Pad and Beam Foundation

[GDAY1122]

Hi everyone, new to this site and appologise if i am duplicating an already existing thread or question.

Looking at extending my three bed detached house, with a 3m by 6m wide, single story extension to the rear of the house.

Architect has sujested a pad and beam foundation as opposed to a traditional foundation, to both reduce cost and time.

Foundation would consist of four 1 x 1 m2 concrete pads with concrete beams placed ontop, with walls built directly off beams.

Has anyone got any thoughts or experience to the pros and more importantly, the cons of this type of foundation?

 

[^woody^]

Normally, non-standard construction is done to get over a technical issue or because of certain ground conditions

I can't see how a pad system - which will incur significant engineer design costs and probably proprietary beams, will cost less than a traditional strip

Also it takes what, 3-4 days to dig and lay a traditional extension foundation, so I don't know what these supposed time savings are

 

[indus]

I had pads and beams done for an outbuilding this year. In terms of cost compared to strip I'm not sure there is much in it. With the former you save on some excavation and skip costs but then you have to factor in the cost of steel.

Not sure if the structural engineering costs will be that high, they will just over spec it as they usually do

Some pics of mine (the pads were very deep due to soil conditions and trees)

 

[^woody^]

It seems that you may have had a need for pad and beam, but just looking at the photos it clear that they are half way there for a normal strip foundation so no real advantage if not actually required

 

[indus]

I think the issue I had Woody is that as well as heavily clay based soil I have got three massive trees within 2 metres of the build.

As a result the other option would have been to dig strip foundations of between 2.5-3m. I think therefore we decided that pads and beams probably represented a faster, easier and perhaps slightly cheaper way to do it.

But as you say that may not apply to the OP

 

[Deluks]

The cost of steel for a concrete beam using rebar cages would be cheaper than using UBs or UCs.
To the OP, what size beam has been suggested? And surely you'd need more than 4 pads for 12m of foundation?

 

[^woody^]

There would be a cost in making up a cage - the steel and the skilled labour to form the cage and maybe shuttering too

Almost anyone can plonk a beam down though

I'm still sceptical as to why pad and beam has been suggested to the OP. Does the designer have an engineer friend who would benefit from the extra work?

 

[TimSmall]

I'm using Pad+beam for a small single storey extension. I can use cheap precast concrete beams (lintels), and it'll save time and materials.

As a bonus, I can wrap polystyrene insulation around the bottom of the beams, and kill the thermal bridge which you normally get between the wall and the ground - making the base of the wall warm.

The walls are a 150mm medium-weight concrete block (single leaf solid wall, no cavity), and will have polystyrene insulation on the outside, rendered over. This is a common building technique in Germany, where it's been used for 30 years or so. That's the plan anyway, I'll let you know how it goes!

 

[Deluks]

Photos please Tim