Foundations/slab advice please

[chrisb]

Hi all,

Over on the old forum, I described my plans for an extension to our semi-detached house in Berkshire. Since then, lots of time has passed, planning issues dealt with and drainage details fought over with Thames Water. I've found a builder to take on the project, and have engaged a TF supplier too. I have BC approval, but want to make some changes, and also would like a sanity check from the collective thought of the forum before finalising things. I've attached BC drawings for reference, and would welcome any comments please.

 

The floor slab is currently to a traditional design - hardcore, slab with mesh, insulation, screed with UFH pipes. We have decided on Karndean flooring and have confirmed with the installer that they will latex screed whatever floor exists to a depth of 3-6mm. On that basis, I wanted to change the design so to insulation under concrete slab containing mesh & UFH pipes as I've read others have done on here. Builder is happy to do it, but I need some guidance on the spec please.

 

Reading all about cold bridges and wall/floor junctions, I'm concerned that I haven't paid enough attention to this area. The plan is to have a brick outer skin to the TF, as per the attached detail from Silvaframe.

 

Thanks

Chris

 

Edited as the attachments didn't list exactly as intended!

Further edited to add a link to the recent images from a Google visit: https://goo.gl/maps/p9vasZwbZov

 

P702-01-master-First Floor Plan Details.pdf

P702-01-master-First Floor Plan Structure.pdf

P702-01-master-Foundation Plan Drainage CB v3.pdf

SilvaFramePackage-SilvaStandardPlus.pdf

P702-01-master-001 Site Layout.pdf

P702-01-master-002 Existing Floor Plans.pdf

P702-01-master-003 Existing Elevations.pdf

P702-01-master-Build Over Detail.pdf

P702-01-master-First Floor Plan Dimensions.pdf

P702-01-master-First Floor Plan Structure.pdf

P702-01-master-Foundation_Plan_Drainage_CB_v3.pdf

P702-01-master-Ground Floor Plan Details.pdf

P702-01-master-Ground Floor Plan Dimensions.pdf

P702-01-master-Ground Floor Plan Structure.pdf

P702-01-master-Posi-joist layout.pdf

P702-01-master-Proposed Elevations 1.pdf

P702-01-master-Proposed Elevations 2.pdf

P702-01-master-Rafter layout.pdf

Edited July 18, 2017 by chrisb

[Ferdinand]

Welcome back.

 

Full marks for supplying all the information . But that may take *some* digesting to comment sensibly. 

 

Can I make 2 3 4 comments / suggestions having just dipped in (very busy for the next 2-3 weeks).

 

1 - Like the way you are using an awkward N/S orientation.

 

2 - I might want to move that shed and deck in a little from the bottom boundary (2-3 feet), so you can get behind to maintain them and have somewhere to store things you don't want seen. We all need an untidy corner.

 

3 - Is it worth considering moving the rear garage door to the back wall so you have an absolutely straight path through when required? This becomes a dead letter if you have a suitable path down the side or alternative, I would want it to let me get a sufficiently large minidigger down the side for any projects I may do in future - how would you get ten bulk bags of sand to the back if needed in future? I would work on a metre wide or a little more (1.5 tonne digger = 980mm usually).

 

4 - I would also say do the floor of the garage such that you can convert it later to an office or grannexe if required. Insulation to domestic standards, capped off water in the corner, perhaps capped off gas, route in, maybe ufh if you are doing it, even enough extra foundations so it will support a second storey.

 

And a question:

 

I see your architect refers to a "covered way" in the Title of the Proposed Development:

 

"Proposal: Erection of a single storey side extension and covered way. Erection of a single
storey rear extension following the demolition of the existing conservatory."

 

Can you tell me which bit is the "covered way", and what your architect knows about them? There seems to be no definition in Planning Law, and we are trying to find out what one is in this other thread:

A Covered Way is exempt from Building Regs and potentially Planning, so it could be a useful concept to use for making small additions less expensive wrt paperwork.

 

Cheers

 

Ferdinand

 

Edited July 18, 2017 by Ferdinand

[Ian]

@chrisb

Hi Chris,

Re the spec - if you do nothing else, make sure you test the relative humidity of your concrete slab before laying the vinyl. If takes ages for a slab to dry out naturally to the recommended 75% RH. Typically a 100mm slab would take about 4 months to dry sufficiently. If it doesn't get down to 75% in time to suit your programme then you will usually need to apply a liquid surface dpm and unfortunately they're not cheap.

If you powerfloat the surface it can have the effect of doubling the drying time needed.

There are also some underlays for vinyl that claim to get around the problem but I have no personal experience of using them.

Edited July 18, 2017 by Ian

[ToughButterCup]

24 minutes ago, Ian said:

[...]

Typically a 100mm slab would take about 4 months to dry sufficiently. If it doesn't get down to 75% in time to suit your programme then you will usually need to apply a liquid surface dpm and unfortunately they're not cheap.

If you powerfloat the surface it can have the effect of doubling the drying time needed.

[...]

 

Pheeeeewwww, thanks for tell me (us)! How do you test for RH in Concrete?

[Ian]

12 minutes ago, recoveringacademic said:

 

Pheeeeewwww, thanks for tell me (us)! How do you test for RH in Concrete?

The only method that is accepted by the British Standard and therefore the only method I'm supposed to accept when I'm working as an architect is the hygrometer test method which involves sealing a digital or analogue hygrometer meter onto the surface of the slab in an insulated box and leaving it in place for a minimum of 72 hours. Other methods are available which are quicker, some of which I'd be happy to accept for personal work - these typically involve drilling into the slab and measuring the humidity of the concrete at the centre of the slab.

 

The only time as an architect where I've allowed vinyl to be stuck on top of a concrete floor which had an RH reading of more than the recommended 75% was on an industrial project where there was a very large packing hall. We had an RH of 80% and managed to save the cost of a surface applied dpm by switching to a spirit based adhesive rather than the originally specified water based one. With the clients full agreement we took a gamble and got away with it as we only had about half a dozen of the vinyl tiles that lifted and they were easily stuck back down again.

 

Vinyl tiles and planks are a little more tolerant than vinyl sheet flooring - I'd never take a gamble with vinyl sheet flooring as I've seen some horrid stinking messes when it's gone wrong. Water migrating from the slab has got nowhere to go and it turns the adhesive into a very smelly soup - if you've ever had a car with a blown cylinder head gasket you'll know what the resulting oil/water emulsion looks like.

[MikeSharp01]

Looking at the wall / slab intersection construction it does have some cold bridges. Perhaps take a look at some standard details and see if you can rid yourself of them if at all possible. Main one is up through the inner leaf off the foundation and into the slab above the insulation.  I am sure, but I cannot find, a detail that includes a thin ring of upstand insulation around the slab between the inner leaf and the slab. 

[Ferdinand]

@recoveringacademic

 

The Concrete Society have a recommended procedure in a £10 factsheet.

http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=778

 

For an *indication* I think it is roughly:

 

Drill a hole 40% of the way through, put an RH sensor in it, and seal it (plastic cap), wait for it to stabilise (48-72 hours?), then take a reading. 

 

For BH-ers it is probably something to install as we do with heat sensors.

 

For an even cruder indicator tape a square of plastic sheet to the concrete floor and see if it is wet underneath after 24 hours => still drying out. Repeated tests give an indication of moisture which is coming out, and perhaps when it slowing down.

 

As @IanR points out, these are not seen as definitive, however. 

 

Ferdinand

Edited July 18, 2017 by Ferdinand

[Jacob]

Re karndean, There are some new click together vinyl planks that are an easy diy job and as a floating floor system are easy to take up when the need arises, they say its waterproof so id agree be aware the concrete will not be dry for a looong time and vinyl will trap the moisture. Having fitted karndean and the click planks i would never use karndean again , for one its impossible to remove and the prep has to be perfect. The click stuff sits on a thin underlay, quite forgiving as its about 5mm thick and works out way cheaper with arguably a better immitation of wood finnish.

 

[chrisb]

13 hours ago, Ferdinand said:

 

1 - Like the way you are using an awkward N/S orientation.

 

2 - I might want to move that shed and deck in a little from the bottom boundary (2-3 feet), so you can get behind to maintain them and have somewhere to store things you don't want seen. We all need an untidy corner.

 

3 - Is it worth considering moving the rear garage door to the back wall so you have an absolutely straight path through when required? This becomes a dead letter if you have a suitable path down the side or alternative, I would want it to let me get a sufficiently large minidigger down the side for any projects I may do in future - how would you get ten bulk bags of sand to the back if needed in future? I would work on a metre wide or a little more (1.5 tonne digger = 980mm usually).

 

4 - I would also say do the floor of the garage such that you can convert it later to an office or grannexe if required. Insulation to domestic standards, capped off water in the corner, perhaps capped off gas, route in, maybe ufh if you are doing it, even enough extra foundations so it will support a second storey.

 

And a question:

 

I see your architect refers to a "covered way" in the Title of the Proposed Development:

 

"Proposal: Erection of a single storey side extension and covered way. Erection of a single
storey rear extension following the demolition of the existing conservatory."

 

 

 

Ferdinand,

 

Thanks, I'm just getting to grips with the forum, I really wanted to be able to display the images inline, but they need converting from PDF first, which I will do later. 

 

1. We wanted to get as much done as possible to a watertight shell and deal with decor etc when funds allow. I particularly wanted to avoid demolition works and building in the future next to a rear extension, so it seemed best to do as much structural stuff in one go. The South gets lots of sun, solar gain is a big challenge as I have 2 x 8'x4' windows in the front aspect - one main bedroom and the other lounge. The back garden gets sun in the summer as there is really onl a 6' fence either side to limit it, but in the spring and autumn, the back wall of the house is quite cold.

 

2. The shed and decking are existing, the decking has already been extended by 3.6m towards the house on the garage side in order to put a temporary shed to house the contents of the garage while I knock it down and rebuild (although my wife would have far preferred much of it to leave in a skip....). The shed has been eaten from the bottom up by rats, so will be replaced in the future with a better one with at least the first 18" in masonry. I'll also move it further from the boundary, after removing the pear tree which currently pushes it that far back.

 

3. I wanted to try and preserve a 1m wide access straight through the garage, utility room and study/playroom for exactly the reasons you describe. However, as the rooms are only ~8' wide in the side extension, I can't really afford to put an outside door in one end of the study and an inside door to the utility in the other as the remaining space would really suffer usability-wise. I'm planning on making the internal doorway garage => utility 1m wide and the same utility => kitchen/diner. French doors or bifolds from there to the back garden should still give me the capability to move big things if I really need to in the future. Additionally, the boundary to the garage side is my fence (panels & concrete posts) with the front gardens to the neighbours houses end-on to it, so with some negotiation that route is also available.

 

4. There is a chance that my son will need ground-floor accommodation & bathroom in the future, so the intent is that the study and utility could be turned into a bedroom with en-suite and the utility moved into the garage. It's too narrow for a car anyway, so will only ever be a workshop (except for the planner who signed it off!). I am planning on keeping the structure the same as the rest of the extension - slab, UFH, insulation etc, so it wouldn't need major works to accomplish this in the future.

 

Last point - where do you see the title of proposed development as single storey? Its a 2 storey side and single storey rear extension, with the existing garage coming down to make way for new garage and 2x bedrooms above. I know the architect who did the BC drawings was a very poor proof reader and has many details left in from other jobs, so I'm busy getting those changed before going back to the BCO for an updated approval. The architect is a friendly chap though, so I can ask him about covered way stuff if you like?

 

Chris

[chrisb]

Still learning about the forum - closing the tab you're editing in is a bad idea it seems!

 

Flooring - we had decided on stuck down Karndean, but that was based on research ~2yrs ago, so maybe worth a revisit now. Especially as it seems the slab drying time measns we don't have to make any quick decisions!

 

On that note, I was planning on putting some DS18B20 temperature sensors in the slab inside lengths of UFH pipe - same method as OnOff used in his bathroom I think? Should I also be thinking of something to measure humidity in the slab, and if so, what?

 

Thanks for all the input so far

Chris

[Ferdinand]

2 hours ago, chrisb said:

Last point - where do you see the title of proposed development as single storey? Its a 2 storey side and single storey rear extension, with the existing garage coming down to make way for new garage and 2x bedrooms above. I know the architect who did the BC drawings was a very poor proof reader and has many details left in from other jobs, so I'm busy getting those changed before going back to the BCO for an updated approval. The architect is a friendly chap though, so I can ask him about covered way stuff if you like

 

Somewhere else, apparently :-). Out of my backside or something I had in a tab from elsewhere.

 

Scrub that paragraph.

Edited July 18, 2017 by Ferdinand

[Ferdinand]

1 minute ago, chrisb said:

Still learning about the forum - closing the tab you're editing in is a bad idea it seems!

 

Flooring - we had decided on stuck down Karndean, but that was based on research ~2yrs ago, so maybe worth a revisit now. Especially as it seems the slab drying time measns we don't have to make any quick decisions!

 

On that note, I was planning on putting some DS18B20 temperature sensors in the slab inside lengths of UFH pipe - same method as OnOff used in his bathroom I think? Should I also be thinking of something to measure humidity in the slab, and if so, what?

 

Thanks for all the input so far

Chris

 

Text being edited usually reappears when you reopen the thread when logged in. 

 

For me, anyway.

 

F

[chrisb]

It did reopen, but with the content of my first reply again, rather than the second. Perhaps I wasn't editing the second one long enough for it to save or something like that.

 

Chris

[chrisb]

Some more work done on this over the weekend. I note @Russ P's thread here: 

The suggested floor construction from @Nickfromwales , @JSHarris & @Onoff is:

 

1. MOT Type 1 whacked

2. Sand blinding whacked

3. 25mm EPS

4. DPM

5. 150mm PIR

6. Thin membrane to stop concrete attacking PIR foil

7. 100mm concrete with mesh and UFH pipe cable tied to it

 

This above seems consistent with @oranjeboom's method too.

 

I would then add:

8. Latex screed done by Karndean (or similar) installer

 

Some questions please:

A. What should go under the MOT Type 1? Is there any alternatives here?

B. Do I really need the sand blind as the 25mm EPS is quite squishy? If so, how thick should the sand be (trying to estimate quantity required).

C. Taking a quick look online, 150mm PIR is more expensive than 200mm EPS, and 200mm EPS will give me more insulation under the UFH than the 100mm that the architect has specified. Does my thinking sound sensible? Should I try and get more EPS in, or swallow the additional cost for PIR?

D. If I do use the EPS, can I put this directly on the sand blind, with DPM on top, directly under concrete? This would save additional labour and cost to lay 25mm EPS and thin membrane.

E. Mesh is currently specified by architect as B785 which comes in at >£100 / sheet. Mentioned here and on the old place is much cheaper A142 (£20/sheet) or similar mesh. Why is my slab likely to need so much more reinforcement? I wonder if adding fibres to the concrete could allow a cheaper mesh? I have 3 intermediate footings in my design - not entirely sure why, neither were the builders when they looked at it.

 

On other points - cold bridges inner leaf to slab - it seems the usual method to alleviate this is to run 25mm EPS all around the edge of the slab, so this should be fine I think?

 

UFH designs - everyone I've looked at / spoken to so far don't seem to understand the concept of UFH pipe tied to mesh. How have others done pipe layouts / circuit designs for this method? Is it worth giving Wunda a call and see what they say? I'm thinking that the proposed rooms (study/utility/garage/new kitchen/diner) will have their own circuits and the existing kitchen will have it's own when I did up the floor and replace it. I'm hoping to be able to do this at the same time as the new floor, but that means dismantling and emptying the kitchen and logistics might not allow that :-(

 

Humidity sensors - worth incorporating into the slab? If so, any ideas on a type / supplier?

 

If I want to run some 15mm PB water pipes and some electrics in the slab, is this going to cause any issues? I was expecting to run the electrics in 20mm PVC conduit (or pull through later). Not sure of the best solution to 15mm PB plumbing though? The flexible conduit offering from John Guest seems very expensive?

 

I'm also likely to want to re-run my mains water pipe to the house in future (it's 20mm black poly at the moment) and possibly the electric cable too. I'm thinking it's wise to run some ducts for these anyway so they are there, even if I never actually use them? Same goes for a water tap in the back garden, I'd like to bring this out underground if possible rather than having to take it through the study and out through the wall. Is there likely to be any issues with this?

 

Thanks for all the advice

Chris

 

[Nickfromwales]

A) Mother Earth mate. You dig a hole, she tells you what's there . 

B) Sand blind is a surface filler for the type one. You don't need much if you've prepped and levelled the type one properly. I suggest the 25mm as an additional protection for the DPM and it can be counted into the insulation layer, making 100mm of eps up out of the 25mm blinding cover, and a further 75mm of eps for the difference. 

C) What is the cost of excavating the extra 50mm depth? If more than the difference in cost of the insulation then PIR it is ?

D) I wouldn't, but eps doesn't absorb water so not really anything to stop that. Tough one to call tbh. 

E) Question the architect, but if that's come from him asking the SE for a design then there must be some merit to it. If not, go direct to the SE and ask him to justify its requirement. 

 

Perimeter insulation. 25mm should be fine, for the money though, go PIR. 

 

Humidity sensors, what for ? When the slab is 'dry', end of the chat.   

 

Run the hot and cold, individually insulated, under the slab. Don't run them in the Ufh heated slab as they'll get warmed and cold water won't be cold. Wrap them with Armorflex, and go around that with duct tape to keep'it sealed. No need for ducting / pipe conduit then. If running electrical conduit go for 25mm so it's easier to pull through. 

 

Others will advice on the ducting for services . 

[PeterW]

53 minutes ago, chrisb said:

Mesh is currently specified by architect as B785 which comes in at >£100 / sheet.

 

Does your architect have steel shares...???!!

 

Never seen B785 in a domestic environment - it is used in heavy slabs for structural floors and other open span designs ..!! If this is a standard enclosed ground bearing slab with a span not exceeding 10m in either direction without support then A142 would be adequate. 

 

If he really wants structural then A252 would be ok but it's overkill unless there is some cantilever..??Has he calculated it as non-ground bearing and spanning over the 3 intermediate walls ..?? 

[chrisb]

@Nickfromwales Thanks. All noted.

 

@PeterW & @Nickfromwales, I think the architect has done a lot of copy & paste, and use of standard drawings, specifically from www.buildingregs4plans.co.uk as the text on my drawings is exactly the same as theirs, and there are other errors too. I suspect that the huge overkill is to use standard details and avoid the need for any SE work. I'll give him a call tomorrow and discuss. http://forum.buildhub.org.uk/ipb/applications/core/interface/file/attachment.php?id=o_1bl8hto6bk0o72l9kpesa4ol16 shows the drawings - I can kind of understand some extra concrete where he *thinks* the TF guys might want to place some steels, but I can't understand the need for an intermediate footing halfway across the rear extension, and under the internal wall between workshop and utility room.

 

The longest dimension is 14m, but in his drawings this is broken up into three separate sections, max 4.6m long each.

[PeterW]

Overkill is an understatement..!!

 

SE's aren't expensive - I'd be concerned at what else he has missed if that's an example. 

[Jeremy Harris]

Our structural slab (the whole house sits on it) is 100mm thick with A142 fabric, plus a narrow, deeper, reinforced ring beam around the edge.

 

PIR is a lot better than EPS, typically 200mm of PIR = 300mm of EPS, in terms of thermal resistance.  150mm of PIR = 225mm of EPS, not 200mm.

[Nickfromwales]

9 hours ago, chrisb said:

@Nickfromwales Thanks. All noted.

 

@PeterW & @Nickfromwales, I think the architect has done a lot of copy & paste, and use of standard drawings, specifically from www.buildingregs4plans.co.uk as the text on my drawings is exactly the same as theirs, and there are other errors too. I suspect that the huge overkill is to use standard details and avoid the need for any SE work. I'll give him a call tomorrow and discuss. http://forum.buildhub.org.uk/ipb/applications/core/interface/file/attachment.php?id=o_1bl8hto6bk0o72l9kpesa4ol16 shows the drawings - I can kind of understand some extra concrete where he *thinks* the TF guys might want to place some steels, but I can't understand the need for an intermediate footing halfway across the rear extension, and under the internal wall between workshop and utility room.

 

The longest dimension is 14m, but in his drawings this is broken up into three separate sections, max 4.6m long each.

Time to start stamping your feet. Get an SE's input ASAP before moving an inch. £££'s

[Onoff]

I'd had enough of excavating one room by hand the after breaking up the existing 60mm too high concrete floor; sub base then the "dirt". Think I went down about  400 mm in all then to build it back up by 325mm and I've still got to tile.

 

So I've got 150mm PIR + 25mm EPS. The kicker, is that with hindsight, if I'd have gone down a bit further aiming for 200mm I might have discovered the mains in water pipe, seen the state of it and replaced it earlier. As it is I've had to reroute because when the old iron pipe started leaking I'd already done the floor.

 

Another "with hindsight" I'd have maybe brought the bathroom soil stack internal and then out under the footings which would have made fitting EWI easier in the future. 

 

I found when excavting a previous clay soil going out. Tbh I'd have needed it further left in the corner but that was the time to have done it!

 

(Most of these pics were on my old eBuild thread).

 

 

To give something square to insulate up to / over I cast a bit of a shuttered ring beam over the old rough footings. The DPC you can just see above the breeze block. That incidentally was the old FFL thus we had damp edges.

 

 

Doing the sand blind and getting that EPS down was pretty satisfying tbh. Felt like things were getting level, clean and tidy. 

 

 

Folding the DPM  was a game!

 

 

And then another milestone to lift the spirits, getting the PIR down:

 

 

I'd never done anything like this before btw and couldn't / wouldn't without the help on here.

 

To core down now in that far left corner for an internal soil stack wouldn't be impossible now but still a pita.

[chrisb]

So, discussed with architect: "this is the way slabs should be built" and many variations on that. Then came around to "if the BCO will accept a ground-bearing slab then there's no reason not to do one". Lots of chat about old-fashioned builders, experience in designing slabs, etc etc.

 

BCO said he'll accept a ground bearing slab, providing soil looks OK when we do the foundation dig. He's familiar with the locality and can't recall any issues with the nearby works done recently. He did suggest making the slab 150mm thick though as its "such a large span" and otherwise it might crack. Given @JSHarris's house is sat on a 100mm slab with A142 fabric, that seems unlikely and 150mm thick is excessive? Unless the ring beam on Jeremy's slab means that there is more support and less tendency to crack?

 

Could adding fibres to strengthen the concrete be an option rather than increasing the thickness?

 

Also, my current design includes three 'intermediate footings' marked in blue rectangles on the image below. I can't see any reason for them if changing to a ground-bearing slab as intended. Am I missing something? What about the 14m long section - do I need to split it up with expansion gaps or one of the footings?

 

(attempting to insert an image below here...)

[chrisb]

So, the build is underway. Old garage removed and taken away, guys are working on drainage right now.

I have been asked about the slab detail. The builder would usually use 150mm hardcore under sand blinding then DPM then EPS. I have the above design. He's confident that the 25mm EPS under the DPM is unnecessary as the blinding will be 25mm.

 

The biggest issue is the hardcore thickness. What is acceptable? Does it really need 150mm hardcore? How should this be calculated?

[Onoff]

I see it that the 25mm EPS provides protection from sharps coming up through pthe sand layer and penetrating the DPM. The first foot fall on the DPM will leave a big depression. At least with EPS you've some protection against that.

Edited September 20, 2017 by Onoff

[chrisb]

1 minute ago, Onoff said:

I see it that the 25mm EPS provides protection from sharps coming up through pthe sand layer and penetrating the DPM. The first foot fall on the DPM will leave a big depression. At least with EPS you've some protection against that.

 

I don't disagree @Onoff, and I may get what I asked for. I'm more concerned in the first instance on the hardcore thickness as there is significant cost /work involved.