Basic garden room insulated slab

[daiking]

There are some threads on this sort of thing but those look at a more complicated design. Extra floor loading or higher insulation spec.

 

Im looking at a 4.15 x 3.3m log cabin, 45mm walls, DG windows, approx 1t weight on supplier details
 

I was intending to insulate the floor and roof

 

My wife has a preference for a concrete slab floor and I am keen to prevent this being a large heat sink 

 

There is scope for 25mm PIR floating floor on the slab but I would like to put something in the slab as I don’t think that will be enough.

 

I am not trying to build a passive house just making cheap simple gains.

 

The slab would just be the exact size of the cabin.

 

What I am interested in knowing (and what I need to specify to a contractor) is the slab build up.

 

It’s fairly easy to say something like 100mm compacted hardcore, 100mm EPS 100 and 100mm concrete but are there any details to be aware of?

 

Should the insulation be tapered down at the edges so there’s thicker concrete? (But 1t mass over a 14m perimeter sounds like very little load even with extreme load cases). How would you protect the EPS edges without a foundation wall?

 

I realise there will be a cold bridge around the edge but 14m x 100mm = 1.4 sq m of cold bridge vs approx 13sq m of floor insulated. Doesn’t that make the significant difference?

Edited November 16, 2020 by daiking

[PeterW]

I would use an engineering brick wall all the way round on a 300 x 150 foundation, 3 courses (225mm) deep. Then level and blind this with 25mm EPS and create an upstand with 25mm EPS too, DPM, 100mm EPS and finally 100mm concrete in the middle. Solid but insulated and cheap to do. 

[daiking]

14 minutes ago, PeterW said:

I would use an engineering brick wall all the way round on a 300 x 150 foundation, 3 courses (225mm) deep. Then level and blind this with 25mm EPS and create an upstand with 25mm EPS too, DPM, 100mm EPS and finally 100mm concrete in the middle. Solid but insulated and cheap to do. 

This sounds like a proper job and therefore  will not be cheap. What price would you expect for that? 
 

Dig out founds 

mix and pour 0.7m3 concrete

lay 200 bricks 

dig out base

eps/dpm

Mix/pump 1.5m3 concrete and level (I also have doubts about the competence of the screeding too)

I think I would be lucky to see change out of £2500

 

Is there a slightly less thorough option?

 

(This is why I have previously favoured a timber frame base but her indoors is not having it)

Edited November 16, 2020 by daiking

[joe90]

13 minutes ago, daiking said:

This is why I have previously favoured a timber frame base but her indoors is not having it

why is she so against a wooden floor?, will she be using the room? Tell her concrete will be much colder!, tell her concrete blows the budget (unless she pays fir it ?).

[PeterW]

There is 3 days work there maximum - it’s a conservatory base in effect and they don’t take any longer and tbh it sounds like you’re digging in garden rather than patio which is easier. 
 

Bricks and mortar are £150, concrete (on a volumetric mixer) would be £350, 6 sheets of EPS £100. 
 

If that was more than £1500 tops I would look elsewhere. 

[daiking]

5 minutes ago, joe90 said:

why is she so against a wooden floor?, will she be using the room? Tell her concrete will be much colder!, tell her concrete blows the budget (unless she pays fir it ?).

I was asked if a timber floor would hold 20 people...

[joe90]

4 minutes ago, daiking said:

I was asked if a timber floor would hold 20 people...

Tell her Timber floors have been used in houses fir donkeys years with no problems!,!

[daiking]

6 minutes ago, joe90 said:

Tell her Timber floors have been used in houses fir donkeys years with no problems!,!

I know, our house has downstairs timber suspended floors. 

[daiking]

12 minutes ago, PeterW said:

There is 3 days work there maximum - it’s a conservatory base in effect and they don’t take any longer and tbh it sounds like you’re digging in garden rather than patio which is easier. 
 

Bricks and mortar are £150, concrete (on a volumetric mixer) would be £350, 6 sheets of EPS £100. 
 

If that was more than £1500 tops I would look elsewhere. 

?‍♂️ Round here, money does not go far.

[Bitpipe]

Just now, daiking said:

I was asked if a timber floor would hold 20 people...

 

I was reading log cabin blogs during the summer. A dead flat concrete base was recommended but the cabin itself sat on a ring beam. This firm only supplied wooden floors as an extra over as not everyone wanted them, but they were not structural and just rested on the slab.

[joe90]

13 minutes ago, daiking said:

I was asked if a timber floor would hold 20 people...

so the answer is YES!

[Bitpipe]

6 minutes ago, daiking said:

?‍♂️ Round here, money does not go far.

 

I'd get a spade then and tell her to get digging

 

What is your plan B, posts on slabs with a timber ring beam & deck?

[PeterW]

19 minutes ago, daiking said:

?‍♂️ Round here, money does not go far.

How much does your average brickie charge up there ..??! That’s £300 a day for grounds/bricks. That isn’t a cheap price (I would be paying £210 in comparison in the Midlands) so it may be worth looking around. 
 

None of that is not doable on your own - replace the bricks with blocks and clad with board and you’ll never see it. 

[joe90]

Concrete pads and these.....https://www.strongtie.co.uk/products/detail/adjustable-elevated-post-base/290

[daiking]

22 minutes ago, Bitpipe said:

 

I'd get a spade then and tell her to get digging

 

What is your plan B, posts on slabs with a timber ring beam & deck?

 

It was always my plan A as its more easily DIY-able than that quantity of concrete and easy to fully insulate - although I don't know what I would prop it on nor how easy that would be to level as a first timer. but for now a concrete slab is the line of enquiry.

Edited November 16, 2020 by daiking

[daiking]

10 minutes ago, PeterW said:

How much does your average brickie charge up there ..??! That’s £300 a day for grounds/bricks. That isn’t a cheap price (I would be paying £210 in comparison in the Midlands) so it may be worth looking around. 

 

Who knows but things are never cheap here - I bet the upmarket bit of the Midlands isn't the same as the normal bits. That's the problem here, a location premium. 

 

Not sure show these compare but: 

 

We've just paid £1150 to dig out 6 fence posts, put 7 or 8 tall ones back in with new 1' gravel boards and 12 new 5'6" panels. Took 2 guys about a day. 

£150 to level some ground for a 8x6 shed, laying 6 redundant gravel boards as a base (no sub-base) + chops a couple of small trees (5" holly and 3" laurel the biggest)

£400 to dig out a fence post and 3 new ones with 2 new tall panels, trickier position. They stretched the other jobs over 2 days

 

10 minutes ago, PeterW said:

None of that is not doable on your own - replace the bricks with blocks and clad with board and you’ll never see it. 

 

Realistically, its not doable by me on any sort of acceptable timeline. 

Edited November 16, 2020 by daiking

[PeterW]

24 minutes ago, daiking said:

Realistically, its not doable by me on any sort of acceptable timeline. 

So you’re back to TQC and choose two - and cost is your only option. 
 

Suspended floor, 6” joists at 450 centres and 22mm floor boards is your other option all resting on concrete pads and it won’t go anywhere - that’s your only other option

[daiking]

11 hours ago, PeterW said:

So you’re back to TQC and choose two - and cost is your only option. 
 

Suspended floor, 6” joists at 450 centres and 22mm floor boards is your other option all resting on concrete pads and it won’t go anywhere - that’s your only other option

 

If there was an almost as good way to concrete without the wall I might look at managing it myself

[Temp]

You dont really want the timber sitting on a slab unless the slab is above ground level and the shed is larger so water doesn't sit on the top and wick under.

 

What I would do is build a slab slightly oversize at ground level or slightly above. Then lay two courses of engineering bricks, a DPC and shed on top. Then inside put a sand blind and DPM, then however much insulation you want, and an osb floor or similar on top. Some sites recommend a VCL layer under the OSB.

 

One course of engineering bricks is about 75mm tall allowing for say 55mm insulation and 18mm OSB or 50mm insulation and 21mm OSB. Two courses would be better at about 150mm high allowing 120mm insulation and 20mm OSB.

 

The cost difference is really just the engineering bricks. I calculate one course is about 135 bricks and two about 270. So around £70/£140 at Wickes retail price. 

 

 

[Temp]

13 hours ago, daiking said:

 

If there was an almost as good way to concrete without the wall I might look at managing it myself

 

A course or two of engineering bricks is easier than doing the concrete. 

 

I had to do mine twice - after laying a course of bricks i discoved the bag of "ready mixed mortar" i thought I was using actually had a bag of cement in the bottom.  ?

 

[daiking]

26 minutes ago, Temp said:

You dont really want the timber sitting on a slab unless the slab is above ground level and the shed is larger so water doesn't sit on the top and wick under.

 

What I would do is build a slab slightly oversize at ground level or slightly above. Then lay two courses of engineering bricks, a DPC and shed on top. Then inside put a sand blind and DPM, then however much insulation you want, and an osb floor or similar on top. Some sites recommend a VCL layer under the OSB.

 

One course of engineering bricks is about 75mm tall allowing for say 55mm insulation and 18mm OSB or 50mm insulation and 21mm OSB. Two courses would be better at about 150mm high allowing 120mm insulation and 20mm OSB.

 

The cost difference is really just the engineering bricks. I calculate one course is about 135 bricks and two about 270. So around £70/£140 at Wickes retail price. 

 

 

 

The idea is to get the slab the exact size for the cabin so it shouldn't have the problem of water pooling at the base timber and put a french drain around it so I'm not that worried about the base in contact with the slab. 

 

21 minutes ago, Temp said:

 

A course or two of engineering bricks is easier than doing the concrete. 

 

I had to do mine twice - after laying a course of bricks i discoved the bag of "ready mixed mortar" i thought I was using actually had a bag of cement in the bottom.  ?

 

 

As someone who has never mixed concrete or mortar before I find it hard to believe that it will be easy to build approx 15m of very short wall completely level. Creating a level shuttering actually seems a lot easier.

[joe90]

4 minutes ago, daiking said:

I find it hard to believe that it will be easy to build approx 15m of very short wall completely level

not everyone can lay bricks level, yes poured concrete with shuttering will be easier to get level. Just get some rebar in it to make it strong.

[Temp]

12 hours ago, daiking said:

As someone who has never mixed concrete or mortar before I find it hard to believe that it will be easy to build approx 15m of very short wall completely level. Creating a level shuttering actually seems a lot easier.

 

My outbuilding base was about 6mx5m. The shuttering was easy and I had ready mixed concrete delivered. The hard part was raking and levelling the concrete. Wife and I tried to  slide boards back an forth across top of shuttering. That was much harder work physically than expected. We struggled. I had to frequently wade in and rake concrete to move it around otherwise we couldn't move the levelling board. Got it done but it took more than one beer to recover.

 

The two courses of engineering bricks was the first bricklaying I had ever done. I set up strings on posts as guides for line and level. It was pretty easy. Put a bit too much mortar on then tap the brick down until level with the string and scrape off excess mortar. Easier than tiling a wall or floor. 

[PeterW]

If you’ve never laid bricks then buy one of these.
 

https://bricky.com/building-tools/bricky/?gclid=EAIaIQobChMIoMTU7NeL7QIVIe_tCh39GQ7FEAQYASABEgJ9dvD_BwE

 

I was pleasantly surprised how good they are - no use for anything non-standard or reclaims but great for smooth engineering bricks. 
 

 

[joe90]

Or you could cast a ring beam on an mot base, with rebar in it instead of bricks. Blind with sand, DPC, infill with foam insulation and floating wood floor on that.