Long shot. Any structural engineers around?

[InTheRoseGarden]

Very long story short, we're having a loft conversion. We're using a loft conversion company who asked us to get a structural engineer's report on the party wall before going ahead. After saying it would be completely fine for weeks, the engineer said the calculations hadn't come out as well as he expected and we'd have to underpin the wall. Everything I've read says it is extremely unusual to need to underpin for a loft conversion and we wonder whether the engineer is being over-cautious. Does anyone know anything about this stuff? Questions coming up in next post...

He has given the ground an allowable bearing pressure of 50 kN/m2. He said the ground under the foundations is a bit soft and it is just coming into more solid gravelly ground. I've tried to google it to check and I think it might be a very low figure but I'm not sure. Any idea?

Our foundations are 490mm which he uses to give an allowable line load of 24.5 kN/m2. Presumably if the weight of the bricks, roof and floors is less than that figure we shouldn't need to underpin?

I realised he had made a couple of mistakes in thinking we have a suspended floor on the ground floor (it is a concrete slab) and that the 1st floor joists hang off this wall (they run the other way). If I just delete those numbers the number comes in under the 24.5. Would that be how it works? He is looking at the figures again but I'm losing trust at the moment.

Thank you to any clever sod who can make sense of any of this!

It is a bit unusual to underpin for a loft conversion. 50kN/m^2 is a pretty low pressure so the ground must be poor, and 490mm is narrow too which is probably why he is concerned.

Ultimately engineers have to be conservative because if something does go wrong they get the blame. Ground conditions especially are always a bit of an unknown. If he's rechecking the numbers then hopefully he comes back with more what you want. (Might sound a bit odd but in some ways I'd be happier if the wall was supporting the floors - it means that the proportional increase is less and your foundations have been proven for a higher load. On the other hand unless there were signs of previous foundation movement then I probably wouldn't be too concerned.)

It would be very unusual to underpin for a loft conversion. Can you check if your engineer is a chartered structural engineer. I've had a recent case where an "engineer" was actually a technican and he had oversized some steel beams by a factor of 4 (when you compared weight of steel). I would be concerned that he didn't check the span direction of the floor joists and of the ground floor slab and made the wrong assumptions. Did he check the existing load and compare it with the proposed load and provide a percentage increase. It should be pretty low if he has assumed everything spans onto the wall? If there really is an issue there may also be options to rejig the loft structural layout to spread the additional load onto other walls which might add some cost but would be significantly cheaper and less hassle that underpinning.

If you now know he has made some incorrect assumptions, can you not ask him to revisit his calculations?

For an allowable bearing pressure of 50 your house would be sitting on ground as soft as butter. I would check 1. Where has he got soil information from and 2. Have the foundation calcs been checked. If the ground was really that soft the house would likely have subsided already. its possible but not common.

Thank you so much for the replies.

notme
That's what my googling suggested. We dug a trial pit to expose the foundations (is that what you mean by where he got his soil information from?). In the report he says:

• "the base of the hole was sufficient to support heel pressure"
• "the material contained a degree of organic matter and can be considered to be in a topsoil transition medium into the underlaying material"

Does that sound like 50 kN/m2 ground? The house is about 115 years old and we've had no subsidence. The whole thing was kicked off because our neighbour has a crack on their side of the party wall where they knocked out a wall (and put in a steel) but the engineer says that wasn't factored into the calculations and that the crack was probably because they'd done a poor job of taking out the wall.

When you say "have the foundation calcs been checked" do you mean by him, as in, has he checked his sums basically?

fudge you've just missed it in my original posts, he is redoing the calculations. He has said he doesn't think it will make a difference which baffles me because it's removing all/almost all of the weight of 3 floors.

ladypie he is a "Chartered Civil Engineer (MICE)" and is a Director at the company (they have a couple of offices in the country). I was surprised he screwed up about the floors too. He also forgot something quite crucial earlier on in the process too. He takes ages to do anything and I suspect ours is one of his smallest jobs.

Yes on the % increase. His original calculations say it is a 9.55% increase. I think if we ran the loft conversion joists the other way so they didn't go on the party wall the increase would be either nothing or tiny (we've got a knocked through wall with piers the other way and I'm not sure if the weight on that wall would be taken to be transferred into the party wall in any way).

We asked him repeatedly whether he couldn't suggest something that would avoid underpinning and he said no. It was only when I read the report for a third time that I spotted about the joists. Surely suggesting running the loft joists the other way has to be the most basic possible suggestion. If I can think of that why didn't he suggest it!? I really like the chap but I'm starting to feel like he either can't be bothered to spend any time on this or he just wants to recommend underpinning no matter what so as to cover his arse.

whatsthe I am so desperate for him to come back and say "actually it's fine*. I completely understand about being conservative but my worry is whether he is being overly conservative. The bearing pressure thing in particular. That's the lowest figure on the charts I looked at and yet when he was here looking at the test pit he didn't look in and say "bloody hell!"!

These are the "existing" calculations. The loft addition is just a repeat of the 1.875 floor under the heading "LL" and 0.75 under "DL".

It does seem odd. I wonder whether he thinks that the foundations are basically inadequate as they are so he doesn't like the idea of doing anything that will change it. Running the joists the other way will reduce the load on the party wall but increase it on other walls so I'm not sure that would necessarily help, not if he's taking this attitude anyway!

I wonder whether it would be worth your while getting a second opinion, maybe from a one man band operation rather than a company. I know for us we don't like to do small domestic jobs because there just isn't the money in it but for someone working for themselves they will tend to make up a lot of their income and they will have a lot of experience. Might feel like wasted money on fees but if it helps save 1000s on underpinning it could be worth it.

MICE is Civil Engineer, as you say, not Structural Engineer. The professional qualification you need is MIStructE. A civil engineer normally would not look at this.

He has not had the soil tested and just exposing the foundations is not the same as soil testing. A good local structural engineer would know what the soil was likely to be through experience but would ask for testing if concerned.

I would ditch this Engineer and talk to one who knows about this sort of work. My DH is qualified in both and you will find others who are. If they are just Civils they tend to work on much larger projects: think designing roads, railways, docks, drainage etc. The structural engineer makes buildings stand up and you need someone with a track record of this type of work. The Civil Engineer designs the roads but the structural engineer designs the bridges.

Engineers really do not design belt and braces schemes these days. Clients do not want to spend vast amounts of money on unecessary steels. The reason you use a good structural engineer is to save money and come up with solutions that are safe and meet the needs of the client. Conservative design is poor design.

It's tough when clients don't know the difference between branches of engineering but MICE is more like a GP, MIStructE is more like a surgeon. Far fewer of them and a bit niche.

Be careful if you go for a one man band. No checking of calcs because no senior colleagues, possibly inadequately insured and what about CPD? More likely to be belt and braces! I wouldn't advise this in your case because it is not a straightforward design of a beam.

I think you're overstating the case a bit there Bubbles. I'm IStructE but there are plenty of engineers working on structures who are members of the ICE. And there are lots of IStructE members who wouldn't have much of a clue about domestic projects. Without knowing this engineer it is impossible to say. But what is clear is that they have lost the clients trust and are not being overly clear with their reasoning. For that alone I think it worthwhile getting another engineer involved.

It's a loft conversion Bubbles. That is what one man band engineers do all the time. They do have insurance and mostly a vast amount of experience. Obviously you need to make sure you get a good one. And I have no idea why you think they are more likely to be over conservative.

I'm a geotechnical (soils) engineer so I wouldn't comment on the load takedown but the report sounds quite cautious about the soil conditions. They were right to do a trial pit but the descriptions aren't as I would describe a soil. If they are going to check anything they should maybe do another pit and get a geotechnical engineer to look at it and/or bring some basic soil strength testing equipment. Or re-check their interpretation of the soil bearing capacity.

I'm not sure MICE/MIStructE comes into it. Its more about their experience/knowledge of this kind of work and people who design house foundations a lot will have a good grasp.

Who do you think design house foundations where there are tricky soil conditions?

Sole trading engineers can be over cautious because they are not at the cutting edge of anything. They just want a nice little income designing beams. Nothing wrong with that but when things get a bit tricky, they are not always the best. Plenty of work is picked up from other people's cock ups!

It is always interesting that no-one would want an unqualified technician to remove their appendix, but when it comes to building design, any old bod will do. From the OPs description it is not a straightforward loft conversion. It is a question of whether underpinning is needed or not and what could be done differently (if anything) to prevent this. There is also a query about the soil analysis. Straightforward it is not - but it could be if the engineer is wrong. We don't know.

If a client asks an engineer what work they have done, then they may well get the person for them. Clearly going to Ove Arup isn't the solution but saying anyone will do isn't the best idea either.

I didn't say one man bands were not insured, I said they could be under insured. Premiums are massive for Consulting Engineers and anyone designing for £100 may have questionable insurance for a larger job.

I'm also a geotechnical engineer, and completely agree with NotMeNoNo. I'd suggest getting a geotechnical specialist (who could be MICE or FGS) to revisit the foundation conditions, assumptions and calcs.

Bubbles, to be honest you don't sound like you know what you are talking about. Nothing the OP has described sounds particularly tricky in the slightest. Any engineer with good experience in domestic work should be capable of doing it.

I know a number of sole traders and none of them meet the stereotype you have described. No one has suggested going to just anyone. The reason I suggested sole traders is precisely because they often do this sort of work all the time, and are able to put the effort into the work that someone at a company with overheads etc isn't going to.be willing to do unless they are charging on a time basis.

But hey, what do I know. I only do this for a living.

And yes agreeing with BRS. If there is something that structural engineers might not be good at it is properly understanding the soil conditions.

Thanks so much for the comments.

My jaw almost fell off my face when I read your first post bubbles I had absolutely no idea he isn't a structural engineer. I've looked him up on the company's website and he heads the infrastructure and highways team and it says his background is in that type of work. It does say something about structures too though and I remember him saying he used to have his own company presumably working on domestic buildings (?).

He has said a number of times before that he is not someone who over-engineers stuff and will always look to recommend the simplest and cheapest solution. That was reflected in the original work he did for us (small domestic extension) and everything he said right up to the day he sent through the report. He's also now saying he'll have to revise the calculations for the extension (to be built after loft conversion). I can't bring myself to think about it yet but that redesign will probably be as much as a nightmare as the underpinning is now (e.g. steel goalposts design that he told us would add £4K to the build cost).

I knew absolutely nothing about soil testing and now feel a bit stupid that I didn't think there might be more to a test pit than just having a poke with a pen.

I'm hoping to hear back from him tomorrow (I've been saying this every day Monday - Friday for the best part of two months now...). If he still says to underpin I think in the first instance I'll do exactly as you say notme, partly because I'm still clinging to the trust I originally had in this chap and partly because I'm hoping that's cheaper than starting from scratch with someone else. It seems to boil down to the interpretation of the soil conditions so getting a specialist to look at that would presumably put the whole question to bed (which would also be useful for the extension).

Why is he saying he needs to decide the extension calcs??

Revise not decide.

I've just been looking at them. They were prepared before he produced this report (and looked at the test pit) and he based his original calculations on a higher allowable bearing pressure (75kN/m2) and slightly wider foundations. Those calculations said the existing foundations were fine for all the work.

I just feel sick about the whole thing. It's dawned on me that his decision to give the ground a 50 kN/m2 bearing pressure will cost us over £10k between underpinning and building the extension with two huge steel goalposts on concrete pads. We'll also be living in the house while they underpin the party wall which is an internal wall in our kitchen. And we'll have a newborn baby when the rest of the work is done.

Do you mean that the goalposts themselves are new? Or that their foundations have had to be increased?

Don't feel stupid - it's not everyday stuff, it's what you employ engineers for.

In my experience, if a solution seems unusual (compared to others of the same type) there should be an understandable reason , like the structure is particularly heavy or all supported at one end or the ground exceptionally bad (river bed, old pit etc). If it just seems over engineered there is no shame in asking for a double check of the figures and assumptions. (Much larger projects have failed to do this). It's worth spending a little to avoid £10k.