Calculating slab height using RLs
Page 2 of 3
Just get a good building inspector in to have a look at your slab before you pay for it and you should be fine.
Gee, sounds much like ours...the blind leading the blind! What a mess
We are now stuck with slab that is too high, has no current engineering, contours or house plans that match what is there...currently under investigation with the local council and NSW Fair Trading so I won't comment further....may the force be with us
Don't be paranoid just be alert is all and get the independent inspector if you think there is a problem... before the piers are laid or any time before the slab is poured...once poured it's a whole another mess!
We are now stuck with slab that is too high, has no current engineering, contours or house plans that match what is there...currently under investigation with the local council and NSW Fair Trading so I won't comment further....may the force be with us
Don't be paranoid just be alert is all and get the independent inspector if you think there is a problem... before the piers are laid or any time before the slab is poured...once poured it's a whole another mess!
Actually Lenny171, I'd say get an inspector regardless of whether you think there is a problem or not. You don't know what you don't know.
Aint that the truth!
I just started my own post on a similar issue but now going to jump in here too.
So when a designer designs a slab RL I'm guessing there are two main constraints
1. Flood level
2. Overall building envelope height
So I can understand how a house built too high can impact council height restrictions, but a house too low?
1. Flood or water ingress
So who sets the min freeboard height if your not in a flood prone area?
If your not on a flood prone area but your RL is too low, does this mean that you now have a higher risk of water ingress?
If the answer is yes, will this affect our ability to get house insurance? Or if found upon investigation, possibly void an insurance policy because the house was not built to specification/approved plans?
Remember insurance companies will find anyway to get out of paying for a claim.
So when a designer designs a slab RL I'm guessing there are two main constraints
1. Flood level
2. Overall building envelope height
So I can understand how a house built too high can impact council height restrictions, but a house too low?
1. Flood or water ingress
So who sets the min freeboard height if your not in a flood prone area?
If your not on a flood prone area but your RL is too low, does this mean that you now have a higher risk of water ingress?
If the answer is yes, will this affect our ability to get house insurance? Or if found upon investigation, possibly void an insurance policy because the house was not built to specification/approved plans?
Remember insurance companies will find anyway to get out of paying for a claim.
Part of the problem I see with a lot of plans that get posted on these forums is that very few if any referenced ground levels are placed on the floorplans.
The first thing I do when a client rings me is to ask them whether they have had a contour survey done - if not that is my first phone call.
Next I use that as a reference when drawing the floorplans - keeping in mind maximum cut and fill, maximum height, building envelope etc allowed by the council and when they are dimensioned I make sure I have a ground level shown at all four corners of the house at a minimum plus the floor level of the house. This makes it very easy for the client, the builder, plumbers for their drains, concreters, landscapers etc to see what the finished levels should be - inside and out. Any draftie, architect or building designer not doing this is remiss in their professional competence.
Any builder or any other tradies who can't follow those levels shouldn't have a licence.
Any changes to these heights that they want to make should be run past the client then structural engineer first for their input.
Stewie
The first thing I do when a client rings me is to ask them whether they have had a contour survey done - if not that is my first phone call.
Next I use that as a reference when drawing the floorplans - keeping in mind maximum cut and fill, maximum height, building envelope etc allowed by the council and when they are dimensioned I make sure I have a ground level shown at all four corners of the house at a minimum plus the floor level of the house. This makes it very easy for the client, the builder, plumbers for their drains, concreters, landscapers etc to see what the finished levels should be - inside and out. Any draftie, architect or building designer not doing this is remiss in their professional competence.
Any builder or any other tradies who can't follow those levels shouldn't have a licence.
Any changes to these heights that they want to make should be run past the client then structural engineer first for their input.
Stewie
So if a builder has a contour survey and then has a subsequent house plan drawn reflecting RL for slab and setout reference points, then there should be no excuse to have the slab poured too high or low.
Nope. None whatsoever.
I don't know what other councils require but all our local ones need a contour survey for any DA ( unless it's just for a deck etc )
Stewie
I don't know what other councils require but all our local ones need a contour survey for any DA ( unless it's just for a deck etc )
Stewie
I mean I don't know why we can't just demand it be pulled up and fixed to the drawing specification and why it's always a fight to get the spec changed to the varied design to suit the builder.
Having the slab too low can also affect the site stormwater and sewer drainage.
I cannot understand why some slabs mentioned on H1 have been laid too low when it should have been bleeding obvious that in many cases the drains would then be too low to connect to their LPOD if they were laid with the required fall.
I cannot understand why some slabs mentioned on H1 have been laid too low when it should have been bleeding obvious that in many cases the drains would then be too low to connect to their LPOD if they were laid with the required fall.
Having the slab too low can also affect the site stormwater and sewer drainage.
I cannot understand why some slabs mentioned on H1 have been laid too low when it should have been bleeding obvious that in many cases the drains would then be too low to connect to their LPOD if they were laid with the required fall.
I cannot understand why some slabs mentioned on H1 have been laid too low when it should have been bleeding obvious that in many cases the drains would then be too low to connect to their LPOD if they were laid with the required fall.
Many builders don't source out design work to competent registered architects or architectural draftspersons. Instead they do it in house employing people with insufficient skills (even basic). As you say, even bleeding obvious isn't obvious when they are clueless.
Secondly, many builders don't supervise their concrete contractors, so they do what they like on site. How many times have I been standing next to SS ringing the concreter " has this slab been done?"
Just now I have one of my client's whose slab is significantly lower than FFL, it will create drainage issues.
On many of my pre purchase inspections I come across houses where slabs have been set too low to the ground.
That is why we always recommend a pre slab inspection. Even though I don't take levels I will spot if there is significant variation and make recommendations that levels be verified before pour.
Hi Liliana,
Sounds like quite a shemozzle.
A few notes of interest. I understand that R.L's are only to be used if they refer to AHD levels carried out by a land surveyor. The correct term to use was probably Top of Slab Level or finished floor level and this in turn should relate to some datum designated on the site plan as Bashworth was intimating earlier in the thread.
As to levels and garage wall height on the boundary, the builder is still responsible to make this happen on most occasions, because it is 99% of the time written on the plans by 'drafties' with any sense that... 'The contractor shall verify all levels and heights and dimensions on site prior to commencement of any work'... or words to that effect.
Then you might be able to start to sort out who is to blame. Sounds like more than one person. Hope this helps,
Cheers Leonardo_23
Sounds like quite a shemozzle.
A few notes of interest. I understand that R.L's are only to be used if they refer to AHD levels carried out by a land surveyor. The correct term to use was probably Top of Slab Level or finished floor level and this in turn should relate to some datum designated on the site plan as Bashworth was intimating earlier in the thread.
As to levels and garage wall height on the boundary, the builder is still responsible to make this happen on most occasions, because it is 99% of the time written on the plans by 'drafties' with any sense that... 'The contractor shall verify all levels and heights and dimensions on site prior to commencement of any work'... or words to that effect.
Then you might be able to start to sort out who is to blame. Sounds like more than one person. Hope this helps,
Cheers Leonardo_23
Hi Leonardo_23,
The building designer appears to have worked off the estate's layout plans. Her "datum", if you can call it that, was one corner of the block which may well have changed over time since the house on that side was being built while our house was being designed. Heavy machinery going over wet clay soil - a little bit of clearing up of sand that might take a few centimetres of soil with it - and where's your datum?
The builder told us that the surveyor who did our house setout couldn't work out where the designer had taken her levels from. This would indicate that he knew before he even broke ground that there was a problem and, yes, the plans do say:
The builder and subcontractors shall check and verify all dimensions, setbacks, levels and specifications and all other relevant documentation prior to commencement of any works. Report all discrepancies to this office for clarification.
It would appear that the builder did do this and was told by the designer, as I posted earlier, to lower the whole house and raise the slab in the garage if he wanted to, though she claims he ignored that advice because he thought he could just drop the garage floor a bit not realising that the roof was not going to follow, being in one piece with the house. How is it possible that the builder would not realise that the garage and house roof were in one piece??????
The building designer appears to have worked off the estate's layout plans. Her "datum", if you can call it that, was one corner of the block which may well have changed over time since the house on that side was being built while our house was being designed. Heavy machinery going over wet clay soil - a little bit of clearing up of sand that might take a few centimetres of soil with it - and where's your datum?
The builder told us that the surveyor who did our house setout couldn't work out where the designer had taken her levels from. This would indicate that he knew before he even broke ground that there was a problem and, yes, the plans do say:
The builder and subcontractors shall check and verify all dimensions, setbacks, levels and specifications and all other relevant documentation prior to commencement of any works. Report all discrepancies to this office for clarification.
It would appear that the builder did do this and was told by the designer, as I posted earlier, to lower the whole house and raise the slab in the garage if he wanted to, though she claims he ignored that advice because he thought he could just drop the garage floor a bit not realising that the roof was not going to follow, being in one piece with the house. How is it possible that the builder would not realise that the garage and house roof were in one piece??????
Leonardo_23, as you say even if the RL's are not to AHD they should be referenced to something on site like a Telstra pit on the nature strip, top of a brick retaining wall etc. The builder or anyone else on the site shouldn't be able to arbitrarily choose the floor level because it suits them better.
All my jobs have a contour survey done first and they are always to AHD. All our local councils demand one as part of a DA. They also have to show the floor levels and sill heights of all windows of neighbouring dwellings facing the property in question which is what I use when I am doing the relevant shadow diagrams.
Stewie
All my jobs have a contour survey done first and they are always to AHD. All our local councils demand one as part of a DA. They also have to show the floor levels and sill heights of all windows of neighbouring dwellings facing the property in question which is what I use when I am doing the relevant shadow diagrams.
Stewie
Having the slab too low can also affect the site stormwater and sewer drainage.
I cannot understand why some slabs mentioned on H1 have been laid too low when it should have been bleeding obvious that in many cases the drains would then be too low to connect to their LPOD if they were laid with the required fall.
I cannot understand why some slabs mentioned on H1 have been laid too low when it should have been bleeding obvious that in many cases the drains would then be too low to connect to their LPOD if they were laid with the required fall.
Many builders don't source out design work to competent registered architects or architectural draftspersons. Instead they do it in house employing people with insufficient skills (even basic). As you say, even bleeding obvious isn't obvious when they are clueless.
Secondly, many builders don't supervise their concrete contractors, so they do what they like on site. How many times have I been standing next to SS ringing the concreter " has this slab been done?"
Just now I have one of my client's whose slab is significantly lower than FFL, it will create drainage issues.
On many of my pre purchase inspections I come across houses where slabs have been set too low to the ground.That is why we always recommend a pre slab inspection. Even though I don't take levels I will spot if there is significant variation and make recommendations that levels be verified before pour.
BE, I am going to get you (or your company) to do a prepour inspection on my Simonds Homes build at Wollert, Melbourne. My soil tests are due to be done in the next few weeks and then it is off to the drawing board for slab engineering and site drainage engineering hopefully after that contour level survey being completed. Is it worth consulting you as soon as I have the slab design and drainage design drawings to look over even before you do the prepour inspection? Cheers!
I take it that you have not yet signed building contract. If so I strongly recommend pre contract review where we can go over your drawings and contract. It could mean you go back to the builder with questions or requests for amendments and even re negotiate terms. You can PM when ready
BE, PM sent. Thanks.
This topic answered a lot of my questions on reading the floor levels. Thanks for all the input guys.
Below is extract from
NCC 2011 Volume Two Australian Building Codes Board
3.1.2.3 Surface water drainage
Surface water must be diverted away from Class 1 buildings as follows:
(a) Slab-on-ground — finished ground level adjacent to buildings:
the external finished surface surrounding the slab must be drained to move surface water
away from the building and graded to give a slope of not less than (see Figure 3.1.2.2)—
(i) 25 mm over the first 1 m from the building in low rainfall intensity areas for surfaces
that are reasonably impermeable (such as concrete or clay paving); or
(ii) 50 mm over the first 1 m from the building in any other case.*************
Hi BE,
I have always thought that the slope must be 50 mm over the first metre for all class 1 buildings but when reading the above, this is an urban myth!
What is defined as "low rainfall intensity"? The intensity would obviously also have to pertain to the region's ARI but which one, 1:10, 1:20, 1:50, 1:100 or one of the others that I haven't mentioned?
In Melbourne for example, it says that the low rainfall intensity is to be calculated over a 20 year period for a 5 minute duration of rainfall.
Below is extract from
NCC 2011 Volume Two Australian Building Codes Board
3.1.2.3 Surface water drainage
Surface water must be diverted away from Class 1 buildings as follows:
(a) Slab-on-ground — finished ground level adjacent to buildings:
the external finished surface surrounding the slab must be drained to move surface water
away from the building and graded to give a slope of not less than (see Figure 3.1.2.2)—
(i) 25 mm over the first 1 m from the building in low rainfall intensity areas for surfaces
that are reasonably impermeable (such as concrete or clay paving); or
(ii) 50 mm over the first 1 m from the building in any other case.*************
Hi BE,
I have always thought that the slope must be 50 mm over the first metre for all class 1 buildings but when reading the above, this is an urban myth!
What is defined as "low rainfall intensity"? The intensity would obviously also have to pertain to the region's ARI but which one, 1:10, 1:20, 1:50, 1:100 or one of the others that I haven't mentioned?
In Melbourne for example, it says that the low rainfall intensity is to be calculated over a 20 year period for a 5 minute duration of rainfall.
Close but no cigar. B-E answered my query not long after I posted it.
A "low rainfall intensity" is a region where the 1:20 Average Recurrence Interval (ARI) is 120 mm/hr or less. The one hour figure is based on a 5 minute average rainfall intensity, for example, in a 120 mm/hr rainfall intensity area, total rainfall of 10 mm over a 5 minute period would qualify the storm as a 1:20 ARI.
AS an aside, eaves gutter compliance is based on a 1:20 ARI and box gutter compliance is based on a 1:100 ARI. A 1:100 ARI has a 5 minute intensity that is approximately 50% greater than a 1:20 ARI.
Related
25/09/2023
2
Broker here - legislation says that every true broker must put the clients best interests before theirs so in theory they must offer you the best options for you on their…
7/02/2024
0
Anyone using Bellriver bulder and doing the turnkey projects
27/06/2023
2
I'll look into different shower heads and ask the plumber about some engineering and see what he says. Thanks