Upstairs extension on a waffle slab?

[Fozzy]

Hi,

We are thinking of creating an upstairs extension but our house is on a waffle slab which someone has told me aren't designed for two story houses. Does anyone know if it is possible to build an upstairs extension on a waffle slab house?

Thanks.

[barney118]

I would suppose on the design of the waffle, Assuming the perimeter is going to be load bearing and you would be using this for you footing, do you have plans to tell how deep x width current footing outside/under walls? is and type of reinforcement. What do you plan on doing? is currently a veneer? clad? going up to be brick or clad? Type of current roof and what are you planning? steel/tile? What can you currently tell..more info needed.

[Bloss]

No more info needed - you need to seek professional advice as it is not a simple yes/no question. The answer at one level is simple - so long as you pay the money and design correctly you can build pretty much what you want. Whether you will think that the cost is a good value is another question! You'll need the existing specs and plans and then someone to talk to you about what you want and they'll be able to guide you as to what is realistically possible.

[Fozzy]

Thanks for the reply guys. I do have the specs for our current slab. It's a waffle pod design with 300 mm wide edge beams, which only look to be about 160 mm deep. The freeboard of the slab is 310 mm. The current house is brick veneer with colourbond roof. If we did an upstairs extension we would use lightweight cladding with a colorbond roof and have the walls inset from the external lower floor walls.

I realise I need to seek professional advice. My main reason for posting was that the builder said you can't build upstairs with a waffle pod slab, but he has since said you can it's just a matter of cost.

I will look into it further but it may be that the extra cost won't be good value. We might look at options for extending on the ground floor instead.

Thanks again.

[barney118]

The issue of going up always seems to be costly but it can allow a lot of scope change to the downstairs plan. There is always the issue of that space lost in your staircase void too. Have a talk to an engineer cost $180 well worth piece of mind. For instant I payed for one to get approval to put up piers on an existing slab saving a lot of heartache.

[barney118]

Sounds like you may be able as the std states the stem width of the slab has to be 300mm for 2 storey which I assume is the base width of the edge beam and this requires extra reo. An engineer will confirm this, it also depends on your soil type ground classification

[Fozzy]

Thanks Barney,

What do you mean by putting up piers on an existing slab?

I spoke to another builder who explained that the design of waffle slabs means that the edge beams are generally the only load bearing part of the slab so if we went up over the existing walls it could possibly be ok, but if we want the upper walls inset then it would need to be engineered. He is coming out and will speak to an engineer on our behalf to see just what is possible and what the likely costs are. If it all looks like it is going to be too expensive we should have enough room to extend out rather than up.

[barney118]

I made a slab to level off an area with a 300mm high edge beam, 100mm slab and the engineer gave the ok to put up piers to support a deck on the slab 3m off the ground.

[Bloss]

The comment from the builder is correct - so long as you are willing to pay then it can be done. It is not all that complex - depending upon the design and layout of the new upper floor and how you plan on taking the loads down to the ground. There are also a number of options - you can create news footings by rein firing and enhancing the exiting slab edges (normally on just two sides) or you might design the supports for the upper floor to be taken out beyond the current slab edge line and be taken down too new piers - the number depends upon the design and size of the upper floor (and maybe the soil too).

For example, I have used relatively simple steel construction using beams to steel posts - effectively creating a new platform independent of the current building. This has the advantage that much of the structural work is down off-site and the cut-in and removal of the editing roof section and replacement with the new space can happen very fast - depending on scale, but my example took a long summers day to have the frame up and metal roof on (not gutters act, but pretty much weatherproof).

But all those cost money and the way it is done will influence the cost. Again, steel framing as described needed crane etc - a more conventional approach of increasing the slab edge size, strength, underpinning etc might take longer, but could cost less with an equivalent result. The design off the existing residence and how you want it to look afterwards also will have an influence. If you are happy to have a 'simple box on top' that will cost less than something the has multiple roof and/or wall components etc.

[Fozzy]

Thanks for your replies guys. Seems we have lots of options. I'm talking to the builder this week so I will let you know what we decide to do.