John’s workshop

[JohnH]

Mike, would your ‘concrete lintels laid on compacted hardcore’ method be suitable for a heavier roof (eg concrete-tiles) on a workshop my size? Sorry if that’s a stupid question!

 

[Mike G]

Mike, would your ‘concrete lintels laid on compacted hardcore’ method be suitable for a heavier roof (eg concrete-tiles) on a workshop my size? Sorry if that’s a stupid question!

Yep. They'd need to be chunkier than the ones in your drawing.....but then they'd need to be bigger anyway to get the timber frame further off the ground.

 

[JohnH]

Yep, understood - 100x150 lintels. This certainly seems like the most economical route; just been looking at the cost of ground screws - even as a DIY installation it ain’t cheap…

Anyway, plenty to occupy my thoughts with for the time being!

Cheers, have a nice evening.

 

[JohnH]

Evening all, thought I’d share some updates. Well, still no actual building work has taken place - but I have dismantled the old shed at the back of the garden and carried it down to the allotment, where it’s been re-erected and re-purposed as a potting shed. So the existing slab is now clear(ish) and thoughts are once again turning to how best to go about casting a new slab in its place… A big consideration will be height/headroom, if the workshop is to be constructed within PD limitations. However, I’ve applied for planning permission to gain some extra height (3.2m ridge line) so we’ll see…

For the planning application, I did the drawings myself and cobbled together a supporting statement. I consulted the neighbours and the eventual plans were based on (understandable) concerns that one of them had about being overshadowed. I had originally designed a fairly standard-looking symmetrical dual-pitched roof but ended up with an asymmetrical one so as to move the ridge line away from this neighbour. I guess it’ll take a bit more head scratching to construct, but I quite like the look of the new design:


The resulting off-centre doors also suit the situation of the workshop within the garden better (away from the beds and more central to the lawn).
Here’s a pretty rough ‘n’ ready rendering (from back when the garden was much neater and less full of junk…):



Anyway, the Planning Officer came over the other day and literally spent two minutes in the garden, taking a couple of snaps. I should get a decision within the next 4 weeks. Fingers crossed!

 

[JohnH]

Here are the plan views too, might be of interest to someone who’s wondering what level of detail/dodgy draughtsmanship was considered acceptable by the planning dept:

 

[Steve Maskery]

I hope that your BCO is as helpful and constructive as mine was when I built SCW* He solved more than one unfrrced errorsfor me. Great bloke.
Famous quote: "I go to see houses that are not being as well-built as this is".
I'll take that.

S
*Steve's Cathedral of Woodwork, (Copyright Pete Maddex).

 

[JohnH]

I hope that your BCO is as helpful and constructive as mine was when I built SCW* He solved more than one unfrrced errorsfor me. Great bloke.
Famous quote: "I go to see houses that are not being as well-built as this is".
I'll take that.

S
*Steve's Cathedral of Woodwork, (Copyright Pete Maddex).

Compared to your cathedral, Steve, my humble workshop* is a mere priest hole (about 13.5m2) - so no BCO involvement. I’m relying on you lot to solve - or, even better, prevent - any errors (of which there’ll be many) on my build!

* referred to as a ‘garden room’ in my planning application, in an effort to sound less noisy, dirty, dusty, etc

 

[JohnH]

Afternoon all, some thoughts about the workshop base I’d like to run past you. I had been thinking about using Mike G’s concrete lintel approach (ie no concrete pour), but I’ve got my heart set on having a brick plinth upon which to plonk my timber frame (for durability and aesthetics), so have been thinking of options for a new slab. This is my current thinking about it…

The footprint of the proposed workshop currently covers:

1. an existing concrete slab (that the old shed was on). No cracks, good condition. Around 100mm thick (judging by the only side I’ve so far exposed).
2. a paved patio area. Paving stones cemented in. Rock solid.
3. Three railway sleepers laid to ground level; one to separate slab and patio and two as a kind of border between these and the lawn.
4. Lawn
5. Raised bed

All these areas are level with each other except for the raised bed (obviously!).

Rough sketch:



My plan is basically to retain the existing slab and patio and cast a new slab on top of them. They both appear to be sound and plenty solid enough, so it seems unnecessary to break them up…

I’ll need to remove the raised bed and sleepers, of course. Then excavate under the latter and dig out the lawn within the area of the footprint to a depth of 150mm, and fill these areas with hardcore (not forgetting to embed some kind of trunking for utilities first). This will bring hardcore, slab and patio to the same (ground) level.

Then sand blinding on top of everything, DPM, cast new slab.

‘Work flow’ sketches:



So, before getting into too many specifics, the big question is: What do you think of this general plan?

 

[Mike G]

I would only do that with a reinforced conc slab. You have three main bearing surfaces, all of a different nature. There's every chance that if you cast a slab over them, that you'll get structural cracks at the junctions. Again, I'm an architect, not an engineer, but I suspect our stuctural engineers here will be saying the same thing, or will just tell you to excavate in the normal manner. And with the need for reinforcement at the top and the bottom of the slab thickness, it will beed to be an absolute minimum of 150mm thick.

 

[JohnH]

Hi Mike, many thanks for your input. A reinforced slab, minimum 150mm thick is pretty much what I was anticipating (from my research on the topic). So, good to have confirmation

For the reinforcement, are we talking wire mesh or thicker rebar, would you say?

 

[Mike G]

Again, wrong profession....but I'd be happy enough with A142 mesh for a little outbuilding like that, without any calculations. Unless there is a big tree/ trees near by, in which case, consult an engineer. As always with mesh, big overlaps tied in to each other, and spacers to hold them up above the bottom. Normally you'd want 50mm concrete cover (top, bottom and edges), but if you are doing the job yourself, and you are assiduous with the placement such that there was no possibility of a route in for moisture, then you could relax that to 30/ 35mm.

I'm not sure there's a lot of point in a DPM under a slab like that, standing entirely above ground. It could actually do more harm than good.

Reinforced concrete should be vibrated. My experience is that the little electric jobbies are useless, and that the pneumatic ones are overkill. See what your local hire shop has available, but I have heard tell of petrol powered back-pack-type pokers.

 

[JohnH]

Thanks, Mike. Really useful info. There is a big cherry tree about 10m away, and an increasingly big sycamore a bit further in the other direction. But there doesn’t seem to be any cracking/movement in the existing slab.

Regarding the DPM: its secondary - or, given what you say about it being unnecessary in terms of keeping moisture out above ground - primary purpose would be to keep the new concrete from bonding to the old, as I’ve read that’s good practice (to avoid any issues in the old telegraphing to the new). Although, as I’ve said, I’ve not noticed any cracks in the old... What harm do you think it could do?

 

[Lons]

Hi John
Why the double doors onto the alleyway?

 

[Mike G]

Another purpose a DPM can serve is simply to retain the pour. Otherwise, leakages can occur around your formwork. It's downside is that it can't be exposed to light, so it has to be cut off at the bottom edges of the slab, and that can mean it can actually trap damp.

There isn't a perfect answer to this design. It's a whole series of compromises. If you want a perfect job, you'll need to excavate, build dwarf walls on strip footings, and have a ground bearing slab with the DPM lapped with a DPC in the walls. That's a massive overkill for a garden shed.

 

[JohnH]

Hi John
Why the double doors onto the alleyway?

Hi Bob,

Double doors onto the alleyway to make it easier getting bulky things in and out - including bikes, which will be stored in the workshop (and when you’re wheeling a bike out - ie not sitting on it - it takes up a chunk of space!). Our house is a Victorian mid-terrace, no side gate or anything.

I already have the doors, incidentally - nice chunky oak ones, possibly older than me, that were someone’s front doors till recently. So they’ll also be pretty secure. I’m really looking forward to giving them a new lease of life.

 

[JohnH]

Another purpose a DPM can serve is simply to retain the pour. Otherwise, leakages can occur around your formwork. It's downside is that it can't be exposed to light, so it has to be cut off at the bottom edges of the slab, and that can mean it can actually trap damp.

There isn't a perfect answer to this design. It's a whole series of compromises. If you want a perfect job, you'll need to excavate, build dwarf walls on strip footings, and have a ground bearing slab with the DPM lapped with a DPC in the walls. That's a massive overkill for a garden shed.

Hmmm, hadn’t thought about the DPM actually trapping moisture. I can see how that might happen. Maybe there’s some product I can paint onto the old concrete instead that’ll stop the new concrete bonding. Or maybe I shouldn’t worry about it too much - I dunno…

You’re right that this design is a compromise. I’m just hoping to avoid, if at all possible, the backbreaking and disruptive (to us and also our neighbours, to whom we’re in close proximity) work of smashing up what’s there, when it seems perfectly sound. But I take your points and appreciate the wisdom in what you say - many thanks for taking the time to consider my ideas and share your thoughts. Decisions, decisions…

On a more architectural note, what do you think of the asymmetrical roof design, Mike?

 

[RogerS]

Hmmm, hadn’t thought about the DPM actually trapping moisture. I can see how that might happen. Maybe there’s some product I can paint onto the old concrete instead that’ll stop the new concrete bonding. Or maybe I shouldn’t worry about it too much - I dunno…

......

Acrypol, perhaps.

 

[JohnH]

Acrypol, perhaps.

Thanks, Roger. I’ll look into that.

Thinking about it, though, I’ll need a DPM anyway for those parts of the footprint outside of the old slab & patio area (ie where it’s just hardcore on top of the soil), so might as well cover the whole footprint with the DPM. As Mike pointed out, it’ll be necessary anyway for containing the pour within the formwork. I’ll just have to be careful to trim the DPM afterwards in such a way that it doesn’t trap any moisture, or seal round the perimeter after the pour somehow.

 

[Lons]

Hi Bob,

Double doors onto the alleyway to make it easier getting bulky things in and out - including bikes, which will be stored in the workshop (and when you’re wheeling a bike out - ie not sitting on it - it takes up a chunk of space!). Our house is a Victorian mid-terrace, no side gate or anything.

I already have the doors, incidentally - nice chunky oak ones, possibly older than me, that were someone’s front doors till recently. So they’ll also be pretty secure. I’m really looking forward to giving them a new lease of life.

Ah Ok John, just seemed like a security risk and of course loss of a wall.