Foundations part deux

Superdaddy

My previous thread was derailed so can people please stay on topic.

I am about to dig out my site to comply with my commencement notice. Site is on a slight slope, it's a 140m2 footprint.
Passive raft foundations appeal to me. They appear a simplistic and logical way to eliminate cold bridging.
People keep telling me strip foundations are cheaper. But why are they? Surely you use the same amount of insulation under the concrete to get the desired u value, same radon barrier. So what's left only concrete, steel and labour?

The floor slabs I've seen in passive rafts are 100mm concrete on 300 mm of eps100 and where wall support is required eps300 is used to form a type of ring beam. There can't that much of a difference in the concrete and steel required can there?

You still have to put down a slab with steel and a ring beam with the strip foundation.
I will talk to my engineer too but I would appreciate your experience and expertise.
Has anybody been down this route. What is the price difference between raft and strip with comparable cold bridging detail and u value?

4Sticks

Superdaddy wrote: »

I am about to dig out my site to comply with my commencement notice.

Did the commencement notice look like pages 22-24 from this ?

So just consult with the persons you entered details of at 5 , 6 + 7

5. Building Designer Details:
Name: Tel: Fax: ——————
Address: ———————————————————————————————
————————————————————————————————————
Email: ————————————————————————————————
6. Information: Person(s) from whom such plans, documents and any other information,
as are necessary to show that the building or works will, if built in accordance
with design, comply with the requirements of the Building Regulations, may
be obtained.
Name: Tel: Fax: ——————
Address: ———————————————————————————————
————————————————————————————————————
Email: ————————————————————————————————
7. Drainage System Foundations: Person(s) from whom notifications of the pouring
of any foundations and/or the covering up of any drainage systems may be
obtained.
Name: Tel: Fax: ——————
Address: ———————————————————————————————
————————————————————————————————————
Email: ————————————————————————————————

Superdaddy

Mod Note: No back seat moderating. Thanks. This is an open forum and anybody is free to post. Use the ignore button if there is advice from certain posters that you do not want to see/hear

wirehairmax

4Sticks wrote: »

Did the commencement notice look like pages 22-24 from this ?

So just consult with the persons you entered details of at 5 , 6 + 7

What he said.

sydthebeat

should you not have posted in the prices / costs forum if thats what your looking for?

Drift

Ok.

I'm going to have a go at this OP. Please don't bite if you hear something you don't like!

Posters here are wary of expressing an opinion because the way you phrased previous posts it sounds like you are listening to everyone's opinion except those of qualified experts so they fear you will just take what you're told here and rush off and throw in a "Sure it'll be grand" type of foundation agreed on the back of an envelope with the "digger man."

The point everyone is trying to make is that saving money on a foundation is only a good thing provided the foundation you end up with is still suitable structurally. Is is phenomenally expensive to fix a faulty foundation!

With all that in mind and taking into account the fact that we know nothing about the loadings/weights from your house or the ground conditions on your site I think I could say the following:

1. There are many places in Ireland that are suitable and unsuitable for strip or raft or both foundations so step one is to get a qualified, experienced engineer to check the subsoil conditions and with your house type and construction style in mind tell you what types of foundations would be suitable.

2. Assuming they confirm that it is suitable for either type (NOT guaranteed!) they can then recommend one or the other and provide you with a design for whichever one you decide between you. They will then supervise the installation. This is very important because God is in the detail with foundations. Don't rely on your "digger man" to install the correct steel and tie it, fix it correctly or to select the appropriate concrete strength, etc., etc. Get an engineer to check everything. There is no doubt that this will make it more expensive but you will always find that if something is too cheap there is something wrong with it!

3. I think this is really what you want to hear: On paper a strip foundation normally looks cheaper than a raft foundation because supporting a like for like load the strip uses less materials and requires less excavation (volumetrically). I have found that the amount of labour is often underestimated for strip foundations. Firstly the excavation has to be much more precise - tell the digger man he'll have to tidy the edges with a shovel and he'll soon start thinking that a raft is a better idea. Secondly in many places in Ireland particularly in Autumn/Winter/Spring conditions strip excavations collapse necessitating extra work, often by hand. So it may be cheaper or it may not depending on a very unique set of factors that vary from build to build. Listen to other people's experiences but don't rely on them! The only way to find out for sure is to get a full design done for each and then get a fixed price from a contractor for each one - bearing in mind that providing a fixed price for work involving unknown/hidden ground conditions is not something that many/any contractors will do!

Anyway my main point is don't skimp on the foundations and follow the advice of the one person who is qualified to design the foundations - your structural engineer. You can listen to everyone elses experiences and take them on board but the engineer is the only one who has years of design and supervision experience (not to mention insurance!) behind him backing him up.


One last point: Passive raft foundations are quite specialist. I don't know the numbers of them in the country but I'd say it is less than 1000. With that in mind be very careful that you are covered with defects insurance and supplier certificates should some unforseen problem develop in future.



Edit: Note to Mods - I don't think any of what I've said would count as Structural Specification. If it does please feel free to delete the appropriate bit and I'll try to refrain from similar comments in future. Thanks.

Superdaddy

Thank you very much for your time Drift. That is all good advice and very welcome. It would appear that my site is suitable for both foundation types.

The digger man is not an idiot, he expressed a view that surely a strip foundation is cheaper and that i should be sure that i am not going to end up going for a strip foundation in the end as it would cost more to bring in fill for it if i level the site now.

I have an engineer and i will be meeting him on site in the next couple of days. I am trying to gain some info before meeting him. This is not a DIY project, but i can make phone calls and get quotes as well as any architect. I intend to hire professionals to install the foundations and all other components of the house. I am just trying to get the right people at fair prices.

I would love to have a architect come in and run the project. Build my perfect house stress free. But at the prices i was quoted and how i have seen them operate, i cannot afford them and certainly wouldn't trust them to do their best for me.

I have a quote from a passive house company already for a raft. I have to say they were very good to deal with, the price is a bit more than i expected. I am waiting on more quotes.
Like most people i am working within a tight budget and i am trying to get the most for my money while still maintaining quality.

Personally i think there is nothing in price difference between both foundation types once you take everything into consideration. But i do think there is a perception that the raft is more expensive and therefore a premium is added, but if it was all broken down to materials and labor they should be near enough to each other. I'd rather spend my money on quality materials than an extra few days labor.

I hear what you are saying about the certificates, there's a lot of new technology and methods floating about and while i want to be up to date, i don't want to be a test case.

Superdaddy

sydthebeat wrote: »

should you not have posted in the prices / costs forum if thats what your looking for?

In hind sight you are probably right. But i will leave it to the mods to decide that, they have already edited my post so they are obviously aware of it.

galwaytt

Superdaddy wrote: »

My previous thread was derailed so can people please stay on topic.

I am about to dig out my site to comply with my commencement notice. Site is on a slight slope, it's a 140m2 footprint.
Passive raft foundations appeal to me. They appear a simplistic and logical way to eliminate cold bridging.
People keep telling me strip foundations are cheaper. But why are they? Surely you use the same amount of insulation under the concrete to get the desired u value, same radon barrier. So what's left only concrete, steel and labour?

The floor slabs I've seen in passive rafts are 100mm concrete on 300 mm of eps100 and where wall support is required eps300 is used to form a type of ring beam. There can't that much of a difference in the concrete and steel required can there?

You still have to put down a slab with steel and a ring beam with the strip foundation.
I will talk to my engineer too but I would appreciate your experience and expertise.
Has anybody been down this route. What is the price difference between raft and strip with comparable cold bridging detail and u value?

Don't forget there are two types of raft out there, not just one, so you need to compare apples-with-apples, as they say.

1. Strip foundation, as sub-floor
2. Raft (conventional), as sub-floor
3. Raft (Passive - i.e. insulated) as finished floor.

As Drift above very well says, 1 & 2 are directly comparable, and in my experience 1. is not cheaper. YMMV and all that.

Now, #3 is fundamentally different. You cannot compare the costs of #1 or #2, with #3 without adding in the finishing costs to the other 2 to make them comparable. (finished floor screeds + insulation + DPC etc etc ). Only when you do that will you have a true comparison.

Again, you must get an S.E. to look at your ground conditions/site before determining on anything.

Superdaddy

galwaytt wrote: »

Don't forget there are two types of raft out there, not just one, so you need to compare apples-with-apples, as they say.

1. Strip foundation, as sub-floor
2. Raft (conventional), as sub-floor
3. Raft (Passive - i.e. insulated) as finished floor.

As Drift above very well says, 1 & 2 are directly comparable, and in my experience 1. is not cheaper. YMMV and all that.

Now, #3 is fundamentally different. You cannot compare the costs of #1 or #2, with #3 without adding in the finishing costs to the other 2 to make them comparable. (finished floor screeds + insulation + DPC etc etc ). Only when you do that will you have a true comparison.

Again, you must get an S.E. to look at your ground conditions/site before determining on anything.

Thanks for clearing that up,
I may not be very clear but you have explained precisely what i am trying to do. I am trying to work out if there is a difference in price between #1 with finished floor screeds + insulation + DPC etc etc and #3 Raft (Passive - i.e. insulated) as finished floor.

Drift

galwaytt is spot on.

I have no experience with Passive Rafts, so I can't say much about them.

In my post I was specifically talking about 1 & 2 as per galwaytt's post. So in my post any time I said raft I meant: Convential reinforced raft foundation bearing on compacted fill and onto subsoil. Insulation above raft and below screed.

There's a lot of details to consider OP. Among other things ask your engineer where he prefers to see the radon barrier. Some prefer above the raft and some below.

Also there is nothing to stop you managing your own project from start to finish provided you lodged your commencement notice prior to 1st March. But as a self-builder you must be doubly careful with everything you put in because if some flaw appears in future you have a lot less comeback than if you hired an insured (registered) building contractor for a start to finish project. Remember that if the house doesn't comply with all the building regulations you may have serious trouble selling it or closing off your mortgage in future. Your standard sub-contractor (be they blocklayer, chippy, etc.) doesn't know the building regulations inside out, nor should he be expected to know them all. This means that it is up to you and the person who did your specifications to supervise construction and ensure the house meets all the rules and regulations. You should not underestimate how difficult this is, particularly in the case of a direct-labour project.

galwaytt

My personal experience on building 2 x homes for myself, is that 1st one was strip, and 2nd raft (as in, #2). If I got the strip for free I wouldn't do it again. Too slow, not accurate enough, etc etc. And, as you have kind of sussed yourself, when you add it all up, saved nothing really.

House #2 was a raft type #2 ! Think about it: you excavate entire footprint in one pass, stone & blind out in one pass, and then erect shutters, fit radon/steel etc, and pour..........for a circa 200m2 house in Galway on a fair site you'd be looking at that entire process being done in less than 2 weeks.

Raft #3 is essentially the same, but with the floor detail added at the beginning.