How much concrete and rebar is needed over planks/slabs?

Almost everything that needs to be said about concrete, cement, hand-mixing, uses and prices has been said on this forum. Please check out the contributions before posting new questions.

Moderators: Sometimewoodworker, MGV12, BKKBILL, pattayapope

How much concrete and rebar is needed over planks/slabs?

Postby JungleBiker » Sat Jul 09, 2011 9:23 pm

I have tried searching and found only incomplete information :? so let me ask our resident experts...

I want to make an upper floor of dimensions 4m x 4m. Supposed I use solid concrete slabs (not sure if they should be prestressed or reinforced or what ??) 4m long and 50mm thick. So I want to know

(a) How thick the cover pour should be (50mm or ?)...

(b) What size rebar (9mm or ?) and...

(c) What spacing between the rebars..?

(d) And (while I've got your attention :mrgreen: ) what's a typical thickness for the mortar and ceramic floor tiles that will go top of the above floor?

Thank you gentlemen (and any ladies who may be know the answers and be willing to share them with me)!

Posts: 8
Joined: Fri Oct 21, 2005 6:32 am

Re: How much concrete and rebar is needed over planks/slabs?

Postby geordie » Sun Jul 10, 2011 6:52 am

Hello j b the planks have reinforcing in them already so you are suplementing it 6mm or a bit less
should be ok
most people use a ready made mesh but hand made on site i would guess 150mm for the grid
50mm is the top layer thickness usually followed by 50 mm cement for the tiles
don,t forget to support the planks during the pour or they will dip with the weight

Take a look at building in nontaburi rodger ramjet page 32 ther,s some clear pics
my comments may be wrong but never deliberately
If it aint broke, dont fix it
User avatar
Posts: 3867
Joined: Sun Oct 12, 2008 4:39 am

Re: How much concrete and rebar is needed over planks/slabs?

Postby Roger Ramjet » Sun Jul 10, 2011 7:02 am

I have used over 4 metres of reinforced slabs, which must be supported in at least two places when you first lay them. They should be welded together using scrap 9mm rebar, joined to 9mm rebar spaced 12 inches apart, then joined to trench mesh by wire. That should be supported by chairs, which are simple to make out of left over rebar. The reinforced slabs have 4 inches of concrete over the top of the trench mesh and rebar. The slabs should be wet down before the pour and a bonding liquid used.
The typical thickness of concrete used on top of the slab, once it is poured, varies greatly and depends on the tiler. Most of them slap concrete on because the slab is not flat. They'll run a string line up to 4 inches above the slab, then waste concrete when it shouldn't be wasted by slapping it on. They also never use the correct serrated trowel so that if the tiles do break, the tiles are a mongrel to lift and replace.
There are a complete set of photos and specs on my current build.
User avatar
Roger Ramjet
Posts: 5331
Joined: Tue Apr 27, 2010 12:55 pm

Re: How much concrete and rebar is needed over planks/slabs?

Postby JungleBiker » Sun Jul 10, 2011 8:11 am

Thanks Geordie and Roger for your replies. As you can see, the info differs greatly - Geordie (and elsewhere Jazzman also) says 50mm concrete on top of the planks while Roger is talking double that amount (4 inches = 100mm). (Roger is also talking about double the amount of mortar for the tiles above the slab 100mm versus 50mm). Likewise, Geordie is talking about a layer of mesh (or 6mm rebar instead of that), while Roger is talking mesh PLUS a grid of 9mm rebar. Actually, based on what others have described for their (plankless) floor slabs, I have a feeling that the concrete pour and rebar design that Roger is using would do the job even without the planks underneath. What do others think?

Posts: 8
Joined: Fri Oct 21, 2005 6:32 am

Re: How much concrete and rebar is needed over planks/slabs?

Postby MGV12 » Sun Jul 10, 2011 9:47 am

Hi JB ... yes that gives you some idea of the varying opinions you will get in LOS ... even contractors that have been at the game for many years sometimes vary in their opinion and of course the specific details of the actual build ... spans etc ... will also come into play. Very little of what goes on here has a technical basis ... it's mostly learnt by watching others or what their boss or father or whoever used to do before they started up on their own. The passing on of bad, inadequate or simply misguided practise is commonplace ... hard to blame the person doing it ... it's the system that's at fault. I was taught that if you see something that doesn't look or seem right yourself ask questions or check it out for yourself ... don't just carry on regardless or until the customer complains.

If you are using the reinforced 5cm planks then they are mostly used simply as formwork and a 10cm pour over the top with mesh [usually 20 x 20 grid of 3mm or 4mm on wider spans] is fairly normal. If you tie them together as Roger has done and integrate them into the slab above by also tying them into the mesh [forming an homogeneous slab] then you could reduce the pour thickness to 7.5cm; unless the span is over 4 metres then best to stay with 10cm and use the tying in simply to ensure the pour and the planks remain attached over time.

If you go for Hollow Core slabs then you have some real strength in the slabs themselves [see tech specs in link to a manufacturer supplied below] and can reduce the pour to 5cm without the mesh if using a ready-mix delivery such as CPAC ... again good practise is to tie the slabs together ... they are pre-formed with a keyway at the sides that is filled with mortar/grout to ensure the slabs can't flex in the vertical plane relative to each other.

Fixing tiles ... normal practise here is to bed directly onto mortar [which is also effectively their screed] and this can vary in thickness dependent on the need for a slope to direct water flow in bathrooms and external applications, matching to adjoining spaces and often simply what the tilers are used to. It's very rare and quite pointless to go over 5cm in thickness unless circumstances dictate. On my upper floor I used 10cm Hollow Core slabs which were specified to give all the strength required for the spans involved and so the tiles were bedded directly onto the slabs with a minimum 5cm bed. Standard practise where I am from [UK] would be to finish the pour off with a a cement [no stones] screed and then lay the tiles on a thin adhesive bed .. much better should you ever wish to remove them but does require a level and flat screed. Not a common practise here but it does happen and you will find adhesive readily available in all the big stores and even some smaller ones.

Hope this helps a little more ... ask questions if not.

Reinforced planks 5cm nominal.jpg
Reinforced planks 5cm nominal.jpg (39.42 KiB) Viewed 1136 times

Hollow Core slabs.jpg
Hollow Core slabs ... the spaces between are the keyways before mortar/grout inserted. ... -core-slab

“Some days I am an optimistic pessimist ... other days I am a pessimistic optimist”
User avatar
Posts: 5353
Joined: Wed Jan 23, 2008 4:23 pm
Location: Chiang Mai

Re: How much concrete and rebar is needed over planks/slabs?

Postby JungleBiker » Mon Jul 11, 2011 7:36 pm

Thanks MGV12.

Well it looks like my idea to replace a sub-standard wooden floor with a concrete floor and tiles is scuppered as the thickness (at least 15cm it seems) would require me to cut away too much of the main concrete beams to enable the new floor to be level with some existing floor.

Thanks again everyone.

Posts: 8
Joined: Fri Oct 21, 2005 6:32 am

Re: How much concrete and rebar is needed over planks/slabs?

Postby paragon » Wed Aug 03, 2011 4:10 am

As a contractor myself I couldn't of put it better than the above posters did. If you are replacing a base floor you may not need to reinforce the cement if you have running boards underneath. The safest option is to always go with reinforced concrete in my opinion. I would give it about 15cm either way.
Posts: 5
Joined: Tue Aug 02, 2011 7:39 am

Return to cement, concrete and mortar

Who is online

Users browsing this forum: No registered users and 1 guest