EPS Slab Design

[Bitpipe]

Our contractors effectively treated the EPS slab as if it was level ground (i.e. they ignored it) and built their traditional steel lattice & formwork / shuttering on top. Slab and a 100mm kicker first and then built off this as they did the walls in sections. The kicker is necessary to hold the vertical formwork straight on the slab. Water bar on every join (inspected by the Sika rep) and Sika additive in the concrete as this was our sole waterproofing method.

I don't think any serious concrete crew will entertain EPS upstands for a basement slab as they would get trashed when doing the walls - the formwork they use is very heavy duty and is craned into position for each wall pour.

In my mind, EPS formwork only really works for ground level slabs or ICF where traditional formwork is not being used. For a basement you need to use traditional formwork and apply the EPS to the cast walls afterwards. 

[Bitpipe]

This the data sheet that  I shared with our BCO and SE, they used it to agree the required grade for under slab (EPS 200).We used EPS70 on the sides as there is no load bearing there. 

http://www.insulatio...Information.pdf
 

[Bitpipe]

The EPS 200 was for under our basement slab which was 122m2 x 300mm RC with just under 50 linear m of 3m high of 300mm thick RC walls - 12+ tonnes of steel in that plus about 80m3 of concrete. Plus the TF house on top.

I ordered 49 sheets (2.4 x 1.2) which worked out just right as Hillard (not our SE but I used him for advice on the EPS) recommended a min 200mm toe between the insulation and edge of slab which we just about met in places and exceeded in others. Just laid whole sheets (on base of 150mm compacted type 1 and 50mm blinding) didn't bother with any cuts and then covered with a high density DPM, taped at the seams and tucked down at the edges. Groundworker filled in the edges with more type 1 to give a flat surface.

Each sheet weighed about 35kg, not super heavy but tricky to manoeuvre and you need a bit of space to store them.

One note, the footprint was originally marked out on surface by a surveyor using a Total Station. Groundworker then translated the pins outside the excavation area and dug down, with an additional 1m working space all round. When the surveyor re-marked out the slab ahead of concrete works, we saw that the footprint we were working to for laying the EPS was about 100mm off true but it wasn't an issue as we'd allowed for plenty of toe all round. 

[vivienz]

Can I resurrect this topic, please?  OH and I very much want to include a basement in our new build, but it's difficult to find much information on them and what we do isn't that informative.

 

I have spoken to a few kit house manufacturers and they all put this work out to an alternative company (usually Glatthaar) who seem to be phenomenally expensive, albeit for top quality, or so I'm told.  As I understand it, the two main issues are ensuring that it's watertight and also that it needs to be constructed to within very small tolerances if there is going to be a timber frame house built on top of it. 

 

Ideally, I'd like to start trying to price up the cost of a basement to determine how realistic it is to have one, i.e. is it affordable within our total budget.  Can anyone give me any pointers, please?

[TerryE]

Sounds like you need to hook up with @Bitpipe. And also research all of his posts on this. Drop him a PM; he might be willing to have a 1-1 Skype / telephone chat, though I clearly can't make any promises on his behalf 

[vivienz]

Thanks, Terry.  Will do.

[Bitpipe]

On 19/10/2016 at 18:57, TerryE said:

Sounds like you need to hook up with @Bitpipe. And also research all of his posts on this. Drop him a PM; he might be willing to have a 1-1 Skype / telephone chat, though I clearly can't make any promises on his behalf 

 

 

 

Terry is my booking agent, gets a 10% fee

 

Happy to help - I posted a lot on ebuild on our basement and I think some of the content made it over here.

 

We also started down the Glatthar path and while they are a fine company with a high quality product (friends have used them), they are pricey.

 

Here's our story:

 

1) we got a SE to design the basement  - design was simple, an open box that followed the upper wall footprint with some lateral steels to support a suspended floor and point loads. We already had the MBC frame design in hand at this point so that was their point of reference. After construction, we framed out the interior in timber to give us some future flexibility in layout.

 

2) SE needed a ground investigation report and drew up the spec for that, we both shopped around for a few quotes before choosing a reputable firm. Unfortunately they had poor supervision on site and the subsequent report was a bit of a mess, needed to be re-done to collect more data (which I had to pay for, albeit at a discounted rate). But we finally got what we needed and the engineer did the necessary calcs and drawings which I took to building control and shopped round local groundworks contractors  - NOT basement specialists. EPS under the slab is chosen based on the loading per m2 in N - EPS 200 grade met our requirements. The side insulation does not have any vertical load on it so we used EPS 70. Thickness is up to you, makes no structural difference purely thermal. We chose 300mm for the slab and 200mm for the walls.

 

3) waterproofing strategy depends on the conditions. We did not hit water until 4m so well below the slab. Our ground is clay, gravel and chalk so free draining under the basement. Engineer specced a land drain around the slab to soakaway and 1m of stone backfill. We were happy to just use warrantied waterproof concrete (Sika)  - the internal membranes are expensive, especially when pumps etc are required and I was wary of the external membranes (how do you every prove they are the cause of a leak?).

 

4) Engineer was very conservative on the size of the excavation required but we spoke to a few ground workers and they said that 1m working room with a small 45degree batter at the top was standard - this is important as party wall law for basement is concerned with where the excavation starts not where the wall starts. Was pretty easy to conclude that we would be ok based on the 45 degree rule so I took it no further and the neighbours never raised it. Up to this point, it's hard to be definite on costs as you may have some scary ground conditions that need piling or other specialist work - so you take the risk that you may get this far and need to walk away if you can't afford the cost.

 

5) we shortlisted two contractors, both are essentially muck movers and sub to specialist concrete teams. Neither had a clue on the EPS but were happy to prep and let me lay it or do it themselves at a day rate. Excavation and muck away was about 1/3 of cost - if you can loose spoil on site then you'll make a big saving. Concrete prices were broadly similar, complex structures will cost more as there is a lot of skilled joinery in making the formwork. We bought GRP lightwells from MEA in Germany - effectively the contractor leaves a window hole in the wall and the light well is fixed during back fill. Window goes in later with rest of building. Would have been quite expensive to get concrete lightwells cast in-situ.

 

6) We picked a contractor and got to work. I purchased the EPS via a buyer who got me the best deal, but you can get prices from SIG and other places. Laying it was not complicated - ground worker compacted 150mm of type 1 and then laid 50mm sand, blocks were just laid on top, allowing for 200mm min extra all round. We didn't need to cut blocks at this stage. EPS was covered with a membrane, tucked down and taped and the gaps between the EPS and excavation were filled with type 1 to give the concrete team a level working space. 

 

7) Slab was laid out in steel and poured and then the walls were cast. Regular inspections from Sika and a final sign off at the end. You need to give some thought on service penetrations for fouls and water, electricity etc - normally an oversized duct is cast in place and the eventual service will run inside with the gap sealed up.

 

8) I then applied the EPS 70 to the sides using LE foam to adhere and fill gaps. This was clad in corex to minimise damage during backfill. Used a small chainsaw to cut it - messy but quick. Land drain installed and then backfilled with stone as per spec. Half way through this I put the light wells on. Top was 200mm graded flat with muck ready for the scaffolders and frame erection.

 

Was probably one of the most straight forward parts of the build, once all the SE work and GI frustration was resolved. Needless to say, we love our basement, great decision that gives us 50% extra floor space for about £1000/m2.

[vivienz]

Fabulous - thanks, Bitpipe.  We're not after anything complicated for the basement, just a large box that will be subdivided into smaller boxes.  We will want to run some services into it, though, as we're planning to have GSHP and will put the plant room into the basement rather than use any of the main living area on the ground floor.

 

Looking at your method, I've been coming at this from the wrong point of view and trying to deal with the basement first and then the house, rather than the house first which is the logical approach given that the basement may be the thing that has to go if budgets are too tight.  MBC certainly seem to get a good press on this forum for the house construction and if they're happy to work with this type of construction, all to the good.  A few of the kit house manufacturers I've spoken to have been very iffy about basements citing the small tolerances of the floor slab that are needed for any pre-fabricated structure to sit on top.  What was your experience of this with yours and MBC?

[Bitpipe]

2 hours ago, vivienz said:

Fabulous - thanks, Bitpipe.  We're not after anything complicated for the basement, just a large box that will be subdivided into smaller boxes.  We will want to run some services into it, though, as we're planning to have GSHP and will put the plant room into the basement rather than use any of the main living area on the ground floor.

 

Looking at your method, I've been coming at this from the wrong point of view and trying to deal with the basement first and then the house, rather than the house first which is the logical approach given that the basement may be the thing that has to go if budgets are too tight.  MBC certainly seem to get a good press on this forum for the house construction and if they're happy to work with this type of construction, all to the good.  A few of the kit house manufacturers I've spoken to have been very iffy about basements citing the small tolerances of the floor slab that are needed for any pre-fabricated structure to sit on top.  What was your experience of this with yours and MBC?

 

We have electric, water and phone coming into basement plant room and fouls from kitchens and bathrooms exiting. Gas is the only service that needs to come in above ground, ours then runs down into the plant room where the boiler and UVC are, plus elect distribution and MVHR.

 

There are two basement approaches - an open box, where the floor is suspended timber (joists and deck) or a full box where the floor is cast in situ concrete or bison floor (or similar) that is screeded.

 

If done properly, either should give you the tolerances you need. We did the former and had 2-3mm tolerance across an 11m run at the wall top. Pockets formed for steels (we had a steel lattice over the walls that the joists connected into, designed by our SE and supplied by MBC) had 25mm tolerance all round. MBC did come out prior to frame manufacture after the basement was cast to see if we needed to tweak anything, a few pockets needed small adjustment to get everything where it needed to be.

 

Cant' see why the rc lid / screed option wouldn't be every bit as good as a normal raft. I'd say its a toss up cost wise. You will probably need to commit to structural walls or column in the basement to support a cast roof but the walls can perhaps be thinner (250 or 200mm vs 300) as the lit provides additional rigidity.

 

We put UFH in alu spreader plates once the floor was joisted out and before it was decked. Did not put any UFH in the basement itself - its a pretty constant temp there year round, very slightly cooler than the ground floor.

 

I was happy to use MBC but there are plenty of others so do your research and choose who works best for you. Ironically, the reason many choose MBC is because they offer a frame and slab package and remove uncertainty and risk around the interface - this didn't apply to me !