether to use the Dritherm 32 or graphite polystyrene beads.

[Moonshine]

 

9 minutes ago, Matt60 said:

Its a mixture of block and brick. The main expanses have a brick bib at the bottom with k-rendered block above, a full fill Dritherm 32 and block on the inside.

 

This is pretty much my proposed external wall construction, though i am proposing to use dot and dabbed plasterboard on the inside. I have been toying with doing a parge coat.

• Rendered lightweight blockwork 7.3N (0.45 thermal e.g. Besblock insulite)
• 150mm Dritherm 32
• lightweight blockwork 7.3N (0.45 thermal e.g. Besblock insulite)
• dot and dab plasterboard

U-value of 0.18.

 

Using an aerated block internally (0.18 thermal ) gets a U-value of 0.17, but i didn't want to use the aerated due to the reasons given above.

 

Edit: the construction above with 200mm Dritherm 32 give a u value of 0.14 regardless if use the aerated block or the lightweight blockwork due to the rounding

 

 

 

 

Edited March 25, 2021 by Moonshine

[Matt60]

3 minutes ago, Mr Punter said:

I am about to start on a project with brick, 150 cavity, med agg blocks and render / set finish.  Using Hi-therm lintels mostly.  Not sure whether to use the Dritherm 32 or graphite polystyrene beads.

 

I was told that the Dritherm has better thermal efficiency but that also it more or less comes down to personal preference. I gather that Dritherm 32 is just about the best bang for buck you can get. I have also seen some people say that the pressure of the beads being pumped can cause cracks but the professionals I spoke to about that said that it is nonsense. 

 

What stage are you at with your build?

 

[Matt60]

5 minutes ago, Moonshine said:

 

 

This is pretty much my proposed external wall construction, though i am proposing to use dot and dabbed plasterboard on the inside. I have been toying with doing a parge coat.

• Rendered lightweight blockwork 7.3N (0.45 thermal e.g. Besblock insulite)
• 150mm Dritherm 32
• lightweight blockwork 7.3N (0.45 thermal e.g. Besblock insulite)
• dot and dab plasterboard

U-value of 0.18.

 

Using an aerated block internally (0.18 thermal ) gets a U-value of 0.17, but i didn't want to use the aerated due to the reasons given above.

 

 

 

 

 

Thanks. I'm awaiting the architect to call me back as I think I will change to the same 7.3n block too. I'm sticking to my guns re the render and plaster coat though - no parge coat for me to do, less chance of leaks and easier to fix things to including kitchen units, bathrooms and TV's etc.

[Mr Punter]

I am waiting for Building regs signoff.  Just had pre-commencement planning conditions approved but we need a new application to alter a condition and remove another, which holds up the building regs as we need a relaxation on part M4(2).

 

It is a fiddly infill site and we are building 3 flats.

 

I think the Dritherm 32 and the platinum poly beads are the same thermal resistance.

[Matt60]

2 minutes ago, Mr Punter said:

I am waiting for Building regs signoff.  Just had pre-commencement planning conditions approved but we need a new application to alter a condition and remove another, which holds up the building regs as we need a relaxation on part M4(2).

 

It is a fiddly infill site and we are building 3 flats.

 

I think the Dritherm 32 and the platinum poly beads are the same thermal resistance.

 

Good luck with your project. I guess another reason for Dritherm over beads would be that you don't risk losing insulation if you ever need to open the cavity. I dunno, I'm sticking with that as I can do without giving myself another thing to look in to to be quite honest! I expect the brickies prefer the beads though as they don't have to fit regular insulation boards. 

[PeterW]

7 minutes ago, Matt60 said:

The maximum single span wall lenth is 3 meters.

You don’t need expansion joints on an internal wall that size. 
 

On the external expansion joints what you need to understand your render panels and their maximum length - you normally split them around windows or changes in profile at around 6m or so ( @nod is that correct ..??) 

 

 

[Matt60]

21 minutes ago, PeterW said:

You don’t need expansion joints on an internal wall that size. 

 

1 hour ago, Matt60 said:

Apologies if I said expansion, I meant movement joints - I have lots of unrelated issues right now and my brain is fried tbh. The maximum single span wall lenth is 3 meters.

 

22 minutes ago, PeterW said:

On the external expansion joints what you need to understand your render panels and their maximum length - you normally split them around windows or changes in profile at around 6m or so ( @nod is that correct ..??) 

 

Nothing has been mentioned about expansion joints, just movement joints. That said, for reference, the maximum k-rendered external finish is about 12.5 meters long.

 

Thanks for your help.

[PeterW]

Movement joints are only required where differential movement is expected between structures such as extensions on existing buildings or where substantially different materials are used such as steel and blockwork.  
 

Brick and block have such similar linear expansion coefficient that from -10°C to 30°C it’s not really relevant. The only time it makes a difference is with very dark materials against very light such as dark grey renders. 
 

 

[nod]

1 hour ago, PeterW said:

You don’t need expansion joints on an internal wall that size. 
 

On the external expansion joints what you need to understand your render panels and their maximum length - you normally split them around windows or changes in profile at around 6m or so ( @nod is that correct ..??) 

 

 

Correct