# What blocks (5N, 10N or 20N) for concrete roof slabs?



## NoIdentity (4 Jun 2009)

I am building a house in concrete blocks with a flat concrete roof. The roof slabs are 7m x 1.2m @ 316kg per sq m (2.654 metric tonnes per slab). The roof slabs will sit on the inner leaf of the walls. 

What I would like to know is what Newton strength block should I use? 5, 10 or 20?

I would greatly appreciate your help.

Thanks in advance.

NI


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## NoIdentity (4 Jun 2009)

Meant to say that the walls are quite tall (3m).  I know that will impact the type of block needed.


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## CharlieR (4 Jun 2009)

what is the dif in cost between the 10 and 20n. We have just built a double lift shaft, 15 high for 2 lifts, blocks laid on their bellies out of 10 newton blocks.

If not much diff go for the 20n


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## NoIdentity (5 Jun 2009)

The 20N blocks are twice the price of the 5N blocks.  I want to avoid the additional expense if at all possible.

It was suggested to me to build the inner leaf with 5N blocks on the flat but that would have required twice the blocks (and twice the expense!!). I was also worried about losing floor space inside the house so I decided to build them normally on their sides. 

I am getting conflicting information from engineers and manufacturers as to the block requirements. Engineers are saying that I have to use the 20N blocks, manufacturers are saying the 5N block is sufficient. The roof slab spec sheet lists the Self Wt. = 26.54kN 3.16kPa. The slabs are 7m x 1.2m @ 316kg per sq m (2.654 tonne).  Is there any easy way to calculate the load on the blocks?

Thanks.

NI.


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## NoIdentity (5 Jun 2009)

I just read that the compressive force rating for a concrete block refers to Newtons per sq mm. If I lie a block on its side, the area of the face is 100mm x 440mm = 44,000sq mm. If I use a 5N block (that's 5N per sq mm) is the total compressive force for that block 44,000 sq mm x 5 N = 220 kN? 

Since 10N is roughly equal to 1kg, 220kN = 22,000kg or 22 metric tonnes.

Does this mean that a 5N concrete block on its side can withstand the pressure from 22 tonnes?


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## NoIdentity (5 Jun 2009)

Going back to my original post, the roof slab only weighs 2.654 tonnes. That weight will be spread across 2 walls, so 1.327 tonne per wall. 

The slabs are 1.2 m wide so they will rest across almost 3 blocks so there will only be ~ 400kg resting on each block. 

If the blocks can withstand 22 tonnes why the hell would I use any more than a 5N block?


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## sfag (5 Jun 2009)

OK - from experience ...

a 10 metre tall house with three floors of concrete slabs will require 10N blocks - definately not 20N.

its up to you to choose either 5N or 10N - either will do.


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## NoIdentity (5 Jun 2009)

That's what I'm thinking. I'm not a builder myself. I'm building my house by direct labour so I'm just looking for some reassurance. The 20N blocks are twice the price and I have a very limited budget. But at the same time I don't want to risk my family's safety. I just want to make sure I'm making the right decision.


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## Structural E (13 Aug 2009)

the loaded area of a block on its flat is 215mm x 440mm and not 100 x 440. But really a  wall will buckle before it crushes due to its slenderness. Your real issue here is the shape of your slab and not the strength of the blocks. you need to cast it in sections due to 1 length being far greater than the other and make sure the bigger diameter steel bars are going in the correct direction.
you know a good structural engineer will save you the cost of his fee with a reduction in materials. you should consider one for this job.


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## baldyman27 (13 Aug 2009)

Structural E said:


> *the loaded area of a block on its flat is 215mm x 440mm and not 100 x 440.* But really a wall will buckle before it crushes due to its slenderness. Your real issue here is the shape of your slab and not the strength of the blocks. *you need to cast it in sections* due to 1 length being far greater than the other and make sure the bigger diameter steel bars are going in the correct direction.
> you know a good structural engineer will save you the cost of his fee with a reduction in materials. you should consider one for this job.


 
The OP wants to build a 100mm leaf, not a 215mm leaf.

The OP says that he/she is using precast slabs, not casting the slab in-situ.

OP, presumably the supplier of the precast slabs should be able to supply you with the info you need, they employ (or should) design engineers. Otherwise, there are a few good engineers on here who will hopefully soon answer your questions.


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## wexford dude (14 Aug 2009)

Hi NoIdentity,

I am a civil engineer.The general rule of thumb is that blockwork can be used for up to 4 stories before it becomes price uncompetitive.
I will give you some examples: 4 storey building, all load bearing walls laid on flat - 1st storey = 20N block, 2nd & 3rd storey 10N block, 4th storey 5N block.All floors precast slab & screed.
Two storey on my road: 1st & 2nd storey 5N block on flat.1st floor precast slab & screed - v. good builder.
Two storey in Cavan: 1st & 2nd storey 5N block on edge.1st floor precast slab & screed - self build.I have been in the house and there is no issues and house was 3 years old at the time.
The reality is that there is a lack of consistency generally among structural engineers & Architects.I understand that you have a tight budget but it sounds like you don't plan on getting it signed off.A house without a cert of compliance from a professional with P.I insurance will be questioned as regards it resaleability.If you choose to get it signed off then that professional will advise on his/her requirements for the blockwork in question.
Personally I would go for the 5N block on flat option for strength purposes as once you have solid foundations you will never have an issue with your home - you won't get a silk purse from a pig's ear.


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## onq (14 Aug 2009)

wexford dude said:


> Hi NoIdentity,
> 
> I am a civil engineer.The general rule of thumb is that blockwork can be used for up to 4 stories before it becomes price uncompetitive.
> I will give you some examples: 4 storey building, all load bearing walls laid on flat - 1st storey = 20N block, 2nd & 3rd storey 10N block, 4th storey 5N block.All floors precast slab & screed.
> ...



Couple of comments:

#1

Is there not a requriement for a minimum of 150mm bearing for structural elements in the Building Regulations.

I have never specified anything less than 215mm blocks on the flat for an inner leaf where precast concrete floor elements are concerned. 

#2


A flat roofed private house is an unsual thing in this country.

Does the OP have permission for a flat roofed house?

Usually rural builds expect harmony with adjoining or indigenous architecture, most of which is pitched roof for a reason.

A flat roof, properly weathered, can cost a significant amount.

A timber roof is usually preferred not least because you can use the attic to site water tanks etc, and allow for some family storage.

The OP may think that simply "laying on a few PC planks" will save him the cost of a carpenter, but the finishing work to make a PCRC plank roof watertight and properly insulated is significant.

There is also a significant amount of preparation to the top of any wall to ensure that the load is properly distributed and an engineer should advise on this detail as well as the wall specification and sizing.


FWIW

ONQ.


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## onq (14 Aug 2009)

wexford dude said:


> Hi NoIdentity,
> 
> I am a civil engineer.The general rule of thumb is that blockwork can be used for up to 4 stories before it becomes price uncompetitive.
> I will give you some examples: 4 storey building, all load bearing walls laid on flat - 1st storey = 20N block, 2nd & 3rd storey 10N block, 4th storey 5N block.All floors precast slab & screed.
> ...



I echo your comments and add a few of my own:

#1

I think there is a requriement for a minimum of 150mm bearing for structural elements in the Building Regulations.
I have never specified anything less than 215mm blocks on the flat for an inner leaf where precast concrete floor elements are concerned. 

When the OP says he's builidng a house in block, is he suggesting a single leaf of block or a cavity - what is the proposed method of weathering especially at the eaves?

#2

A flat roofed private house is an unsual thing in this country.
Does the OP have planning permission to build a flat roofed house?
Usually rural building are supposed to be harmony with adjoining or indigenous architecture.
Most of this is pitched roof for reasons of overall cost, ease of installation, maintenance, usefulness and weathering.

A flat roof cannot be used to site water tanks etc, except in an unsightly way or behind screenage which will need permisison. and fails to allow for some dry areas for family storage.
The OP may think that simply "laying on a few PC planks" will save him the cost of a carpenter, but the finishing work to make a PCRC plank roof watertight and properly insulated is significant.
There is also a significant amount of preparation to the top of any wall to ensure that the load is properly distributed and an engineer should advise on this detail as well as the wall specification and sizing.

#3

Many problems arise from inadequate detailing of foundations, the lack of Radon Gas barriers and collector boxes, and the setting of house levels relativel to site levels to cater for Part M access.
It may not be safe to assume that the foundations are sound in this case, not unless a trial hole has been dug and an engineers comments sought.

#4

Many issues arise in relation to Health and Safety compliance, Health and Safety competence of the Direct labour employed, the preparation and updating of the safety file, safe working practices etc.
Self-builders AFAIK are not immune from these legal requirements.

#5

When questions like this are being raised by a self-builder, it may suggest a total lack of input by building professionals in the work.
The OP would be well-advised to take professional advice and not rely on cost as a justification for not so doing.

FWIW

ONQ.


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