# Forum Home Renovation Structural Renovation  Digging out under house.

## Andrew_T

Hi Everyone, 
I have been lurking around here for a while and found alot of useful information but thought I would finally sign up to ask a question of my own. 
At the moment, my partner and I live in a 3br weatherboard house that was built in around 1960's. We are slowly renovating the whole place and getting rid of some dodgy work carried out by the previous owners!  
Our garage is underneath the house, between the piers (as shown in the attachments). Now the problem here is that on average, the height underneath is only 1.82m, not very high and means you have to do a bit of ducking to miss your head! Over the next year or two we have plans of putting a garage down the back of the property and redirecting the driveway which means we have the chance to do something with this garage downstairs - mainly just want another room down there at the moment but just want to focus on dropping the slab enough to get some decent height under there. Also, see the attachment that i drew up - it shows how the land slopes away and after speaking to my old man (surveyor) he thinks we should be able to get *at least* 400mm out of the garage/driveway and still ahve the water run towards the back of the property (hope that makes sense). 
Lastly, no services run under the garage slab. 
Keeping in mind that I would like to do most the works (labour wise etc) and I have access to a bobcat, excavator, jack hammers and a few mates are tradesmen (bricky, concreter, plumber, carpenter) - what is the best way to go about this? 
I am assuming the main cost will be actually underpinning the existing pier/footings? 
I know that the question is quite broad, but I would just like to see peoples opinions/experiences with this sort of work. We don't really want to carry out the work if it means it will cost us 50k+! So any help would be great, or information about who I should talk to would be even better! 
Cheers,
Andrew.

----------


## Moondog55

I think it would be cheaper in the long run to just build the new garage as big as needed, even making it 2 stories high ( with rear access to the lower floor ) would be cheaper than digging out and underpinning.

----------


## Zoe

> I think it would be cheaper in the long run to just build the new garage as big as needed, even making it 2 stories high ( with rear access to the lower floor ) would be cheaper than digging out and underpinning.

  
I think maybe Andrew is talking of lowering the floor of the existing garage, with the view of using this as another room in the future.. am I right? 
We are thinking of extending our home, using the space of an existing carport, and doing a 'sunken room', with another room/s above it- either for use as an actual room, or to use as a storage room/cellar in the future, so it would be very interesting to see the progress of your project! 
Zoe.

----------


## r3nov8or

Andrew_T, it looks like you've got lots of capable mates, but what I think you really need is structural engineer who can inspect your current foundations and design a solution for you. It'll be difficult for the forum to comment with confidence on such an undertaking, and hence maybe why there haven't been many replies so far.

----------


## Andrew_T

Thankyou for the replys. 
Zoe, you are right. We want to convert the space to be able to use it as a room! 
r3nov8or, thanks for your comment. I will have to have a look into getting a structural engineer in to have a good look at the foundations. It's a shame that under the house wasn't just done at 2.4m to begin with! An extra room would just be very handy and would be great for being the 'poker room'  :Biggrin:

----------


## Bloss

If you can dig it out to take it to 2.1 or 2.2 that will make it a usable space and you might get away without underpinning - but need a good site inspection to know that.

----------


## Andrew_T

> If you can dig it out to take it to 2.1 or 2.2 that will make it a usable space and you might get away without underpinning - but need a good site inspection to know that.

  I was thinking of digging down on the outside of the house to see how deep the footings lie but the only option at the moment seems to be getting a structural engineer in to check it out!

----------


## Buzzman

The other issue is whether or not it will need to be Council approved as living space - they normally require a minimum height of 2400mm from underside of ceiling to top of slab. 
This would indicate an excavation of not less than 700mm in total (1850 down to 2400 + new slab + sand blinding underneath). 
Your existing footings are probably 400mm wide and either 300 or 400 deep, starting one or two courses below ground level. Possibly the existing slab sits on the top of those footings, which was normal practice back then. 
So the base of your excavation will probably be between 200 and 300mm below the base of the existing footings at best. 
So, yes you need a structural engineer and you will need to take soil samples and have them compaction tested. As the top surface of your new slab will still be below the base of the existing footings, removal of the foundation inside the garage bay result in a turning moment developoing for the existing single skin walls, which I assume are load-bearing. 
If this turning moment results in movement the walls could potentially collapse. But that's worst case scenario. 
One way to underpin the brick walls would be to remove the existing slab and then hammer out a series of holes in the base of the wall just above the footing. 
Using railway sleepers or similar (maybe RSJs) as bearers, excavat the holes in the brickwork about 700mm into the foundation and place 2.5m bearer in holes. Place sleeper or other suitable stable bearer under opposite end (near opposite garage wall), then using a 20T builder's jack place strain on the undereside of each beam, and pack accordingly. 
A beam every 1.2m is heaps. The cross beams are now supporting the wall and the house, not the footing. Excavate the footing and continue down to a depth of 1.0m below existing floor surface. The bottom 3-400mm is where you place the new replacement footing, then brick up to match up with the remainder of the wall. 
When concrete and mortar hard, remove tension from beams and remove. Repeat for the other side. 
Did this with a terrace house cellar in Sydney 20 years ago. Big job. Good luck.

----------


## phild01

Did the almost exact same thing a couple of years ago.  The house was a 60's w/board and brick piered to about 1.9 at the rear like yours.  However I left the outside corner pier in place to maintain stability with the earth works.  I dug down deep outside each of the outer point load piers below intended floor level and concreted in some hefty sleeper supports angled in towards the bearer with a bit of a birdsmouth.  These were outside the intended slab and angled in a bit.  I braced existing bearers carrying floor load only with two F27 190x35. All piers then strategically removed.  The area worked on was a quadrant of the house so the area remained as fairly well braced by the remaining house structure.  A dingo was used to excavate soil.  The new walls went in using a reinforced cement and grouted blockwork.  This was designed to retain remaining ground as it can shift due to the pressure of the nearby piers over time.  Drainage around the work was achieved using ag pipe, a plastic crate like panel (forget name) with a fabric barrier.  This was to ensure no moisture breached.  One of my temporary supports was in the midst of the concrete pour but it was shored up for later removal.  Also the new pour had trench mesh for the new house footing.
Not a job for the feint hearted at all and I would not encourage you to do it unless great care is exercised, particularly with temp supports and bobcat excavation and you understand everything you are doing.  Use a dingo instead as these are more nimble.  Ensure that the excavator operator is experienced and not likely to bump a temporary support. 
Get some professional help if you are uncertain about anything as this has a lot of potential to injure or kill.

----------


## Moondog55

> I think maybe Andrew is talking of lowering the floor of the existing garage, with the view of using this as another room in the future.. am I right? 
> We are thinking of extending our home, using the space of an existing carport, and doing a 'sunken room', with another room/s above it- either for use as an actual room, or to use as a storage room/cellar in the future, so it would be very interesting to see the progress of your project! 
> Zoe.

  No I really meant that it may well be cheaper to build a new 2 story garage with the extra living room as the second story; than to do all the "afterwards" work under the house. 
Building a habitable room with a toilet as a second floor on a garage is doable and may be cheaper than all the work involved in underpinning and digging down etcetcect

----------

