Calulating Rebar needed

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Calulating Rebar needed

Postby hkexpat » Thu Dec 14, 2006 11:46 am

If calculating the amount of concrete required is easy, calculating the amount of rebar has been nearly impossible!

I spent ages trying to locate an internet site that gave a good explanation of how much rebar was needed for footings and floors. In the end I gave up and came up with my own method.

Assume the rebar in laid as a matrix with the rebar at 0.25cm centers. So that's a matrix of 0.25 x 0.25.

Find the length and width of the footing or floor to be poured.

Assume that the rebar is constructed as a sandwich i.e. one matrix/grid on top of another. Both matrices held apart by say 0.1m or 0.25m spacers.

Plug in the length and width numbers and out comes the mtrs of rebar needed.

As for size or rebar, 9mm seems popular. This I am sure is overkill for a non-load bearing floor, where probably a 4 or 5mm wire mesh could be used. But is probably correct for load bearing footings and columns. But at less than 100baht/10m length. More rather than less is on the right side of caution and it's cheap.

If anyone has a better method, please feel free to educate me.

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Postby jazzman » Sun Dec 17, 2006 10:38 am

Rebar is not the mystery you presume. I have built my foundations, footings and floor and pillars all within the last 20 days.

Standard footing size: 40 cm deep x 20 cm wide. Vibrated CPAC.
Don't use expensive wood shuttering (formwork) BUILD your form with two rows of the standard Thai concrete 19 x 38 bricks (3.50 baht each.) These will stay in place afterwards and will add even more strength to the footings.
In your footings you use Grade A steel serrated 10 mm diametre rebar, sold in 10 m lengths for 130 baht per peice.
To build the cage you use 30 x 15 cm rectangular steel spacers preformed for this purpose from 5 mm steel. These are sold by the Kilo, at 28 baht / kilo at GlobalHome or 30 - 38 baht from a small local hardware store. 7 pieces of this weigh 1 Kg, and you need one every 20 com. The cage is made by tying the spacers to the rebar with iron wire.

If your floor is to be non load bearing (your footings should already run everywhere where there is gong to be a wall), the free areas between the footings will never be more than about 3.5 x 3.5 metres if you are building to standard, modular dimensions (I push this sometimes to 4 x 4). If the ground is prepared correctly, filled with earth then vibrated with a whacker plate, PVC damp course, 2cm sand, rebar mesh, CPAC concrete (using Portland cement - boon daeng)in that order, then a 6" (10 cm) thick floor will be more than strong enough for domestic use. rember that the Thai builders will lay another mesh of chicken wire and about 2 cm of mortar for your tiles. They do NOT use tile adhesive and a metal comb like we would back home.
I have built floor like this for over 30 years, some will argue that you need 15 cm thickness, but concrete is expensive and a better investment is in the time it takes to prepare the ground properly.

Reinforcing the floor is done with Wire Mesh (waimesch) in a 20x20 ckm matrix which is also sold by the 100 sq.m. roll for 2,000 baht specially for this purpose. When you lay your concrete, be sure to keep giving the wiremesh an upward tug to bring it to the centre of the mix.

PILLARS: are made usually 20 x 20 cm, again using a cage of rebar made the same way as the cage for the footings, the same steel and 15 x 15 cm square spacers, also sold by the kilo.
You will make your pillars using wooden formwork and wooden struts to keep them perfectly upright. You will need to calculate about 1,000 baht's worth of wood per 2.6 metre column.

Do not remove the formwork for 4 or 5 days. During this time you can be spraying your roof steel with red-oxide primer, and cutting it to length.

For the rest, see my posting on CPAC and how to prepare, lay, and cure it.

I 've got pix of all this but unfortunately this forum won't let me upload them for some reason. I can email them to you, they are only .jpg and only about 150k each. The detail is ecellent.
I hope all this helps. Dont' hesitate to ask more.

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Rebar and Floors

Postby hkexpat » Tue Dec 19, 2006 10:13 am


Thanks for the info. I would be interested in seeing the photos, you can email them to me at

I am on the learning curve for rebar and reinforced concrete and found out a bunch of new stuff recently.

I assume your footing width is for one storey dwelling. For a two storey domestic house the rule of thumb for footing width is 3 times the width of the wall it supports. So a 20cm thick block wall, requires a 60cm wide footing, as for depth, 50cm seems to be the number. Locate it up to 1m below ground level. Use 9mm rebar wired as a cage like you suggest or an upturned "U". Since the strength is needed at the base of the footing where the concrete would tend to be in tension

For a load bearing floor (1st floor) for spans up to 4.87m (16') use 1/2" diameter rebar at 10" centres laid as a matrix on the bottom (lower side) of the slab. Use further 1/2" rebar at load bearing edges formed in a 'UA' shape to wrap around the bottom of the slab and top surface. This makes sense as the bottom side of the slab would be in tension and the top in compression. Since concrete's compressive strength far out weighs it's tensile strength, the rebar is where you need it.

Incidentally, 6mm mesh seems to be the right number for a typical domestic floor.

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Postby jazzman » Tue Dec 19, 2006 11:15 am

Hi Expat

You are probably right, and I must admit I have never been overly concerned about the structural dynamics of two storey houses. It was always my intention to find a big enough plot of land (which I did) to be able to spread all my living space over one floor. Also, building my house the Thai & Southern European way which obviates the need for load bearing walls, the 40x20 footings only need to support the lightweight brickwork of the walls and serve as anchors for the concrete floor.

In modern domestic 2-storey homes in Thailand, I have never seen the upper floor made of anything else other than prefabricated pan peun concrete slabs which are used to span up to 4m between the beams. They are laid over concrete beams which have been cast in shuttering (formwork) which is held up by by a scaffolding of a huge number of eucalyptus poles. The beams are an exact replica of the footings on the ground floor, and the same 40x20 X-section. I seem to remember that for these upstairs beams, they use 12mm or even 15mm diametre serrated rebar.

A friend in Pattaya tells me he used pan peun which were 10 cm thickand he laid another (he cant remember exactly) either 7 or 10cm concrete - with wire mesh- on them. Plus a bed or mortar for the tiles.

In countries where wood is good and cheap (Western Europe) it can make sense to build a house with load bearing walls and lay wooden upstairs floors over wooden 4x2 joists, which is in fact what I did in the house restored in Avignon. (Tongue and Groove, class A Norwegian Pine florboards).

If you had, for example, a 14x12 metre house, a grid of 4x4m footings at 60x50 cm will cost about 72,000 in CPAC for the ground floor, not including filling the foundation holes, which depending on the design, could need 1 cu. or more each.
The wood for the formwork for the upstairs beams would cost another 35,000 baht or more. It is rarely possible to reuse wooden formwork, unless one goes to all the trouble of using detaching grease.

I have 18 holes 1,5 m deep, but my builders showed me a trick which saves a lot of very expensive concrete, but which is just as strong - you'll see this in the photos.

The photos will be on their way shortly - there will be several e-mails as my (let this be a warning to anyone considering the same Internet solution) iPstar broadband satellite connection is not stable enough to handle more than one attachment at a time for the 2,000 baht a month which it costs me.

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More Rebar Chit Chat

Postby hkexpat » Tue Dec 19, 2006 4:01 pm


I am used to timber joists that's why I have the concrete / rebar learning curve. I am pretty sure that the 2nd storey floor is a cast in situ slab (at least on the constructions I have taken a look at). So it will be interesting to compare notes once my final drawings and construction is underway. I'll be sure to photograph each stage.

Interestingly and related directly to the footings and blockwork, there is a hell of a lot of weight in the roof. Concrete tiles weigh in at around 50kg sqm, plus the steel, say another 10kg/m2. So with a 500sqm roof that's over 30tons of dead load, add wind loading (no idea how much to add here) etc and that's a lot of weight to keep from crashing down.

On a related issue, how have you managed/managing the services and ductwork that need to be put in place before the floor is poured? One tip provided to me by a guy who had to have is floor kango'd because the builders forgot to put in the pipes was to write up a schedule, check the formwork and ductwork BEFORE the pour and don't pay them until after the pour.

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Postby jazzman » Wed Dec 20, 2006 8:31 am

Any services that required to be under floor were of course laid before pouring the floor. As much underfloor pipework is ossible is avoided of course by following building tradition and placing kitchen sinks, toilets, bathtubs, washbasins, showers etc, up against an outside wall, usually at the rear of the house. You can then have your pipework entering/exiting the kitchen and bathrooms the British way by just going straight through the wall and down/up the outside of the wall.

I already made some comments on this site about how to protect in-ground pipework. Basically I always lay them in the hardcore or earth and never allow them to be encased in the concrete. In my Thai house, because - as you will see from the photos - the footings are above ground, we ran the pipes under the footings to enter/exit the houses. In this house there is an absolute minimum of underfloor pipework. One is the central drain for the kitchen floor, and there are floor drains in the showers, placed in the corners of the bathrooms, close to the outside wall.

Having 'Italian' showers - the term means that the whole bathroom floor is your shower, as again, shower cubicles with their square floor pans is very much localised to the UK, France and Germany only and Australia (and maybe the USA) - it is of course essential to calculate a slight slope in the floor. The slope is imperceptible to anyone in the room. It can be calulated by using a slope level, which can be purchased in most good DIY stores.

I'll send you a pic of the pipework.

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Postby dozer » Fri Jan 05, 2007 6:15 pm

Jazzman; would like to see the pic you mentioned.
Site Admin
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Attn Jazzman

Postby Pluma » Sun Jan 07, 2007 10:05 am

I too would like to see your photos.

This is a very interesting topic to several of us.

Would it be possible to upload your photos to one
of the FREE photo hosting sites, and sharing the URL
with us? is a pretty good site.
or any others would be OK also.

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bathroom plumbing pic

Postby jazzman » Wed Jan 10, 2007 6:27 pm

Here is the pic of the plumbing of the two bathrooms which were built side by side. There is of course no dividing wall yet. Note how all the plumbing is close to the outside wall. If we want to add anything more at a later stage, we'll just plumb straight in/out through the outside wall, the British way. If you can't see the image below, use this link:

bathroom plumbing.jpg
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Rebar Calculation - A formula

Postby hkexpat » Wed Jan 31, 2007 9:52 am

In pursuit of refining my cost estimate and in particular determining the amount of rebar needed in load bearing floors I came across a set of lecture notes on how to calculate rebar. It seems that the big boys buy rebar by the kg and not by the meter. In any case it’s pretty easy to find out how much a ½ inch and ¾ inch dia rebar weigh.

So for whoever is interested here goes.

1, Calculate the area of the slab (lets say 15m by 18m) = 270m2
2, Determine the spacing of the rebar (typically 0.25 centres)
3, Determine the unit are of rebar (0.25 X 0.25) = 0.0625 sqm
4, Determine the unit length of rebar (0.25 + 0.25) = 0.5 lm (linear meters)
5, Determine the kgs of rebar per sqm. Here I assume a ½ rebar (no. 4 bar) which weighs 0.5kg/m
So 0.5lm X 0.5kg = 0.25kg

We now know that there are 0.25kg of rebar per 0.0625 sqm of floor

Thus 0.25/0.0625 = 4kgs per m2

Therefore total rebar weight = 270m2 * 4kgs/m2 = 1,080kgs of rebar.

If you brought this by the meter, this would equate to 2,160 meters of rebar or 216 10 meter lengths.

If this helps anyone then great.

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Postby jazzman » Thu Feb 01, 2007 9:37 pm

Hi Xpat,

It was not not quite clear from your original posting whether your main interest is in fact in the science of the subject, or just trying to work out a cost for your project. If the latter is the case, it's easy, it's sold by diametre and length (prices already mentioned in this thread, but see below) with the exception of the rectangular braces* and there are many on this forum who have that at the tip of their fingers and are ready and willing to help.

I still really feel
that rebar is not the complex equation which you feel it needs to be. Unless one is one of the 'big boys' (and we aren't) who is considering building a flyover (overpass), a multi-storey car park or a hospital, I would suggest that your excellent information is probably of real interest to a structural engineer who is still pursuing his studies. For the purposes of building a family home here (or in Europe), to know the diameters and tensile strengths of reinforcing wire to be used in the different part of house construction is no more difficult than finding out which kind of oil to put in which part of your car whenever it needs changing. The actual composition of the oil or its lubricating qualities is of little interest to the average driver who, like me for example, just wants to get from A to B without the engine or the transmission seizing.

As far as steel is concerned, we just need to know, like the oil, which is the right one for the job. And there is plenty of empirical experience and lots of leaflets which tell us that.

The most expensive
piece of rebar for a normal house is a 10m length of serrated 15mm dia, steel, and will cost about 180 - 220 baht. You could use this in foundations, footings and colums.
For a single storey house, 10mm diametre will do and costs about 130 baht per 10m. Anything requiring a grid is constructed of a 20cm lattice.
The load bearing qualities and the math of upper floors is something that even the most academic of us will leave to a full time professional. The results of trying to work something like this out for ourselves could be catastrophic.

*The rectangular braces which tie the rods together in footings or colums are usually of a thickness a couple of hun less than the main steel and they ARE sold by the kilo. The 6mm ones we use for standard 40 x 20 cm footings and beams are placed every 20cm and are sold between 27 and 35 baht per Kilo. Seven pieces weigh one Kilo.

Most people start their upper floors with pan puen, prestressed reinforced concrete planks over which they then pour another 10 cm layer concrete over another wire mesh. This is how Mr Cruising, who is an architect, constructed the raised ground floor of his single storey home, and how Attila consructed the upper floor of his house. It is also how we constructed the upper 300m2 floor of the large sports and conference centre at Castle Howchow in Kranuan (Khon Kaen). The owner of the local hardware store just told us what we needed and we used it.

I remember translating some technical sales manuals for Lafarge in France some years ago which contained a table where you look at the load you want to support (i.e. the number of kilos per m2 based on the weight of the people + furniture or machines, etc) and just read off the appropriate slabs in the catalogue. It also gave the recomendations for the matrix of beams and their sizes, using either prestressed concrete, or free pour.
If you are going to prestress your own mix, you must use serrated bar, cut threads at the ends and use the special stretching plates and screws which you remove when the concrete has cured in its wooden form .
rebar for footings.jpg
Footings: 40cm deep x 20 cm wide. Portland cement + sharp sand + 1.5 - 2.0 mm graded, washed aggregate.
Rods: Dia. 10mm. serrated. Braces: Dia.6mm, smooth. Tied with iron wire.
How to build a $20,000 / £14,000 house and a $???? MOTEL Updated 21 March 09 - with BOQ and costs
Don't let this happen in YOUR house.
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My love of rebar

Postby hkexpat » Fri Feb 02, 2007 11:34 am

The original reason for my apparent fascination for rebar is to get a reasonable level of accuracy for my BOQ. I am then in a position to knowledgably challenge the contractor should it be necessary. Also if I pay for 10 tonnes of rebar, I want to see 10 tonnes put in to my house and not finding its way into another project.

Originally, I drew a blank when asking questions on the forum about rebar, this seemed to be a ‘balck art’ so I went off and did my own searching. If you are that way inclined, it is a pretty interesting subject.

After my research, I agree it’s pretty straight forward and you can always go and copy your neighbour and even add a bit extra for good measure. (I’d wager this is how many of these buildings are built.

On the other hand, for those that want to delve a little deeper into the ‘black art’ then I thought I’d share my findings, I figured it would be a useful check for those inclined.

Correction to my post –

The figure I gave for #4 rebar weight was in fact for #3. #4 (4/8ths of an inch) or 12.5mm weighs in at 0.988kg/m.

HKexpat – Rebar enthusiast

P.S. membership to my newly formed rebar enthusiasts club is free and comes with a free sample of #4 rebar and a complimentary copy of Rebar Weekly!
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Postby jazzman » Fri Feb 02, 2007 1:01 pm

Consider Jazzman your first full subscribed member 8)

BTW: I would hate to think of you needing ten tonnes of the stuff - that would cost you almost the entire BOQ for my house :shock:

I promise I will make my BOQ asvailable very soon. We are just putting the finishing touches on the house then it's all over bar the shouting.
The XL will show what I estimated (and this was often up to 10% under or over); what I was quoted; and what I actually paid and where.
How to build a $20,000 / £14,000 house and a $???? MOTEL Updated 21 March 09 - with BOQ and costs
Don't let this happen in YOUR house.
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Online Slab Calculator

Postby atlas_shrugged » Tue Feb 06, 2007 1:59 am

For my project, I found this nifty slab calculator:

It does calculations for suspended slabs, but that just gives you a nice margin of safety for a ground slab. Cracked slabs are nasty.

US code for live load is 40 pounds/square foot, or 195 kg/square meter. Don't forget to add about 100 kg/square meter for tile & grout.



P.S. My construction photos are online at: ... struction/
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Postby prufrock » Fri Jun 01, 2007 3:43 pm

'I assume your footing width is for one story dwelling. For a two story domestic house the rule of thumb for footing width is 3 times the width of the wall it supports.'

The above quote is from someone who stated he's on a learning curve. To each his own.
In my experience the footing is as thick as the column is wide, and forms a square each side being twice the column's width. Thus a 90cm sq column is supported by a footing which is 90cm thick and 180cm x 180 cm square.

We have all seen the cracked walls on most Thai buildings. This is not simply due to poor rendering and poor quality cement it is essentially because the footings which are usually 50 cm thick by 1.5m to 2m square are actually curling upwards like wings in the soil below and sinking.

To support my view I offer you this site for your perusal: The 'New Total Masonry Home 2-Story with Concrete Block Extending From Footing To Roof' which utilizes - 1 foot thick times 2 foot x 2 foot wide footings:

Total Masonry Home 2-Story w Concrete Block Extending From  Footing To Roof.gif
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