Way ot: house building questions
sdh2903 - 7/3/14 at 08:58 PM
The story so far, we decided we were going to move as we need more space. Many moons ago I had suggested we extend rather than move, this was
dismissed by SWMBO. Today after we have missed out on 3 properties she has had a wonderful idea of building an extension
Anyhow we will be consulting a builder + architect at some point in the near future but I thought id ask the font of LCB knowledge over the weekend
for some pointers.
We have a gulley down the back of the house and the garden is 3 feet higher than the floor of the house, my architectural sketch below illustrates:
house 1
Now we want to do a single story extension to the rear, so obviously some earth moving is required, but my main question is can we have the extension
structure as the retaining wall or is this going to be problematic with regards to damp etc? the light issue would be remedied by a load of
velux's.
house2
Also very rough ideas of cost for a single story extension approx 10ft x 25ft? I had thought somewhere in the £20-30k region??
Any thoughts most welcome!!
[Edited on 7/3/14 by sdh2903]
JoelP - 7/3/14 at 10:05 PM
I'm pretty sure it's cheaper to dig it out than leave it in, as leaving it makes it harder to ventilate the cavity. Can be done but
you'd need a good reason why you couldn't just dig it out to make it worthwhile.
loggyboy - 7/3/14 at 10:11 PM
If you abut the retaining wall you will need to tank or move the retention away from the extension.
big-vee-twin - 7/3/14 at 10:14 PM
Pretty standard arrangement the wall will need to be tanked with a drained cavity.
However always best to avoid construction below ground.
sdh2903 - 7/3/14 at 10:33 PM
I thought this would be the case. I was more curious to see if we didn't need to waste any more of the garden by moving the wall further back.
The other option would be dig out a shed load of earth and tier the garden away from the extension.
Sam_68 - 7/3/14 at 11:42 PM
quote:
Originally posted by sdh2903... my main question is can we have the extension structure as the retaining wall or is this going to be
problematic with regards to damp etc?
I'd suggest that it's going to be both cheaper and less problematic in terms of potential damp to dig back beyond the extension (by another
1.2 metres minimum, so you can get a path down the back) and put in a separate retaining wall, rather than tanking the rear wall of the extension so
that it serves double function as both rear wall and retaining structure.
Tanking is both expensive and prone to defects, so I'd avoid it if you can.
quote:
Originally posted by sdh2903Also very rough ideas of cost for a single story extension approx 10ft x 25ft? I had thought somewhere in
the £20-30k region??
Very rough figure for extensions is between about £800-£1,200 per square metre, so 3.05m x 7.6m (10' x 25', approximately) means your
estimate is in the right ballpark. It depends on the function of the extension, though - obviously the cost of kitchen and bathroom extensions is
disproportionately high compared to 'average' domestic work because of the cost of the kitchen units and bathroom fittings. Also, how you
arrange the build makes a dramatic difference - at one extreme, you could do all the work yourself and just pay for materials, at the other extreme
you pay a builder to sort the whole lot for you as a single contract; in between, you could project manage the job yourself but employ direct labour
or individual subcontractors to do the work for you.
Contact me via my profile: This is my line of work, these days and I may be able to provide both more detail on likely costs and a very cost-effective
solution for the architectural design.
v8kid - 8/3/14 at 12:50 AM
Im at the second fix stage of an extension almost exactly the same as yours and it was quite straightforward. Tanking and drainage were new skills but
my architectural technician detailed them on the drawings and it was hardly rocket science. I had no option as the tanked wall was on a boundary and I
reckon it cost me an extra £2k compared to a level build.
The biggest disadvantage was the delay working weekends and sourcing the correct tanking materials and knowledge cost me 3 months and I just managed
to get watertight before winter.
Summarize as easy but a pain in the arse
Cheers!
Sam_68 - 8/3/14 at 01:28 AM
quote:
Originally posted by v8kidTanking and drainage were new skills but my architectural technician detailed them on the drawings and it was
hardly rocket science.
They all say that, until they wake up one morning with a swimming pool in the basement!
Seriously, though... there is a very high incidence of latent defects with tanking. I would always avoid it as a design solution, if there is a
practical alternative.
Texan - 8/3/14 at 04:56 AM
Make the extension an underground house and reap all the benefits of reduced utility costs.
cliftyhanger - 8/3/14 at 06:27 AM
When we moved, I had the garden levelled and a garage built. Used a retaining wall, so not dissimilar to what you are doing.
What I would say is you can spend a fortune on groundwork. I was lucky, found a decent diggerman who could only work weekends ( I never asked why, but
suspect he was employed by a company weekdays, borrowed the digger weekends) who put me in touch with a cheap grab lorry chappie. Reckon that saved me
about 3k over a contractor. ie half price (200+ m³ of chalk went away)
Just a thought.....
snapper - 8/3/14 at 07:24 AM
I have a garden that rises less than yours but the overall problem is the same
French drain never worked properly and when I built on the side the natural flow of water that used to go around the house had nowhere to go
I extended the patio built a retaining wall had a drain fitted to far corner of patio which is also fed by a perforated drain in gravel behind the
wall garden side
There are some water channels where patio slabs get near retaining wall
The result of this is that I don't get water lapping at the back of the house even in the worst of this years rain
So plan your drainage and don't just rely on the tanking
v8kid - 8/3/14 at 08:09 AM
quote:
Originally posted by Sam_68
quote:
Originally posted by v8kidTanking and drainage were new skills but my architectural technician detailed them on the drawings and it was
hardly rocket science.
They all say that, until they wake up one morning with a swimming pool in the basement!
Seriously, though... there is a very high incidence of latent defects with tanking. I would always avoid it as a design solution, if there is a
practical alternative.
Anything will fail if it is badly designed and badly executed. If you plan to fail you surely will.
Tanking is a common building practice well proven and there are many different solutions available. In particular on this case where it is possible to
drain the cavity I would strongly dispute that any small defects would lead to failure.
But you may be right, after all I was daft enough to build a car from scratch and how many opportunities were there for failures with that - only a
madman would try!
Cheers!
richardR1 - 8/3/14 at 08:30 AM
Just finished building something very similar for a customer. Dug the garden back and put in a retaining wall with steps up onto the lawn, imho a much
better solution than tanking the dwelling wall. This came in at £29k for 10m x 4m so your budget idea is certainly about right.
Sam_68 - 8/3/14 at 09:11 AM
quote:
Originally posted by v8kid
Anything will fail if it is badly designed and badly executed. If you plan to fail you surely will.
Tanking is a common building practice well proven and there are many different solutions available.
You're absolutely right about it being common practice with many different solutions available. I'm not so sure about the 'well
proven' bit: well proven to demonstrate an unacceptable number of defects, perhaps!
Talking from the perspective of 30 year's experience in the building industry, including several years as Design and Technical Director of a
regional division of one of the country's biggest housebuilders, I can tell you (and I repeat...) that the incidence of latent defects for
tanking is unusually high. Even when they've been designed properly and executed by experienced operatives. That's just a fact, born
out by the warranty claims data (and I accept that the link is saying that 28% of failures are attributable to inappropriate detailing for the
prevailing ground conditions and water table, but that still means that 72% are not):
http://www.nhbc.co.uk/NewsandComment/Name,52466,en.html
And, of course, to do it properly is very expensive: more expensive than simply digging further back, erecting a simple, untanked retaining wall that
is easily accessible for future maintenance, then building a conventional, above-ground extension in front of it.
I should add that it's not always direct water penetration that's the issue: it's impossible to adequately ventilate a tanked cavity,
so if water starts to leach into the cavity in any quantity, the air within the cavity becomes so permanently damp (even if the majority of
the standing water is carried out by the drain) that the inner leaf of the wall will eventually transmit the damp through to the dwelling. It can take
years to exhibit itself, of course, but that's why it's called a latent defect!
There are obviously situations where you can't avoid tanking, but this does not appear to be one of them, so I can see no good reason for
accepting the additional costs and risks associated with below-ground construction.
[Edited on 8/3/14 by Sam_68]
sdh2903 - 8/3/14 at 10:18 AM
Cheers for all the input guys. Looking at it again I do agree for maintenance access etc it will be preferable to dig it out and build a separate
retaining wall. However as the house is a semi the end wall at the property boundary is definitely going to need tanking unless I can convince our
neighbour to allow us to dig out some of their garden too.
A builder from down the road had a cursory glance this morning and thought 25 k would be a rough idea. The extension itself will contain a bedroom and
kitchen/diner. I have a tame plumber and electrician already so it will just be the building work and plastering the rest I will fit out myself.
Sam_68 - 8/3/14 at 10:40 AM
quote:
Originally posted by sdh2903However as the house is a semi...
You failed to mention this!
You wouldn't necessarily need to tank if you didn't run the extension full width, of course, but that will limit the width of the
extension.
If you do want to build full width (and possibly even if you don't, if you're putting a retaining wall on or close to the boundary
with your neighbour), you need to be aware of the Party Wall Act:
https://www.gov.uk/party-wall-etc-act-1996-guidance
...and I would still limit the extent of the tanked wall to the minimum you can get away with (ie. the section that lies on the party boundary), if I
were you.
sdh2903 - 8/3/14 at 11:12 AM
Cheers for the link Sam however doesn't apply up here north of the border. In fact after googling I'm struggling to find a scottish
equivalent. We are on very good terms with our neighbours so we would discuss and get their thoughts before planning or doing anything.
Extension would have to go full width to make it viable so approx 5 ft of the end wall would be acting as a retaining wall.
Sam_68 - 8/3/14 at 11:23 AM
quote:
Originally posted by sdh2903
Cheers for the link Sam however doesn't apply up here north of the border. In fact after googling I'm struggling to find a scottish
equivalent.
Doh! Yes, of course, you're Scotland, aren't you?
You won't find anything, as there is no specific legislation equivalent to the Party Wall and Access To Neighbouring Land Acts that we have here
in England, to the best of my knowledge.
As far as I am aware, your law up there is in pretty much the same state as it was here in England before we enacted these two bits of legislation:
ie. in complete disarray and dependent on complex and often contradictory judgements on civil case law.
The good news for your neighbour is that any tanking defects are likely to affect only your property
v8kid - 9/3/14 at 08:43 AM
Ta for the link Sam but basements in London are not relevant to these circumstances since they are wholly below ground, drainage to lower the water
table in towns is often overlooked due to access difficulties and tanking is usually internally applied to walls and floors.
The OP is completely different to this. Drainage can be installed to make sure the water table is below floorlevel , the tanking can be on the outside
of the retaining wall and drainage can be installed behind the retaining wall.
I appreciate your extensive adverse experience but perhaps it is colouring your judgment to the whole range of circumstances.
The point about high relative humidity in the cavity is excellent but we have ventilated cavities here, bit surprised they are not in England too, and
although the humidity will be higher than outside due in part to the temperature increase I'm not sure if would be saturated i.e. 100%. It would
be a fascinating research topic for someone..
To summarize I feel in the circumstances here tanking is viable but agree with you its a bad idea in a London basement..
My experience in almost identical circumstances is positive and if the OP wants to visit and see how we tackled it please u2u
Cheers!
v8kid - 9/3/14 at 09:10 AM
Regarding party walls in Scotland the system as I found it seems to work really smoothly. The foundation for the wall cannot encroach past the
boundary and there are standard sections approved by building control for this. Apart from this its up to you to agree with your neighbor.
I dug back at 45degrees, built the wall and tanking, installed drains, backfilled with pea gravel and turfed over it. I made sure all the work
affecting my neighbor was finished first and was in a better condition than I found it. She was delighted.
However from the building control point of view they would not recognize the work that was done in my neighbors garden as contributing to the
effectiveness of the tanking and I had to install a system that would be effective without the added drainage. Perhaps from Sam's comments this
was good insurance.
The trouble with planning excavations is you can't see what is there before you start . In our case we pocked up live field drains, myneighbours
roof drains and we had to dig an extra half meter to get solid bearing. Consequently we ended up with 3 drains rather than 1 with all the associated
access. By friends thought this was overkill but during the recent wet weather 2 of the drains were a third full so I recckon I was right!
Hope this is of help
Cheers!
Sam_68 - 9/3/14 at 09:35 AM
quote:
Originally posted by v8kid
Ta for the link Sam but basements in London are not relevant to these circumstances...
The NHBC statistics are not limited to London. You're fixating on a single statement in the report, which is that a large proportion of the
basements built in this country are in London (due to the land values: it's the only part of the country where the very high cost of basement
construction makes sense for developers). That's a simple statement, from a simple report, but if you're extrapolating from it that defects
only occur in London, or only in full rather than partial basements then your scientific standards are slipping: they don't, and the report
doesn't say that.
For what it's worth, my personal experience is mainly with steeply sloping sites with an undercroft storey built into the slope, and I can tell
you that defects are just as prevalent.
quote:
Originally posted by v8kid
The point about high relative humidity in the cavity is excellent but we have ventilated cavities here, bit surprised they are not in England
too...
Yes, we have ventilated cavities here in England, too, but as I said before, it's impossible to adequately ventilate a tanked cavity - the
airflow is simply insufficient to disperse the water vapor down to acceptable levels if any water finds its way into the cavity, and the cool face of
the retaining wall tends to condense out any moisture in the external air that you're trying to ventilate the cavity with, so cavity ventilation
tends to be pretty ineffectual. Drainage does very little in this respect, either, because you're not getting enough flow for the drains to be
self-cleansing, so (eventually) they tend to silt up and accumulate standing water in the bottom.
A short section of tanked wall, such as the OP will need to accept for his party boundary, is a fairly trivial risk. But to consider building the
whole rear of the extension back into the slope as a tanked structure makes no sense to me, either technically or financially, if you have the ability
to build a separate retaining wall... you're just asking for additional cost and problems for no real benefit.
[Edited on 9/3/14 by Sam_68]
v8kid - 10/3/14 at 09:28 AM
I think we are talking about different things Sam,
I was trying to answer the OP question which as you say "A short section of tanked wall, such as the OP will need to accept for his party
boundary, is a fairly trivial risk". I fail to see the point in continuing quoting cases which are not trivial tasks.
We are not interested in extensive tanking here and it is a disservice to the OP to quote figures that are simply not relevant to this case. The
figures quoted comprise the vast majority of cases in London or other cities with similar problems that do not apply here. Just because a minority
were outside towns does not prove the point - they may be the ones that did not fail!
I appreciate you have had bad experiences in the past but this is not an undercroft development - its just not relevant.
I, and the local building control officer, disagree that it is impossible to adequately ventilate a cavity in the circumstances here. Remember we are
only talking about half height tanking, say 1m with the end walls ventilated full height.
Anyhow I'm sure you have a fixed point here, and I'm not pressing to alter it. However this thread started as a a request for advice and
I'm sorry for the OP who has such conflicting opinions.
Cheers!
Sam_68 - 10/3/14 at 08:43 PM
quote:
Originally posted by v8kid
I think we are talking about different things Sam,
I was trying to answer the OP question which as you say "A short section of tanked wall, such as the OP will need to accept for his party
boundary, is a fairly trivial risk". I fail to see the point in continuing quoting cases which are not trivial tasks.
I'm not sure where you get the 'as you say' from: personally, I've been talking about tanking the full seven metre rear width of
an extension built back into the slope at a depth of at least a metre, because that's what the OP's initial sketch shows, and what his
question related to.
The 'short section of wall, such as the OP will need to accept for his party boundary' is the minimum compromise that I suggested as
an acceptable alternative, not the OP question.
quote:
Originally posted by v8kidAnyhow I'm sure you have a fixed point here...
That's rather a case of the pot calling the kettle black, don't you think?
I'm not sure why you feel the need to defend tanking, unless, perhaps, you feel it necessary, bizarrely, to justify your own decision to
adopt it as a solution (which I'm sure was for good and unavoidable reasons).
Any professional, with any experience in the building industry, knows that tanking is a costly and defect prone, but sometimes necessary
evil.
We do it when we have to do it, when there is no other cost-effective, commercially or technically acceptable alternative, not out of
sheer perversity or a misplaced sense of bravado.
In the OP's case there does appear to be a more cost-effective and technically acceptable alternative, so any competent professional would advise
taking that alternative. It really is that simple...
sdh2903 - 10/3/14 at 08:57 PM
Well to put an end to the discussion the retaining wall will be moved back rather than tanking the full weight after speaking to a structural engineer
and a builder today. a. Due to the possible future issues and b. Cost. The neighbour was consulted too and was fine. Also due to size no planning
permission required.
Only prob is the mrs has found another house which has just come on the market.......
I give up
v8kid - 10/3/14 at 11:03 PM
