Class H Slab Edgebeam depth

Hello All,
This is my first time posting in a forum.

We are about to build our first family home and have been looking around at the major buillders aswell a couple of private builders.
The major builders ALL offer the Waffle Style Slab and the private builders offer the conventional Raft Slab.
We like the raft slab so we decided to get a quote from the builder.
The builder's standard inclusions was a CLASS H SLAB.
(the builder said that most of the major builders offer only CLASS M)

Cut a long story short - we paid a small fee for soil tests and contour reports.
The report says;
SITE CLASSIFICATION: CLASS H
FOOTING SYSTEM: RAFT SLAB
SLAB CLASS: conventional CLASS H TYPE on ground
SITE INVESTIGATION LOG:
0mm - 100mm from Ground level is GREY/BROWN CLAYEY SILT and CLAY FILLING - MODERATE COMPACTION strength / density - MOIST moisture condition
100m - 200mm from Ground level is GREY/BLACK CLAYEY SILT - STIFF strength / density - MOIST moisture condition
200mm plus below Ground level is GREY/ BLACK CLAY PLUS FLOATERS - HIGHLY REACTIVE, AUGER REFUSAL at 0.5mt - STIFF to VERY STIFF strength / density - MOIST moisture condition

Our block has a fall of 200mm across width of the house which about 13mt wide not including garage.
We prefered a couple of steps before entering into the house.
(one additional step or 2 brick courses on top of the standard single step onto a porch then small (lower height) step into the house.)

Here's the rub,
The builder wanted to charge 13K for the upgrade, but only wants to make the trenches 100mm deep.
Is this normal?
One would think that you would have to be a least on STIFF to STIFF/VERY STIFF grade meaning a depth of 200mm?
Can anyone shed some light?
Also there will be no CUT and FILL - is this normal too?

As the standard inclusion is for a CLASS H slab, should we be paying only for the extra concrete and fill that required to bring the slab up an extra 200mm?
If we went back to Standard Height (which covers upto 300mm fall), does this mean we don't have to pay for any site costs?

And yes - ROCK allowance was separate to the site costs.
This was 2.5K to allow for 10m3.

Any comments is really appreciated.
CKR

Hi and welcome

Not an expert in this at all, but I also don't understand where is the "upgrade" here and as you say - should you be paying only for the extra concrete and fill that required to bring the slab up an extra 200mm. If H is standard with them, I can't see what's the 13k upgrade for . I think it's just the cost of the slab (plus the profit margins) - although they may say it's "included", builders usually charge you extra for the slab, be it M or H class. Plus, some builders charge the same for M and H class , and also the size of the footprint makes no difference to the cost with some builders - all of this one finds out by chance, looong after it's too late .

Have you tried project builders? They might say their standard slab is class M, but who knows, they might upgrade you to H for similar money.

BTW, we made some inquiries for our future backyard pergola slab - they said the footings need to go into NGL at least 0.5 m - so how does that compare with footings for the whole house that only need to go into NGL by 0.1 m !!! We are on fill section there, but still ...
(NGL = natural ground level)

You need to go at least 100mm into natural ground. Thats not his requirement it would be the engineers he has no say.
ALso how big is your slab? I had a 335 square meter slab poured for about 27K. Thats when steel was at a high price.

I had an experiment done one. I know someone who had two plans for slab. One waffle and the other was conventional. When i worked it out they actually take about the same amount of concrete. Conventional is more labor intensive. The quotes where about 2-3K apart with Waffle being cheaper.

Hi Lex, B Star
Thanks for the prompt posts.
Your comments made it easier for us to understand the builder.
Yes - it is 100mm into natural ground which means it will be at least 200-300mm below surface level

B Star - the slab is 325m2 and the floor level inside the house will be about 450mm (ave) above street level.
ie. one is 350mm (top of slop) and other is 550mm (bottom of slop) from left to right across the front of the house

I feel more at ease that I have heard similar comments from both of you!
Cheers!

I am boggled at why people opt for a raft style slab over a waffle.

Waffle is superior in every way.
B-Star, never seen a raft and waffle of equal size use an equal amount of concrete, waffle will use significantly less every time.

Hi cclarity I agree no two different slab designs are going to use in your terms "equal amount of concrete". However I was supprised that they used very similar amount. The benifit of waffle is that it is much easy to estimate concrete where as convential slab is dependnant on how it is dug out on the day.

I prefer convential slabs as the edge beams go much deeper into the ground and the bottom of the slab can not be seen. Also waffle slabs are only 85mm on top where as converntial are min 100mm.

Waffle slabs are generally better for the builder as it is much quicker to impliment.

Alot of waffles are specified at 85mm thick true. personally id opt for 100mm, and would make 100mm no matter what the detail specified.

If your worried about the minimum specs for a H class waffle slab make it 100mm and thicken the edge another 100mm or more.
Its still better than a raft slab.



I think...
why dig your slab into the ground?, basically encasing large portions of it inside your foundations, when you can isolate the entire thing above it bearing on piers so your slab floats above the reactive soils of a H class site with a very rigid grid of 110mmX400mm beams at 1200mm centers, it makes much more sense to me. Couple that with it using far less concrete, which scales with the size of the slab... the bigger it is the more concrete you will save as per raft style, its faster, stronger, and easier.

The urban myth of car jacks going through a waffle slab are just that.... a myth.

p.s. I have no engineering experience, no responsibility will be accepted for decisions made based on the above statements

Just to clarify, a waffle slab IS a raft slab. Just with differing beam sizes and locations.

The only real difference is that the polystyrene takes the place of the sand in a conventional raft slab.

Waffles are generally cheaper as they require less excavation, less concrete and less work in general but the downside to this is that they ARE weaker.

having said that they are engineered to be strong enough to support the structure placed on top of them so... your choice.

Incorrect...they are structurally different beasts....see by signature for a link.


That is not the only difference.



Slabs are engineered according to the substrate requirements and loadings imposed...saying one is weaker than the other is perhaps factually correct but only because it doesn't need to be. You don't need such a heavily reinforced slab on a non-reactive soil as what you would on a highly reactive Melbourne western suburbs clay for example. Fundamentally they all need to meet the Australian standard...heavy industrial slabs are a completely different design but that does not make a residential slab inferior - simply fit for purpose.


To the OP, if I understand what you are saying you want to elevate the entire slab about 100 mm...which means deeper edge beams as they need to penetrate into natural surface. There may be an issue with the standard polystyrene former not being able to provide a deeper waffle to cope with the increase in height ...still seems steep $ I'd ask for a breakdown of the additional cost. The piering option suggested by cclarity is worth pursuing in the context of the price breakdown.

mrh20 is technically correct. What you end up with is a concrete raft. Just a different way of achieving it.

This is taken from the Government's Building Technical Manual:
Reactive soil sites can be difficult to build on, but ‘floating’ stiffened concrete raft slabs cope well with these conditions. Some stiffened raft slabs known as waffle raft slabs use void formers at regular intervals, forming closely spaced deep reinforced beams criss-crossing the slab underside.

Sorta... if the site your building on is sand....


Waffle Slab
Cheaper, less excavation, less concrete and less work ...... and engineered to hold your house up....

Conventional Raft Slab
more expensive, more excavation, more Concrete, more work ........and engineered to hold your house up....

Choice seems obvious to me...

or am I missing somthing?
enlighten me please.

In my opinion...no.

I don't like the number of volume changes in a waffle slab, also I am a bit worried about the
chances of acheiving minimum cover in an 86mm top.

Just go 90mm off the highest waffle pod... or even 100mm if it sets your mind at ease.

What concerns you about volume changes?

Volume changings have the greatest chance of cracking.
It is even more important to cure correctly.
So has anyone seen plastic cover over waffle pod slabs?

...or any residential slab...present company excluded

I covered my sog and suspended slab, doesn't everyone.

Spoke to 2 engineers last week on this very topic, both were more confident in the waffle pod design, especially on highly reactive soils.

both prefer 100mm cover as opposed to the 85mm min.

I cant see how the conventional raft slab is relevant in residential slab design anymore.... unless your building on sand, i don't see the point.