Foundation systems: Strip vs. Pads

Would appreciate views/advice on the pro's and con's of these two foundation types...

I've looked on the Internet at several articles about building foundations and, whilst there is much of interest, most articles shy away from making definite recommendations so I'd like a builders perspective please.

We've had a design drawn up, by an architectural technologist with structural engineering input, for a small-ish single storey domestic extension to a detached property. The plans specify strip-type foundations. Due to a nearby very deep sewer run, the local water authority (and the structural engineer) have decreed that some of the strip foundations (approx 35-40% of the run) needs to go to a depth of 1.8m to get below the 45 deg sewer load line. I don't dispute this.

Problem is, several builders we've asked to quote have declined to do so stating that 1.8m deep trenches are a real pain and they would prefer to use a pad-type foundation bridged with appropriately spec'd steels encased in concrete. Some builders have been open enough to say that there's currently a lot of building work locally so they can pick and choose jobs - I see their point.

Question is: in these circumstances, is strip or pads the best approach?

PS The soil is most definitely heavy clay, if that makes a difference. Other than that, pretty normal site situation.

I feel for you, I really do.

It's possible that your architect is mistaking you for a mug punter who is ripe for the milking of the £cash.

IMO, it is far better to engage the actual builder of your project first . . . & let them go find the architect.

Never forget that YOU are the customer, YOU are the £money, YOU know what you want.

It's not easy, but all you have to do is find the builder who can give YOU what YOU want at a £cost where you BOTH profit. As the customer, YOU should NOT be worrying yourself over which foundation is best.

A key factor, is it hand dig or is there access for a Digger?

Dork Lard said:

IMO, it is far better to engage the actual builder of your project first . . . & let them go find the architect.

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I'm beginning to see that now. We already have all drawings/approvals etc in place ready to go based on strip foundations. Any changes could delay start and cost more money for new drawings/re-approvals etc (which may not be recouped if I change from strip to pad foundations). Catch 22 situation I think.

micric said:

A key factor, is it hand dig or is there access for a Digger?

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There is access for a digger.

It's interesting to hear that the trade is still as maxed out as it seems to have been for the past few years. I thought the Travis perkins profit warning from yesterday might have been an indicator of things slowing down a bit?

Both are 'correct' solutions but imo if a digger can access just dig the strip footing. The volume of concrete would likely be similar between the two options as the pads would likely be wider than the strip to get the area of load spread required

1.8m foundations should not be a 'problem' maybe you have really hard ground under the clay but then this would mean you would not have to dig down so far.
You'd sill have to go down the depth if pads. Pads will cost more so maybe this why the builder is pushing for them.
I am not sure you would really want to get the builder first then get the drawings done, how would you ever beable to get quotes for the same work if you did it this way? 3 quotes 3 different sets of drawings 3 fees to pay?!

Your builders are wrong, but obviously they would like an easy life.

Whatever the foundation, the requirement is that it goes down to the depth of the drain so that it puts no load on the pipe. So pads still need to go down to 1.8m or whatever. And you probably can't have uneven depth pads on an extension.

If you have poxy lazy builders locally, then consider piles.

Isn't this why groundworks companies exist, because they like doing the muddy 'orrible stuff and get scale from owning all the plant? Maybe I've answered my own question - the builders in question want to keep all the money on their own books.

garyo said:

It's interesting to hear that the trade is still as maxed out as it seems to have been for the past few years.

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Seems so. Just a little way to go before the South East is completely concreted over then I expect a lull.

tomfe said:

Pads will cost more so maybe this why the builder is pushing for them.

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I didn't know that. I guess the cost of all the associated steels (and the extra work involved in treating them) add up.

garyo said:

Isn't this why groundworks companies exist, because they like doing the muddy 'orrible stuff and get scale from owning all the plant?

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I looked into this after a couple of builders (who declined the work) mentioned it. The builder said that if I contracted him to do the work and he then employed a specialist groundworks company, the cost would be 'prohibitive' so he declined to quote.

Another builder thought it may not be a good idea for me personally to employ a groundworks company followed by a builder (my suggestion) because responsibilities could get blurred between the two groups in the event of a problem/dispute. He said it's best to have a single 'principle contractor' to oversee all works. I understand where he's coming from.

tomfe said:

maybe you have really hard ground under the clay but then this would mean you would not have to dig down so far.

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I previously dug a soakaway to around 1.6m depth about 15 metres away from the proposed building site and the wet, claggy texture of the soil didn't noticeably change as I went deeper - just got paler in colour. From things I've read recently about clay soil, it seems that the moisture content remains pretty much constant below a depth of about 1.0 to 1.2 metres so the foundations must go to at least this depth otherwise heave and subsidence could cause problems. But if the UK climate changes as predicted, with longer dry and wet periods, then moisture content variations could occur at even greater depths.

Thanks for all your input - most appreciated. From what's been said, it seems strip foundations are not at all unreasonable in this instance and that pads would be a more costly, though equally viable, alternative.
I'm currently waiting for two builders to get back to me (one has said he'll quote having seen some drawings - he's due a site visit soon), fingers crossed.