Building Standards; Getting It Right!
Mar 21, 2014 10:35 pm
From what I can see most of the volume builders are doing waffle pod slabs around vic. We are building currently, and have a waffle pod slab too. The advice that we are being given to avoid slab heave and cracking later is for a paved area right around the slab that drops by 5cm over the first meter (ie is angled away from the house) to stop rain ponding at the slab.
Some thoughts about this :
1. I have heard that to avoid slab heave, it's most important to have a consistent type of surface right around the slab, ie not go with paving at front/side with lawn at back.
2. I have never seen anyone follow this advice about the path right around the whole house to the letter, but that's one of my main reasons for this post, has anyone actually done it, can you share photos?
3. These days, many designs are based on a garage that sits right on the property boundary, and therefore part of the slab actually borders the neighbours property, so you have no control over what type of surface is present, how do you factor that in?
4. For people that considered this as part of their builds - What was your reasoning for the choice you made in the end?
Re: Protecting your Slab2
Mar 22, 2014 7:00 am
The main reason for protecting the slab as you describe is if the ground is 'Reactive' or 'Desicated' Clay(http://www.anewhouse.com.au/2012/07/bui ... lay-soils/). This can shrink when it drys and swell when it gets wet.
I have never built on reactive clay and have had one house with paths on one side only and the other with virtually no paths around without any problems. ( however being a drainage engineer I did take care in the landscaping to avoid water ponding near the house)
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Re: Protecting your Slab4
Mar 26, 2014 11:29 am
We did it right around the house, except on the garage wall as it forms a boundary.
We needed a path anyway.
Our block is small so on one side we could only have the path 500mm wide but on the other sides it's at least 1 metre.
What else would you put next to your house anyway? You can't put a garden unless it's below the level of the slab rebate and mulch then becomes a termite risk.
Our front 'skirt paving' is exposed aggregate to match the driveway, the rest is plain coloured concrete. The concreters put re-bar into holes in the house slab to stop the skirt paving from floating up and down.
Our soil type was H1.
Re: Protecting your Slab5
Mar 26, 2014 11:42 am
I'm not an expert but wouldn't tying the path/paving to the house slab have the potential to cause damage? like if the path does float up and down it could pull the house slab with it and cause cracking?
I thought it would be better to let the paving move and have a mastic joint between the house slab and paving/path?
Re: Protecting your Slab6
Mar 31, 2014 10:23 pm
I'm in Adelaide where there is a lot of reactive clay soil. I have built three times and on each occasion the Engineer stated that a concrete or paved path had to be around the perimeter.
As the second post says, soil shrinks when it gets dry and swells when it is wet. Depending on how reactive your soil is, this can cause the house to settle (go down) when dry and heave (go up) when wet. The idea of the perimeter path is to keep the rainwater away keeping the soil moisture content more stable.
If you are building on a boundary, you can't factor this in. Or possibly the engineer should when designing the depth of your footings. I would think they would need to be deeper if building on a boundary so that they are more stable. Who knows, your neighbour may put a garden bed up against your garage wall which could mean a lot of watering.
If you are on reactive clay soil, i would follow the engineers advice to the letter.
Re: Protecting your Slab8
Apr 01, 2014 11:06 am
I'm no engineer, but I tend to agree with you that path should not be attached to the slab, albeit up against it.
Let the path settle and heave, and if it is preventing water from getting under the main slab, that's sort of what you want isn't it ?
Re: Protecting your Slab9
Apr 03, 2014 3:20 pm
The following is a pretty landmark case related to waffle pod slabs:
http://www.theage.com.au/victoria/29000 ... 35wdo.html
Re: Protecting your Slab10
Apr 03, 2014 3:20 pm
Re: Protecting your Slab11
Apr 03, 2014 3:22 pm
Obviously there is a clear distinction between houses that are engineered and built well vs those with defects, and the case above is quite specific. I am just looking into paving options that may minimise slab heave and other slab problems
Re: Protecting your Slab13
Apr 03, 2014 4:26 pm
I know from my own experience in the building industry that any slab we put down was well engineered. The structural engineers we used were old school and had no hesitation about calling up piering and/or strengthened edge beams where necessary to mitigate any foreseeable problems.
No problems with any of the jobs we did even though we were often accused of over-engineering our builds unlike some other builders we heard about that had massive problems with the above.
Too often I've gone past new project home sites seeing the slabs being prepared and they always look under designed or to the bare minimum.
Get the foundations strong, level and square and the rest is easy.
Re: Protecting your Slab14
Apr 04, 2014 2:47 pm
To clarify, our skirt paving is only tied to the house slab where it abuts the patio so that if the paving moves it does not create a trip hazard.
It's also tied to the existing driveway paving that the builder put in with the house.
Re: Protecting your Slab15
Apr 08, 2014 10:38 pm
Interesting case with Metricon. I googled a bit further and it turns out that the owner brought to their attention the slab wasnt right after it was poured. Metricon went on with the build anyway.
http://www.propertyobserver.com.au/find ... build.html
Re: Protecting your Slab16
Apr 09, 2014 8:49 am
We had a concreter come and do a quote for us on the weekend and he said he is required to put expansion joints between the house and the concrete and around the downpipes. On the south side of the house, where there is only 1200mm space to the fence, he has to put in three drainage points to get rid of water. However he did also say that the paving alone won't protect our slab as the ground level slopes slightly towards the house - so he will put in agricultural pipes around the perimeter of the paving.
The other thing I noticed when some of our previous paving was ripped up because it wasn't done properly is that the rio was under the paving!!!
So, unfortunately, it isn't just a case of paving around your house and being safe - it depends very much on how it is done. As I said on another post the concreters who did our paving have been in the business for 20 years and the job was very badly done. So you need to question them very closely on how they do their paving and don't pay unless they do a good job.
Re: Protecting your Slab17
Apr 09, 2014 11:28 am
While we are talking about this sort of thing. What is the minimum finished height below damp proof course a concrete path should be? I've searched all over and found about 3 or 4 different answers. 25mm, 75mm, 150mm? Is there an actual standard for this that concreters should know?
Re: Protecting your Slab18
Apr 09, 2014 11:36 am
From the HIA document linked here:
http://vic.hia.com.au/documents/vic_110 ... 0slabs.pdf
BCA states 75mm between damp proof course and paving. You can only leave 50mm between the two if the area is protected from the weather.
Proceed on the assumption that your concreter knows nothing and that you have to tell them. That way you will be safe!
And don't forget the 1:50 fall away from the house. Put a spirit level on it yourself to make sure there is a fall away from the house.
And make sure they lift the rio up and don't concrete with the rio on the bottom.
In other words, be there and take photos as they are working.
Re: Protecting your Slab19
Apr 09, 2014 12:02 pm
Reo should always be supported by bar chairs despite what the concretors say. Too often as you've found Liliana they "forget" to lift it or walk over so often in the course of pouring the slabs that it gets pushed back down to the bottom resulting in a weaker slab.
Re: Protecting your Slab20
Apr 09, 2014 12:42 pm
And one last lesson I have learnt - locate your overflow relief gully and make sure the concreters don't raise it higher than 150mm below your lowest waste outlet (usually shower drain).
And if they say they have never heard of these rules or all of this is not necessary, don't listen. Better still, don't hire them.
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