pipedream

Self build for a growing family

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Hello Everyone,


Just wanted to say hello and If I may add, I am amazed by the collective body of knowledge here and generosity of members who have contributed time to build the knowledge base.

A quick background on our self build, I relocated to London about 15 years back when my job moved from the continent. After few years, when we were expecting our first child, a  run down probate bungalow came to the market around where I was renting , just inside M25.
It is about 80m2, built in 1970s on a small garden plot 17X15 M.

 

It was the only housing option with a garden I could afford at the time.It was followed by manic 4 months to make the place habitable, replacing electrics,flooring,kitchen,plumbing, polycarbonate windows etc in a dire state on shoe string budget .At the end of it, I was pretty sure that I would not ever get involved in a property needing work :)

Fast forward to 2020, we need more space for growing family. The choice is between upgrading for something more suitable or building (planning permitting).Building is likely to end up more expensive but will meet our longer term requirements which might include parents moving at some point. The main sticking point for  build is that self build mortgage borrowing limits seem to be lower than regular ones and SWMBO's has doubt on me pulling it off. 

 

Working from home with two kids helped with the procrastination and we are in the process of applying for planning. The new Boris law making it easy to add a storey will help, but I want a habitable loft so it will be full planning applications with some objections etc. Attaching an Ariel map, thank fully the properties are on south and east and not too close (the adjacent structure is rather large garage/entertainment area of the neighbour on that end.

 

In the beginning I was leaning towards timber frame/MBC route, but increasingly finding the ICF route quite interesting. A lot more research and homework to do and this forum is invaluable.

 


 

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hi and welcome

 

That looks like like a nice re development prospect. Look forward to seeing it progress.

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Welcome. Plenty of experience with both your suggested build methods here and each has their strengths.

 

If you're using one of those energy efficient methods, you should consider Ecology for the self build mortgage as they have a great reputation and are easy to deal with.

 

Given your location, land values and possible constraints on how much you're allowed to grow the footprint, have you considered doing a basement as part of the build for extra space?

 

Planners tend not to pay them too much attention and if you're doing a demolish and rebuild, they are not mega expensive to build and deliver a decent ROI for the additional space delivered.

 

Our approach was to get planning for the above ground bit first and see how much pushback there was. Once achieved, we then put in a new app with just the basement added (big square box with basement on it, no details) and it passed with no issues.

 

We then spent a bit extra on the ground investigation (which you'll need for any foundation system) and got quotes to build the basement vs standard foundations / slabs and only then committed to build it, with the plan to leave it an empty box for a while if the budget got too tight.

 

At each stage of the build (first fix, plastering, second fix, flooring decorating etc..) we made sure we could afford to include the basement vs the must have parts of the house but it chugged along ok and it's been a great asset.

 

 

 

 

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Posted (edited)

Thanks @ProDave and @Bitpipe

 

 

@Bitpipe, we seem to have more similarities than the just display name. I came across your posts and been reading them with great interest even before I posted ,when searching for basement builds  around my location .

 

Your approach said is exactly what I had in mind !  Given the small foot print  that is likely to  be allowed, a basement makes a lot of sense to meet the space requirements.  With the tight budget, I would be mighty pleased, if I can squeeze in an empty basement box , light well and glazing of the basement . The finishing can wait till I have more funding.

 

 

Like you, the plan is to test planning without basement, then do a soil/ground survey to get an idea of the complexity of the basement build. If there are no immediate red flags, then ask planning permission for the basement addition. 

 

Thanks, I have my sights on Ecology, but feel its better to approach them after planning permission . I might be hitting a high mortgage LTV , a good approved design might work favourably for valuation. 

 

If I am not building a basement, timber frame seems to be good option. But with a basement the ICF throughout  looks more straight forward. Any leaks with ICF basement will be tricker to deal with than with formwork, but get the feeling that the ICF basements are evolved enough, where that risk need not be a deciding factor.

 

If I can find a ICF supplier/builder who can build the shell and install windows (supplied by me),it will be great.I am likely to have a lot of glazing and slightly worried that dimensional stability/accuracy of ICF seem to be down to bracing ,which in turn is depends on the quality of builder. https://thermohouse.co.uk/ seem promising as the impression I get is that they can provide all elements of a shell and labour (except glazing and groundworks), I have to contact and find out more.

 

 

May I ask

, if you considered ICF and why you chose formwork basement and timber frame ? 

Was it more to do with your preference for the method or due to  the fact MBC was more convenient/reliable  in getting a shell erected?

Edited by pipedream

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While you wait for @Bitpipe to reply, one thing to keep in mind about ICF basements is that you can't inspect the concrete to ensure it's continuous and void free. Not such a big deal when it's above ground and covered in cladding, but potentially an issue if it's underground and potentially in long-term contact with water. 

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11 hours ago, pipedream said:

If I am not building a basement, timber frame seems to be good option. But with a basement the ICF throughout  looks more straight forward. Any leaks with ICF basement will be tricker to deal with than with formwork, but get the feeling that the ICF basements are evolved enough, where that risk need not be a deciding factor.

 

We considered ICF for the whole house and went as far as visiting builds, shortlisting contractors and getting detailed quotes.

 

However the economics drove us back to a cast in situ (aka formwork) basement (insulated below and to the sides) and the MBC frame above - we just got more for our money that way.

 

I wouldn't say ICF is risky but you have to 'trust' the pour as you can never see it. We were relying on waterproof concrete only as the conditions were good so wanted to ensure that was independently warrantied and so used a Sika system. Also found that many local groundworkers were able to sub out to experienced concrete crews, not so ICF - but that was 5 years ago.

 

 

11 hours ago, pipedream said:

If I can find a ICF supplier/builder who can build the shell and install windows (supplied by me),it will be great.I am likely to have a lot of glazing and slightly worried that dimensional stability/accuracy of ICF seem to be down to bracing ,which in turn is depends on the quality of builder. https://thermohouse.co.uk/ seem promising as the impression I get is that they can provide all elements of a shell and labour (except glazing and groundworks), I have to contact and find out more.

 

We also visited a Thermohouse build, looked to be a high quality system and the owners were very happy with it.

 

11 hours ago, pipedream said:

 

May I ask

, if you considered ICF and why you chose formwork basement and timber frame ? 

Was it more to do with your preference for the method or due to  the fact MBC was more convenient/reliable  in getting a shell erected?

 

As above, primarily cost as we got two fixed price contracts for each and the MBC package left us with a guaranteed airtight shell, battened roof, internal stud walls, floor decks and even temp stairs within a few weeks. Windows were ordered in advance off plan (as the frame is factory made we had high confidence in the apertures being accurate) and timed to arrive as soon as the frame was erected. Roofing and render followed close behind.

 

In all we demolished at end of July 2015, basement was excavated, cast, insulated and backfilled by October (that included services too) and frame erection started Friday Nov13th and the scaff came down mid Jan 2016  - so about 8 weeks to get what looked like a finished house from the outside. Internal framing, insulating and airtightness took another few weeks and then we were ready to start 1st fix. Moved in August 2016, just over a year from starting works.

 

As first timers and relative novices, that's what we needed to build confidence.

 

However, cast or ICF does not make a material difference to the basement structural design so it's a detail you can contemplate once you have planning, a sound understanding of your ground conditions and a SE design of the structure.

 

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Posted (edited)

Thanks @jack.  Agree it is a short coming with ICF basements. However in my mind the key component that makes the waterproofing technique viable for habitable space is the backing of an Independent warranty (like Sikka’s).

I would have imagined, the ICF basement would also come with something of the sort. I am assuming if lenders and others like NHBC are involved, they would require independent third party to take the risk and its not optional.

 

Any thoughts around the path usually taken in these circumstances when manufacturer warranty  is not an option?I suspect builder’s warranty is not worth much.

 

 

 

Thanks @Bitpipe your build timelines for a novice are impressive and is a good example to show what is possible.

 

In my case , I just about manage very busy work and home. So I have to find an optimal build route which minimises  overheads, but is practical/viable and does not require me to be onsite a lot . From my last and only experience of working with trades, often when I was absent, had to pay  dearly afterwards.

I will hopefully have more resources this time around to afford better quality of tradesmen, but suspect it only reduces incidents. A project manager is a consideration, but suspect I wont be able to help  hovering around them trying to double check everything and would never be truly satisfied !

 

Your evaluation on the economics between the shell approach you took and ICF is very interesting ? Intuitively I would have thought ICF would be cheaper as its a single undertaking .It was also one of the factors that is nudging me towards ICF. 

 

Do you recollect a ballpark percentage (or £) cost difference between MBC/ground worker vs ICF route on a like to like basis ?

I suspect MBC has grown more popular in last few years and ICF more mainstream so the figures might move, but still will be valuable info.

Edited by pipedream

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Posted (edited)
4 hours ago, pipedream said:

 

I would have imagined, the ICF basement would also come with something of the sort. I am assuming if lenders and others like NHBC are involved, they would require independent third party to take the risk and its not optional.

 

Any thoughts around the path usually taken in these circumstances when manufacturer warranty  is not an option?I suspect builder’s warranty is not worth much.

 Looks like I cannot edit my post after some time .Please  ignore this question above, it was bit premature. Clearer now that I have gone through some of relevant posts on basement warranty and issues surrounding it .

Edited by pipedream
Hit save too soon.

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2 hours ago, pipedream said:

 Looks like I cannot edit my post after some time .Please  ignore this question above, it was bit premature. Clearer now that I have gone through some of relevant posts on basement warranty and issues surrounding it .

 

If you ever want a post edited, just ask a moderator. I'm one. You can find out the others by looking at the staff tab on the front page.

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5 hours ago, pipedream said:

Thanks @jack.  Agree it is a short coming with ICF basements. However in my mind the key component that makes the waterproofing technique viable for habitable space is the backing of an Independent warranty (like Sikka’s).

 

I'm not sure Sika warranty ICF pours as the final pour quality can't be seen, but maybe they do - should not be hard to find out. They provide the waterbar (goes between pours) the mastic that holds the water bar in place and the admix for the concrete, plus they come and supervise the works.

 

5 hours ago, pipedream said:

I would have imagined, the ICF basement would also come with something of the sort. I am assuming if lenders and others like NHBC are involved, they would require independent third party to take the risk and its not optional.

 

NHBC and the like will ask for 2 waterproofing methods from the three available a) external membrane, b) WP concrete, 3) internal membrane, sump & pump.

 

Your contractor may also issue you with an insurance backed warranty but ultimately there will be something from the likes of Sika or Kryten backing it all up.

 

 

5 hours ago, pipedream said:

 

Any thoughts around the path usually taken in these circumstances when manufacturer warranty  is not an option?I suspect builder’s warranty is not worth much.

 

 

If it's an insurance backed warranty then you have some chance but they are notoriously hard to claim on.

 

5 hours ago, pipedream said:

 

Thanks @Bitpipe your build timelines for a novice are impressive and is a good example to show what is possible.

 

In my case , I just about manage very busy work and home. So I have to find an optimal build route which minimises  overheads, but is practical/viable and does not require me to be onsite a lot . From my last and only experience of working with trades, often when I was absent, had to pay  dearly afterwards.

I will hopefully have more resources this time around to afford better quality of tradesmen, but suspect it only reduces incidents. A project manager is a consideration, but suspect I wont be able to help  hovering around them trying to double check everything and would never be truly satisfied !

 

Your evaluation on the economics between the shell approach you took and ICF is very interesting ? Intuitively I would have thought ICF would be cheaper as its a single undertaking .It was also one of the factors that is nudging me towards ICF. 

 

ICF just gives you walls, you need a flooring system (pozi joists or hollowcore flooring) and a roof system. Steels to span open stretches of glazing etc.

 

Then add internal stud walls, airtightness detail around windows & doors etc. With the TF package from MBC I got all of that in one shot which made life a lot easier - not to say you can't do it just as well the other way but you will need to pull in different trades. 

 

5 hours ago, pipedream said:

 

Do you recollect a ballpark percentage (or £) cost difference between MBC/ground worker vs ICF route on a like to like basis ?

I suspect MBC has grown more popular in last few years and ICF more mainstream so the figures might move, but still will be valuable info.

 

Groundworkers dig holes and fill them back in, required in all circumstances :)

 

It was about a 30% difference in cost - but keep in mind the frame itself is only around 20% of the overall build cost.

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6 hours ago, Bitpipe said:

 

I'm not sure Sika warranty ICF pours as the final pour quality can't be seen, but maybe they do - should not be hard to find out. They provide the waterbar (goes between pours) the mastic that holds the water bar in place and the admix for the concrete, plus they come and supervise the works.

 

 

NHBC and the like will ask for 2 waterproofing methods from the three available a) external membrane, b) WP concrete, 3) internal membrane, sump & pump.

 

Your contractor may also issue you with an insurance backed warranty but ultimately there will be something from the likes of Sika or Kryten backing it all up.

 

 

 

If it's an insurance backed warranty then you have some chance but they are notoriously hard to claim on.

Thanks Understood .

 

6 hours ago, Bitpipe said:

ICF just gives you walls, you need a flooring system (pozi joists or hollowcore flooring) and a roof system. Steels to span open stretches of glazing etc.

 

Then add internal stud walls, airtightness detail around windows & doors etc. With the TF package from MBC I got all of that in one shot which made life a lot easier - not to say you can't do it just as well the other way but you will need to pull in different trades. 

 

 

Groundworkers dig holes and fill them back in, required in all circumstances :)

 

It was about a 30% difference in cost - but keep in mind the frame itself is only around 20% of the overall build cost.

 

Ahh I get it now . The timber frame package coverage is more complete , factory planned/ manufactured  and in your case 5 odd % cheaper overall. 


That is a very interesting addition to my ICF vs Timber frame decision process.

Much appreciated !
 

 

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ICF basements and ICF below ground walls need two of the three zones protected.  Ideally using different systems and one must be maintainable, pretty much as Bitpipe says.

 

In a lot of cases, waterproof concrete is a waste of money.  So external French drain and internal drainage system (maintainable).  A waterbar for good measure but the same can be achieved in concrete slab by creating a step.

 

Remember, not all ICFs are the same or have the same structure so their performance below ground differs.  Woodcrete along with a number of EPS ICFs: lattice work, polarwall: massive continuous slab.  I would also double check to see which ICFs are BBA approved for basements.  It's a moving playing field at the moment.

 

Reading some of the comments regarding "not being able to see it once poured" and I can hear the collective concrete world groan.  It's really not difficult to do right.

 

I would also be interested to find a timber frame that matches the airtightness of mass poured concrete at the same stage without being wholly reliant on tapes and membranes.  Think of a closed box with a window hole in it.  Which is going to be easier to guarantee airtightness? A concrete, mass poured box or a timber frame box?  You can achieve the same with both but one is easier and involves less processes than the other.

 

Yet to find a TF in 2021 cheaper than an ICF.  And I don't expect to.  One ICF brand has put its prices up 10%, timber has jumped 40%.

Whatever anyone says, and this goes back to my comments on cost management elsewhere, be pathological about comparing like with like and make sure you, as the self builder understand the processes involved in getting, say, a TF and ICF to the same build stage. No one makes it easy.  It helps if you've done one of each, next to each other.

 

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7 hours ago, FM2015 said:

Reading some of the comments regarding "not being able to see it once poured" and I can hear the collective concrete world groan.  It's really not difficult to do right.

 

Many things in the building world are "really not difficult to do right", and yet experience says they're often done wrong. People come onto this forum daily and ask for help addressing significant cock-ups by professionals. If it happens so regularly with stuff that's visible, why is it unreasonable to be wary about stuff that isn't going to be visible?

 

I thought the main issue with non-inspectable concrete (as happens in ICF) was the difficulty in getting someone to warranty it when it's used underground, although I can't remember who said that. @Bitpipe, did you ever look into ICF as a possibility? 

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40 minutes ago, jack said:

Many things in the building world are "really not difficult to do right", and yet experience says they're often done wrong. People come onto this forum daily and ask for help addressing significant cock-ups by professionals. If it happens so regularly with stuff that's visible, why is it unreasonable to be wary about stuff that isn't going to be visible?

 

I thought the main issue with non-inspectable concrete (as happens in ICF) was the difficulty in getting someone to warranty it when it's used underground, although I can't remember who said that. @Bitpipe, did you ever look into ICF as a possibility? 

 

Sorry Jack, I totally agree you with regard quality and experience. 

 

With regard below ground ICF, warranties are fairly easy to obtain for some ICFs and less so others.  As a proponent of ICF in general, I think this should be stressed.  Not all ICFs are the same and there's a lot of guff around, surprisingly, a lot of brand Vs brand stuff which is ultimately pointless. 

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43 minutes ago, jack said:

Many things in the building world are "really not difficult to do right", and yet experience says they're often done wrong. People come onto this forum daily and ask for help addressing significant cock-ups by professionals. If it happens so regularly with stuff that's visible, why is it unreasonable to be wary about stuff that isn't going to be visible?

 

I thought the main issue with non-inspectable concrete (as happens in ICF) was the difficulty in getting someone to warranty it when it's used underground, although I can't remember who said that. @Bitpipe, did you ever look into ICF as a possibility? 

 

Yes I did and please don't think I'm anti ICF.

 

We shortlisted two ICF systems, visited sites and did detailed cost modelling on each, TF was a late entrant for consideration. We even chose our basement SE based on their ICF experience. 

 

In 2015 I just got much more build for my money with a TF package, working with one above ground contractor and a very fast progression to first fix, which as a novice was important. Were I more experienced then perhaps I could have found a way to pull in the ICF cost and been able to manage the additional build complexity.

 

8 hours ago, FM2015 said:

ICF basements and ICF below ground walls need two of the three zones protected.  Ideally using different systems and one must be maintainable, pretty much as Bitpipe says.

 

In a lot of cases, waterproof concrete is a waste of money.  So external French drain and internal drainage system (maintainable).  A waterbar for good measure but the same can be achieved in concrete slab by creating a step.

 

 

We were wholly reliant on waterproof concrete for our basement, which is considered type B from an insurance point of view. Also had a french drain externally but that is not recognised as a waterproofing method per se. I wanted to avoid type A (external membrane) if possible and type C (internal membrane, pump & sump). Our water table is at 6m so we were not 'in the water'  so got insurance based on type B alone. 

 

I found that when getting quotes from ground-workers they were exclusively cast in situ concrete - none of them had any experience with ICF. Maybe different now.

 

With regard to airtightness, ours was contractually guaranteed and we achieved 0.56 ACH on a fairly large structure so if the attention to detail is taken and the right materials used then any structure can meet the desired standard.

 

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Posted (edited)
Thanks @FM2015, @jack and @Bitpipe
 
Very good points on ICF vs TF decision. It helps me.
 
This is where I am in the decision process .
 
The build cost is major consideration as ideally I would like a basement to give me the space needed but early indication is that it might be a stretch .Although my location in outer London does not command the dizzyingly high land prices in the centre, but for build costs/quotes, I suspect I will be lumped with London. 
The early foray into prefab routes were an attempt to reduce cost. I.e if the London labour costs are high, a prefab with less onside labour is likely to be cost-effective.
 
Timber frame fits my requirements very well, plenty of resources, suppliers and reviews .Like the fact that it is designed and manufactured in quality controlled settings with plenty of supplier options and service levels. MBC's  package with guaranteed airtightness and foundations stands out. I am impressed by excellent engagement from Fleming and the range of service levels.I liked Hanse Haus who provide turnkey at a premium, but a shell from them might be a sweet spot , hard to tell till I seek firm quotes.
 
My initial uninformed impression of ICF were one similar to that of Steel framed houses at the time . They have their strong points, but not enough mainstream to easily find a reliable contractor who can deliver shell and be able to fund it using mortgage with insurance etc. 
 
Last few weeks I have been looking closer at ICF and finding it very interesting.Good airtightness is achievable without relying on good detailing. I am close to a busy road so the noise insulation is a key consideration for me. I quite like the fact that cantilevers,corner windows are simpler .I also looked at  monolith.com , who provide brick slips that can be stuck directly to ICF insulation. I like the fact that the shell including basement can be undertaken by one supplier/contractor.
 
There may be a subconscious bias/affinity  towards concrete, I spent my childhood/teenage in a humid tropical country, where concrete is used a lot.My parent's roof is massive poured concrete slab. Oddly I don't think they bother with waterproof concrete, instead adjust the cement ratio in the mix, give it sufficient depth and some sort of cementitious coating on top . Never figured out why the practice of building such heavy roofs survived, but that aside concrete is good with termites,ants and other problems specific to that climate.
 
 
The aspects that I am researching now are :-
 
1)Do ICF suppliers also erect ?Thermohouse seems to.
2)Do we have any quality assurance service from ICF suppliers, like for e.g Sikka inspects and provides warranty for their waterproofing product used.
3)On a like to like basis for shell what is  ICF premium for my location.  
4)IWhat is the route for obtaining ICF basement warranty without using type c waterproofing. 
 
I will be visiting a nudura ICF constuction  nearby so that is something I am looking forward to. 
Edited by pipedream
typos

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12 hours ago, FM2015 said:

 

With regard below ground ICF, warranties are fairly easy to obtain for some ICFs and less so others.  

 

@FM2015 would you please expand on this . What are the options for someone who is not keen on type C, and prefer B or a combination of A and B types of waterproofing with ICF basement.

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11 hours ago, pipedream said:

 

@FM2015 would you please expand on this . What are the options for someone who is not keen on type C, and prefer B or a combination of A and B types of waterproofing with ICF basement.

 

The waterproofing is dependent on your ground conditions, until you know where the water table is in your footprint it will be hard to get the SE to sign off on any one scheme over another.

 

When I was engaging with my warranty vendor at quote stage, I had the reports (water below 6m) and SE plans so they were happy to accept type B only. 

 

 

 

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5 hours ago, Bitpipe said:

 

The waterproofing is dependent on your ground conditions, until you know where the water table is in your footprint it will be hard to get the SE to sign off on any one scheme over another.

 

When I was engaging with my warranty vendor at quote stage, I had the reports (water below 6m) and SE plans so they were happy to accept type B only. 

 

 

 

Understood thanks , it’s too early to fix the waterproofing type without GI.

 

Still would be good to know what  are the warranty options with ICF basement, to put a tick against the research box for ICF basement warranty .

Also good to know , if there are any constraints , ie warranty providers insisting on two types, but one of them must be type C, because the quality of pour is not visible .

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1 hour ago, pipedream said:

Understood thanks , it’s too early to fix the waterproofing type without GI.

 

Still would be good to know what  are the warranty options with ICF basement, to put a tick against the research box for ICF basement warranty .

Also good to know , if there are any constraints , ie warranty providers insisting on two types, but one of them must be type C, because the quality of pour is not visible .

 

Suggest you pick up the phone to a few brokers and see where you get. I would be surprised if they are that knowledgable TBH.

 

Outer membrane + waterproof concrete (& water bars etc) would meet the requirements, question is if you can save the expense of using both.

 

Also, a warranty is not mandatory but is advisable if you plan to re-mortgage post build or sell the property in 10 years. Our mortgage company (who we went to post build as Ecology were a bit expensive) asked IF we had a warranty but never asked to see it. They are notoriously hard to claim against.

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On 02/04/2021 at 16:41, Bitpipe said:

 

Suggest you pick up the phone to a few brokers and see where you get. I would be surprised if they are that knowledgable TBH.

 

Outer membrane + waterproof concrete (& water bars etc) would meet the requirements, question is if you can save the expense of using both.

 

Also, a warranty is not mandatory but is advisable if you plan to re-mortgage post build or sell the property in 10 years. Our mortgage company (who we went to post build as Ecology were a bit expensive) asked IF we had a warranty but never asked to see it. They are notoriously hard to claim against.

Thanks that’s very useful info.

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@pipedream we see primarily A (proper French drain) and C as the dominant options.

 

Waterproof concrete is a) expensive and b) in most cases has to get wet to work.  Therefore if space isn't an issue and cost is a factor, A and C (internal maintainable system for which a supplier will supply a warranty) offers the most effective and cost efficient method of satisfying the largest number of structural warranty providers.

 

One critical factor which is often overlooked but can have an impact is that not all build systems are BBA approved for basements.  And the list is being restricted due to issues.

 

From a scientific point of view no A means a real world test for B which then has no secondary defence and is not maintainable.  In the age of litigation, a single method of waterproofing doesn't make sense in my book.

 

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Posted (edited)
16 hours ago, FM2015 said:

@pipedream we see primarily A (proper French drain) and C as the dominant options.

 

Waterproof concrete is a) expensive and b) in most cases has to get wet to work.  Therefore if space isn't an issue and cost is a factor, A and C (internal maintainable system for which a supplier will supply a warranty) offers the most effective and cost efficient method of satisfying the largest number of structural warranty providers.

 

One critical factor which is often overlooked but can have an impact is that not all build systems are BBA approved for basements.  And the list is being restricted due to issues.

 

From a scientific point of view no A means a real world test for B which then has no secondary defence and is not maintainable.  In the age of litigation, a single method of waterproofing doesn't make sense in my book.

 

Thanks @FM2015

it’s a different perspective to what I have seen so far and really useful .

 

With internal cavity membrane , I get this image of mold in the cavity sending airborne spores everywhere . 
 

Maybe I am worried about a improbability due to past challenges with mold.

Edited by pipedream

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