Working off plans?

[Russell]

I got to mark out a job off plans for the first time the other day. It was a bit of fun trying to remember Pythagoras from when I did my electrician training and I was chuffed when it worked.
One thing though, how do you work out the ground height?
The datum point if that's the correct term was a door sill and was marked as 100 or 0.100, I can't fully remember. The other heights were , I think, 0.95.

 

[Smiffy]

The datum point will be ffl or finished floor level measured in meters above sea level
So in your example finished floor level is 100 meters above sea level or 0.1m above sea level
All the other numbers are also from sea level so once you know the datum height you can add and subtract to get back to other levels.
Or using an elevation plan of the house work through everything on the elevation and add it up to get your datums
Some things will be marked on the plans such as inspection chamber covers (IC)
So if that's included on the Arial plan and the elevation plan you can use that as a datum to move between things

 

[Left hooker]

Normally ffl finished floor level
Interesting fact the reason plans have what looks like random numbers the levels are marked at is it's the measurement from sea level

 

[Left hooker]

Ffl is in red and other numbers are the topographic levels taken by a survey engineer

 

[GazCro]

Datum point is a fixed point on site a which all other levels can be worked out from not necessarily ffl. Datum could be existing floor level, a road gulley top of a gate post etc its just a fixed point to work from. Once you have datum established you can then work out how far above or below different phases of excavation or construction need to be.

 

[Russell]

Cool thanks
It's odd this one was bang on 100m above sea level.

 

[Quattromike]

But how do you know where sea level is when you're standing in the foundations of a house

 

[T whiting]

It's probably not they will have set the floor level at 100.00 so they don't get negative numbers anywhere there will be a drain cover off site or some other immovable landmark that can be uses to get to where your finished floor level is supposed to be. I like to set the laser exactly 1m above finished floor level it makes all the levels easy to work out off the staff

 

[GazCro]

But how do you know where sea level is when you're standing in the foundations of a house

That's what the datum is for

 

[Smiffy]

But how do you know where sea level is when you're standing in the foundations of a house

Meters above Sea level is marked on the side of churches and other important buildings so can work back from that with a theodolite if you really want to do it yourself or setting out engineers can get out gandalfs stick and do it for you.

 

[craig]

Is it high tide. or low tide?
Most I`ve done have worked off damp course, or existing floor level as a datum.

 

[Left hooker]

Is it high tide. or low tide?
Most I`ve done have worked off damp course, or existing floor level as a datum.

The ground lifts at high tide by as much as 100mm in UK

 

[Left hooker]

It's probably not they will have set the floor level at 100.00 so they don't get negative numbers anywhere there will be a drain cover off site or some other immovable landmark that can be uses to get to where your finished floor level is supposed to be. I like to set the laser exactly 1m above finished floor level it makes all the levels easy to work out off the staff

Always negative once you start digging

 

[bobthebuilder]

ffl ,that can vary ,architect says 100mm insulation ,sap man says 125mm in flloor ,10 mm tiles or 300mm floot tiles ,ffl can vary a lot,i always work out dpc level and go to that

 

[Smiffy]

ffl ,that can vary ,architect says 100mm insulation ,sap man says 125mm in flloor ,10 mm tiles or 300mm floot tiles ,ffl can vary a lot,i always work out dpc level and go to that

I was always taught the is generally excepted thickness for floor coverings for working out from plans and then what evers actually installed is trivial.
Also if you are given plans and the only height available is ffl
No outside levels or anything else to work from then you have no choice but to work back from ffl.
I doesn't matter what ffl is how thick floor finishing is nor insulation and floor screed.
As long as everyone uses the same numbers everything will tally up. Problems will only arise if different factions use measurements for different parts of the build

 

[T whiting]

I've always taken ffl as the top of the screed (top of structural floor) and not allowed anything for carpets/tiles so it should tie in with doc unless there's something funny going on.

A engineer told me it's the average sea level somewhere in Cornwall but I can't remember the details

 

[Quattromike]

The majority of house build foundations I've been involved with round here on good ground is 675 from FFL to top of strip found. What's the norm with everyone else

 

[GazCro]

The majority of house build foundations I've been involved with round here on good ground is 675 from FFL to top of strip found. What's the norm with everyone else

Same here on good ground, 675 is 3 course of block and then conc often find 2 course of block and conc on existing buildings but doesn't allow drains to come out above conc with usual floor constructions now. You can get away with less but you do need to consider frost cover round here 650mm from finished ground level to bottom of conc found. Tbh other than getting to good ground frost cover is most important as far as building control are concerned.

 

[Left hooker]

I've always taken ffl as the top of the screed (top of structural floor) and not allowed anything for carpets/tiles so it should tie in with doc unless there's something funny going on.

A engineer told me it's the average sea level somewhere in Cornwall but I can't remember the details

Yep was till a few years ago done from a shaft on the end of a pier near Penzance

 

[fred]

The majority of house build foundations I've been involved with round here on good ground is 675 from FFL to top of strip found. What's the norm with everyone else

we do 560mm from conc to FFL, insulation buggers things up as it doesn't work brick. our recipe is from conc:

inside skin
block flat ( across the foot)110
block flat bedded up (lengthways)125
beam 150
insulation 100
screed 75

outside skin
block
block or 3 engineers


no need to lean mix fill cav and you have 235 under beams to fall sewer.