Building into a sloping site. Methods and details.

[Dee J]

Still negotiating on a plot... but I'd like to get my head around current construction methods and detailing.

So in it's most basic form the plan is for a single story flat roofed bungalow, but where the land slopes away having an undercoft (ie partially dug into the hillside) with two bedrooms and a bathroom.

What I'm looking for is some useful, guide/website/book that shows the foundation/slab/retaining wall/insulation/damp proofing interconnections including those tricky transitions at the sides between the buried structure and the visible outer skin. 

At the moment, for the main portion of the house I'm looking at passive slab, structural block walls with external insulation and rainscreen cladding and a beam/block or cast in situ inverted warm roof. Height is key aspect of the planning - so minimising structure and maximising internal height is key.

Suggestions, guidance, criticisms, examples please.

Thanks

Dee

[ToughButterCup]

How steep is the slope @Dee J

I have a gentle slope... no need for any adaptation, @JSHarris blog details how he dug into his hillside, @Bitpipe took things a step further. There's a thread which details a really steep slope  (search > funicular or railway) @jamiehamy has this......

Ian

 

 

 

[Crofter]

Just to clarify, is your access at the top of bottom of the slope?

I'm building on a fairly steep (1:7) slope with access at the top, and chose to raise the house on piers rather than dig it back into the hill. The access would have become almost impossibly steep if the house was any lower.

[Jeremy Harris]

For the dug in part, then I would look at basement-type construction, perhaps an externally insulated reinforced concrete system, from one of the specialist companies that understand how to build watertight and well insulated basements.  There are a few systems around now, a look at how @Bitpipe built his house on top of a large insulated basement might help, even though I'm guessing that you will have at least one open side, the technique is much the same.

 

From then on you just cap the ground floor with whatever system you wish (beam and block might be a good choice) and build the above ground part as you wish.

 

The retaining wall part of the underground part needs careful engineering, and can add a great deal to the cost, depending on the soil type and the local water conditions.  We had to build a large retaining wall, around 35m long and 2.5m high and the cost was over £30k, excluding some of the excavation costs.

[Gone West]

You need to consider orientation carefully and it's effects. As Jeremy @JSHarris found digging in could create an unexpected micro-climate which may have a large impact especially with a low energy house.

[Dee J]

Thanks for the quick replies.

The plot is accessed from the top, and most of the build area is fairly flat, but then drops steeply enough to allow the pair of rooms to be set under the end of the main build... about 2.5m drop in 6m. So the bedrooms will have level access into the garden. - so the undercroft/basement construction is under about 1/3 of the main house. Retaining wall will have to be about 8m run of 2.5m high.

[Bitpipe]

I will say that few contractors seem know much about external insulation but they do know how to build watertight structures appropriate to the conditions, insulation is not that hard to DIY.

 

Rather than go to a specialist basement firm, get your architect and / or SE to do the design and then talk to ground-workers and/or concrete structure contractors.

 

I simply laid a slab of EPS 200 blocks (as speccd by SE) covered in membrane which they built off and then applied EPS70 blocks to the sides once the concrete was cast. This created a 'wrapper' around the basement, mimicking the MBC insulated slab design, and the insulated timber frame structure sat on top.

 

An alternative is to use ICF but you will still need to insulate under your slab first.

 

[Dee J]

5 minutes ago, PeterStarck said:

You need to consider orientation carefully and it's effects. As Jeremy @JSHarris found digging in could create an unexpected micro-climate which may have a large impact especially with a low energy house.

All the main glazing is slightly north of east, so getting enough light (and heat) into the house will be an issue. Already thinking skylights and light-pipes.

[Jeremy Harris]

The sun rises well North of East in summer, so you will get a lot of early morning light and solar gain.  We have found that we get more solar gain from the windows on the East than those on the South or West sides.

[Ferdinand]

Three Grand Designs episodes which may be useful to rewatch:

 

1 - The house in the Cotswold built *under* an old barn - called Underhill House. 1st UK Certified Passivehaus.

2 - The one in Bath where a couple of Doctors spent £300k+ on their excavations.

3 - The one in Penzance where the young Interior Designer such a double garage underground at the bottom of her garden.

4 - Wasn't there also an early strange one in Cheltenham built below ground in the back garden of an elegant terrace?

5 - The one in the Lake District built into an old quarry where they skimped on light-tubes and ended up with indoor gloaming.

 

Ferdinand

(Edit: Item 4 added. Aelf-build projects always expand beyond the design)

(Edit: and item 5)

Edited October 4, 2017 by Ferdinand

[jack]

6 minutes ago, Ferdinand said:

2 - The one in Bath where a couple of Doctors spent £300k+ on their excavations.

 

Ooh, I remember that one. That's the one where they used a German prefab company to do everything except the cladding, which was handled by a British company. The German company finished on time and on budget, but the cladding still wasn't on months and months later.

 

They also insisted on having a fire, against the advice of the German builder. From memory, they admitted they'd never lit it as the house was always warm enough.

[Crofter]

How does a light-tube compare to a PV panel and LED bulb? OK it's not 'real' light but it would be interesting to work it out in terms of installation cost and heat loss.

You need the LED bulb anyway, so that's no extra cost. What's the U-value of a light tube?

 

(Sorry just realised this is probably of no relevance to the OP!)

[Jeremy Harris]

3 minutes ago, Crofter said:

How does a light-tube compare to a PV panel and LED bulb? OK it's not 'real' light but it would be interesting to work it out in terms of installation cost and heat loss.

You need the LED bulb anyway, so that's no extra cost. What's the U-value of a light tube?

 

(Sorry just realised this is probably of no relevance to the OP!)

 

Light tubes seem to work well, but few seem very good thermally, and the one house I visited that had them suffered badly from the top part getting covered in internal condensation in cool weather, enough to drip down into the house.

[ProDave]

4 hours ago, Dee J said:

Thanks for the quick replies.

The plot is accessed from the top, and most of the build area is fairly flat, but then drops steeply enough to allow the pair of rooms to be set under the end of the main build... about 2.5m drop in 6m. So the bedrooms will have level access into the garden. - so the undercroft/basement construction is under about 1/3 of the main house. Retaining wall will have to be about 8m run of 2.5m high.

 

How much have you looked at the plot? Do you know its history?

 

For our first build, we looked at a plot like that, with a higher level close to the road, then dropping off to a lower level, and the PP said the house must be built on the upper level (lower level was a flood plain).  But when looking around the site, it became very apparent that a lot of soil had been tipped over the edge of the slope, to try and make the upper building level look much larger than it was. That loose soil would have been no good to build from.

[ProDave]

3 hours ago, Crofter said:

How does a light-tube compare to a PV panel and LED bulb? OK it's not 'real' light but it would be interesting to work it out in terms of installation cost and heat loss.

You need the LED bulb anyway, so that's no extra cost. What's the U-value of a light tube?

 

(Sorry just realised this is probably of no relevance to the OP!)

We had a light tube (I believe the brand was Solar Tube) in our last house. It worked very well and never had any condensation issues. No doubt it leaked some heat, and was probably not that good for air tightness.  The bit in the ceiling was very much like a low profile flush light fitting with diffused plastic, and confused a lot of B&B guests who said they could not find the light switch to "turn the light off"

[Stones]

@Dee J you could have a look at @iSelfBuild  facebook page which documents his project. He has done something similar to what you are describing.

[ToughButterCup]

3 hours ago, jack said:

 

[...]

The German company finished on time and on budget, but the cladding still wasn't on months and months later.

[...]

 

Naturlich Junge!  Deutsche Firmen sind immer zuverlassig!  (Reine Quatsch)

[Redoctober]

11 hours ago, Ferdinand said:

Three Grand Designs episodes which may be useful to watch:

4 - Wasn't there also an early strange one in Cheltenham built below ground in the back garden of an elegant terrace?

 

I recall this particular one - I think it became difficult to sell and also had an impact on the main house whose garden it sat in. -Sorry for going off topic!!

[mvincentd]

I'm building very much like the scenario you outline. Too early to say if my approach will have been a good one.

 

There's a fair chance your average architect, detailer, building control officer and builder won't have encountered many of the details and as a result those details will potentially take considerable effort to work out.  It depends on your own mindset, skillset and time involvement but i'd caution against getting working drawings from anyone who hasn't 'built this before'.  I've had to trouble shoot my drawings every inch of the way and thats been incredibly wearing and expensive.

 

Your structural engineer will end up as a major factor...dont penny pinch and find one you can get a good rapport with and involve him very early.

 

The size of your house relative to the plot may be a big factor in ease of build-ability and cost on a sunken house in a sloping site and will also potentially dictate some of your 'chosen' building methods.  Don't expect it to be cheap.

 

Someone here a few weeks ago already mentioned Charlie Luxtons current personal project; https://www.youtube.com/watch?v=-hiRazqQFrU&list=PLwMzOUIf2_b6FIG18FbvgQZUrBMxOsO_r

 

[Dee J]

On ‎04‎/‎10‎/‎2017 at 09:16, Bitpipe said:

I will say that few contractors seem know much about external insulation but they do know how to build watertight structures appropriate to the conditions, insulation is not that hard to DIY.

 

Rather than go to a specialist basement firm, get your architect and / or SE to do the design and then talk to ground-workers and/or concrete structure contractors.

 

I simply laid a slab of EPS 200 blocks (as speccd by SE) covered in membrane which they built off and then applied EPS70 blocks to the sides once the concrete was cast. This created a 'wrapper' around the basement, mimicking the MBC insulated slab design, and the insulated timber frame structure sat on top.

 

An alternative is to use ICF but you will still need to insulate under your slab first.

 

I've had a look at many of suggested sources - and this looks the nearest to what I am imagining. ... The whole slab and retaining wall is within the insulating wrapper. 

But there are still some details I'm trying to sort out.... 1) being a semi-buried structure (set into a slope), the detailing of the transition between below and above ground - protecting the insulation at the transition and the detailing of any cladding above ground. 2)The semi basement is only under about 1/3 of the main house - so how to integrate the insulated retaining wall into the insulated slab for the rest of the build - in effect trying to avoid the building 'hogging' or 'breaking it's back' over the hard line formed by the retaining wall.

Edited October 5, 2017 by Dee J

[Grosey]

Not sure if this is any assistance but I’ve attached a few photos of my retaining wall. May help you visualise things as you enter discussions with SE’s and architects etc. 

Edited October 5, 2017 by Grosey

[Bitpipe]

45 minutes ago, Dee J said:

I've had a look at many of suggested sources - and this looks the nearest to what I am imagining. ... The whole slab and retaining wall is within the insulating wrapper. 

But there are still some details I'm trying to sort out.... 1) being a semi-buried structure (set into a slope), the detailing of the transition between below and above ground - protecting the insulation at the transition and the detailing of any cladding above ground. 2)The semi basement is only under about 1/3 of the main house - so how to integrate the insulated retaining wall into the insulated slab for the rest of the build - in effect trying to avoid the building 'hogging' or 'breaking it's back' over the hard line formed by the retaining wall.

 

I may be being simplistic here but I'd expect the insulation layer to follow the horizontal and vertical elements of your substructure.

 

You will need to spec the insulation to deal with the point loading - in our case under the slab was EPS200, sides were EPS70 as no vertical load was presented onto them by virtue of the MBC timber frame design (inner leaf is load bearing and sits on slab perimeter, outer leaf is not and rests on insulation) and the load from the backfill was negligible - we are a flat site.

 

They way I did it was to oversize the under slab insulation to create a 'toe' around the slab, covered in membrane and the team built off of that as it it was solid ground. Underneath was 150mm compacted type 1 and a 50mm layer of sharp sand to get a nice flat surface for laying the blocks. They don't need securing as they're quite heavy and wont slide about.

 

When the walls were cast, the vertical insulation sat on the slab insulation toe and finished flush with the wall top.  Used lots of low expansion foam, a decent gun and soaked the concrete and EPS before application as that improves adhesion. Small electric chainsaw used for cutting (messy!) but I tried to minimise cuts, especially on the EPS200 as it's quite solid.

 

I didn't need to, but you could wrap the external insulation with waterproofing membrane if needed.

 

Whether this approach works for you really depends on the design of the superstructure and where the insulated layer is there - you obviously want them to line up so there is a continuous insulated layer around the whole house. 

 

The render system projects about 50mm off the outer leaf of timber frame so below that I am currently dressing the exposed EPS 70 with a 200mm strip of 25mm EPS, tucking the DPM between the two layers, and will finish this with a 200mm strip of Ubiflex (lead substitute) which will provide a rodent proof barrier. Others have used aluminium strip (the type used to form guttering) or UPVC fascia boards.