Replace suspended workshop floor

[AJE78]

I am typing this thinking to myself that I should have seen this coming so am coming forward feeling embarrassed and a little bit down to ask for a bit of advice.

Having completed the main structure of the workshop, just cladding to go now, and having opted for a suspended floor, and lifted in various bits of kit (including a Sedgwick planer thicknesser) I am being driven potty by the height of the threshold step up into the workshop. Its a pain on all accounts and I am only now beginning to appreciate the importance of Mike's comments in his notes on how to build a shed that he comments on the draw backs of a suspended floor. The principle reason for a suspended floor was that I had a concrete pad at ground level and didn't want to pour a base on this/dig it out and start again. All things being equal its an ok concrete pad, no DPC under it but solid enough for a 3mx3m ish worshop.

Keen to get going I laid three courses of brick with a DPC and then inside this a suspended floor as per Mike's build. The only variation being that I used one course of brick plus a 45mm C24 sole plate bedded on mortar, with DPC, that the 120mm joists sit on with 22mm T&G flooring on this. So three courses of brick plus my door threshold on the top, I've got quite a step (or should I say hop) up in. It's driving me mad already and makes wheeling anything in and out an absolute nightmare. Should have seen it coming. Its got to go.

So, to get to the ask for advice, and hopefully its not too doom and gloom; I was considering three options which I would be really grateful for advice on:

1. Remove the brickwork and timber sole plate the suspended floor joists are fixed to and drop joists directly on the concrete pad. There's noggins at both ends and the centre holding it in shape. This may well need levelling/packing in places to get it level. Then use plates or builders band to hold its position from from possible movement. As per existing suspended floor, retain 75mm celotex between joists, with a 22mm T&G moisture resistant floor on top.

2. Remove suspended floor joists, bricks and timber sole plate. Replace with min 100mm celotex and 22mm moisture resistant chipboard T&G floor on top. Essentially a floating floor. The concrete surface beneath is rough and not perfectly even, not a float finished, just tamped finish if thats the right word. It has high and low spots here and there. Definitely not nice and level but equally not out by several feet from one end to the other. It would need some levelling out though that I am sure of. I have a mixer so could do this or I have read that just plain sand could be used although this sounds questionable.

3. Pour a concrete screed, either to bring the floor level and then go for the celotex floating floor option above or pour a concrete floor as a finished floor surface. There is a local contractor here who gets excellent reviews and mixes to volume so I would be able to pay for only what I need. I also have good rear access to the garden and it could possibly be pumped in or into a barrow for me to put in. If the finished floor level was to be concrete then it'd need to be around 100mm deep approx.

With regards to option 3 I know absolutely nothing about this sort of thing. Is it even ok to pour concrete into the structure I have built? Its a single course of bricks so would the volume of concrete add too much load? In terms of what spec of concrete you should ask for (the contractor lists all their specs they do), what preparation I would need to do? Does a concrete pumped screed find its own level? Would I ask the contractor to power float it or just let it dry and float it up myself? Leave it concrete or put a t&g chipboard floor on top?

Sorry if this is a really wordy description of what is probably a very basic question. I can't help but think if I had stopped for a moment and put a few questions out on the workshop build page in the first place I might have avoided this.

Thanks in advance

Andy

 

[AJB Temple]

Is it not possible to build a shallow concrete ramp that eliminates the entrance step and allows things like machinery to be rolled in and out readily? Or have I misunderstood the problem?

I have quite a step up into my main workshop and I am used to it now. As the winter ground surrounding can get quite wet, the step in is useful.

 

[Mike G]

Well, I'd certainly avoid your option 1. That would give you all sorts of potential problems, with the joists in contact with with a potentially damp floor below, and no good means of ventilation. Really, just forget that one.

Can you tell me about the concrete this thing stands on, Andy. Is the slab the size of the building, or does it extend out beyond it? The reason for asking is that if it extends out, then moisture will pour in through the lowest mortar course, and we're going to have to deal with that before doing anything else. If the brickwork is on the edge of the slab, and the slab is above the surrounding ground level, then we're laughing.

 

[AJE78]

Hi Adrian, a ramp is a good suggestion and you've not misunderstood my post, bringing stuff in and out is definitely bothering me. The problem with a ramp is that I have so little room between the workshop door and the fence next door. I just don't think I would manage to control machines on wheeled dollies.

My other issue is head room and granted I'm not gaining a lot of extra head room by dropping the floor but being able to squeeze just a bit more space above my head would be quite welcome.

Mike - I dug out and poured a new footing at the front of the slab, as I wanted the foot print of the workshop to be slightly larger than the existing pad. So the first course of bricks at the front of the workshop will be ok as it meets the garden and doesn't extend. The remaining three sides though the slab extends beyond the brickwork to the property boundaries. One of my original reasons for a suspended floor was my concerns about damp. I'll take some photos tomorrow to better illustrate the arrangement and hopefully give you a better idea on what my options might be. Thanks for the help and suggestions it's very much appreciated.

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[Mike G]

OK, then I would definitely be pouring a slab inside. This way you can put a damp proof membrane down under the whole thing, and be confident that any moisture that gets through the bottom mortar course is going to cause no harm. Here's how I would do it:

1-Strip everything out leaving just the single skin plinth. Clean up inside, and vacuum around the whole thing thoroughly. Mind your footprints.......we're trying to avoid grains of sand or the like.

2-Paint Black Jack or Synthaprufe or similar liquid damp compound onto the junction between the plinth and the concrete. Two or three coats to the manufacturer's reccomendation. Leave to dry well.
(You could, if you wish, paint the entire inside of the plinth with this stuff if you are worried about damp getting in).

3-Using 1200 gauge polythene DPM, cut a sacrificial piece to fit within the plinth. This is only there to protect the real DPM, which is dressed up the walls. This is going to be awkward, because it makes nasty folds in the corners which will leave a void in the concrete, so do the best job you can. I suggest you staple it to the timber frame temporarily.

4-Pour concrete. Now, you could get away with 2", but I wouldn't risk that. I'd suggest 3" thick. It has no structural requirement (and don't worry about the plinth.....the only load on it is when the concrete is wet, and it's way strong enough to cope with that). You only need about 0.7 cu metres of concrete, which is less than 2 bulk bags of ballast. This is easy for one person to mix (1:6 cement to ballast) and pour, but two would be better. The skill is in levelling it, because this will have top be done whilst walking in it. Float it by hand.

As the concrete is setting it will make the inside of the building very damp. Leave the door and windows open for a couple of weeks if the weather allows. Don't force it with a dehumidifier, at least for the first 2 or 3 weeks. When the concrete is set trim the DPM off to the top of the slab.

Once you have a solid slab you can do a floating floor if you want, but personally I prefer a solid concrete floor.

You'll obviously need to look at your door threshold. This will possibly need some bricks taking out, and you should do this before pouring the concrete.

 

[AJE78]

Cheers Mike, a poured slab it is then. I have attached some pics to give a bit more context and better illustrate things.

I had a few more questions on approaching this then.

1. I will cut the top two courses of brick out and this should give me a finished floor height of around 110mm from the patio slabs in. Nearly a 200mm reduction in height to what I currently have. Does this seem ok to remove the top two? Am I running a bit close to ground level? Would it be better to just remove the top course and take it down to two courses?

2. I purchased an off the shelf door frame and the threshold is pretty thick at 50mm. As per picture below I was going to put up shuttering and then have the slab poured to doorstep threshold height. However, would it be worth planing down the bottom face of the threshold that sits on the brick. Loosing 20mm off this will save me 20mm on concrete. Would it be worth doing this or am I just making things ore complicated? Pour slab put frame back in. Job done. Move on?

3. I would like to get a price from a local concrete contractor as they mix to volume; and post up the cost here for opinion. They do the following specs (actually they do more but these seemed most relevant based on their description)
*Our P180 - equivalent to Gen1/C12-C15
*Our P200 - equivalent to Gen2/C16-C20
*Our P220 - equivalent to Gen3/C20-C25

Which of these would be right. Also I am not sure why they call it our P180 etc

4.When working with concrete to level it out. If I mark up the finished level all around the inside and push the concrete out to fill the inside. Will it eventually settle to a natural level? I know it won't flow like water or a liquid screed. But will it eventually settle level if you've pushed it to more or less the level mark? Or is it a case like dry screeding that I've seen on YouTube where you start at one end and then have to keep working and pushing material, checking levels, going back over until it is bang on?

Thanks very much for the help

 

[Mike G]

Removing 2 courses at the threshold seems fine, so long as you end up with around 75mm of concrete thickeness.

Your choice re the door threshold. The other approach is to cut the bricks under the threshold. Whatever works.

Gen 1 is fine.

Firstly, marking up the level isn't going to be easy, because the plastic is in the way. Second, no, concrete will stay put. You HAVE to level it. And it's not like screed, which stays where you put it. In fact it is more like porridge, or unset jelly. Getting it spot on is a bit of an art form. If you aren't confident with this, then you are going to need to involve a builder. If you use a builder, I haven't the slightest doubt he'll mix such a small quantity of concrete himself on site.

 

[the bear]

Round my way you won't get a volumetric truck (if thats what you mean) for that amount, isn't worth their while. You wouldn't even be able to pay a premium for the pleasure, too busy. As mike says thats an easy amount to do for an average able bodied person with a mixer and barrow.

Mark

 

[AJE78]

Cheers Mike, that answers everything perfectly. From what you've said I am very sure that this is something I am not confident to do. I'll find a local builder to ask to price for it.

Yes that's the right Mark, volumetric mixer truck that the local firm here have. I'll ask them but based on what you say it could be a no or an astronomical cost due to it being such a small size. Like you and Mike have said it's a small volume so the local builder would be able to handle this in any case

I'll make some enquiries next week and see how it goes.

Cheers again for all the advice

Andy

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[Woodbloke]

... the importance of Mike's comments in his notes on how to build a shed that he comments on the draw backs of a suspended floor...

I deliberately built my workshop with a suspended floor and as far as I can make out, there are no disadvantages...and I have some fairly heavy bits of kit. The trick is to plan out where your heaviest items will go and then build a robust 'pad' (concrete/ 50m thick paving slabs etc) just underneath the floor joists; When the joists are eventually laid, the gap is simply taken up by a series of folding wedges.

It's worked in my 'shop for nearly two decades - Rob

 

[Cabinetman]

... the importance of Mike's comments in his notes on how to build a shed that he comments on the draw backs of a suspended floor...

I deliberately built my workshop with a suspended floor and as far as I can make out, there are no disadvantages...and I have some fairly heavy bits of kit. The trick is to plan out where your heaviest items will go and then build a robust 'pad' (concrete/ 50m thick paving slabs etc) just underneath the floor joists; When the joists are eventually laid, the gap is simply taken up by a series of folding wedges.

It's worked in my 'shop for nearly two decades - Rob

Actually Rob you don’t even need to do that, I’ve got big heavy bits of cast-iron equipment on top of eight by twos of 18 mil OSB tongue and groove edge on not very substantial joists spanning 15feet, only 10 years up to now but no deflection on the floor at all, I think the OSB transfers the weight across several joists. Ian

 

[Woodbloke]

Actually Rob you don’t even need to do that, I’ve got big heavy bits of cast-iron equipment on top of eight by twos of 18 mil OSB tongue and groove edge on not very substantial joists spanning 15feet, only 10 years up to now but no deflection on the floor at all, I think the OSB transfers the weight across several joists. Ian

I seem to think mine were 4*2's Ian so I needed to be a bit more careful, but as you say, if it's done with care a suspended floor IMO is brilliant and difficult to beat. Much kinder on the feet and much warmer; easy also to pass cables under the floor to service machines placed in the centre of the 'shop - Rob

 

[Mike G]

It's not what stays in your workshop which makes any difference with a suspended floor or a big step up.....it's what goes in and out that counts.

So, firstly, the worker. Are you OK with a big step, and are you sure you'll always be OK? Secondly, if furniture making is your thing, and you've got to manhandle a big Welsh dresser out, for instance, then you might struggle with steps. Thirdly, if you are constantly bringing big lumps of wood in, as I do, then steps would be a difficulty. As it happens, I regularly find myself wheeling a wheelbarrow into my workshop, and a level threshold would rule that out, but I suspect that won't be a factor for many. However, some might work on a motorbike in their workshop, for instance, and steps would be something of a pain for them.

I'm making no judgement on suspended floors vs ground-level floors, steps vs flush thresholds. All I say is be aware that these are a factor you must think about when designing your workshop, particularly if you have headroom issues internally because of Permitted Development limits on eaves and ridge height to contend with.

A final comment. Andy has built a suspended floor and found it doesn't work for him. I don't think it's up to us to say......"ah, but it works for me so it should work for you". One of the reasons there are two different versions of floor in my sticky thread is that what suits one won't suit another.

 

[AJB Temple]

I agree with Mike here it's a personal choice if a step is a nuisance or not. I must admit, mine has a step (not my choice - I repurposed a pigeon loft), but my workshop is too small (mainly too narrow - Mike knows, he's been in it) and if I bite the bullet and build a new one it will not have a step in. I work a lot with rough sawn green oak which is damn heavy, and getting machinery in an out of my workshop is always a man and a woman job.

On concrete. At our place I have made a hell of a lot of concrete. Also had it chute delivered and pumped. Pumped is super fast and by far the easiest to level, but economically unviable just for a small amount. For under two cube I would probably mix it myself (I own a mixer).

However, if you are mixing two bulk bags worth, and laying, you really MUST have a helper. Doing a good mix takes churn time - it needs to mix really well and this is not a 2 minute thing. To shovel it, mix it, pour, barrow it in and spread it will not be quick if you are not used to it, your first mixes will be going off by the time you do your last and you may well struggle with tamping, floating and levelling on your own. Anyone can do a good job of concreting (and more so if you own or can borrow a mag float) and I encourage you to do your own, but get a mate or your wife to help.

 

[MJ80]

I always used to use a firm in Leeds who mixed concrete on site and barrowed it in. You only paid for the volume used, it was ideal for smaller jobs - footings, slabs etc. I'm sure there must be more similar companies around the UK.
https://newmixconcrete.co.uk/

 

[9fingers]

There is a franchise operation in uk called mixamate who operate on that model
They mix on site and even provide the barrows but you provide the labour to push them.
Bob

 

[Woodbloke]

... getting machinery in an out of my workshop is always a man and a woman job.

It doesn't need to be. I have pretty heavy machinery, all of which has been delivered by Axminster straight into the workshop, steps n'all. When wagon arrives, the only appliance that the driver (singular) will use is a sack barrow with big pneumatic tyres (he was insistent that solid, rubber tyres are no good for moving heavy kit). Quite how he managed to move my p/t and large industrial bandsaw up the step and into the shop I still don't quite understand, but he did it quite easily and didn't even break into a sweat.

It's not what stays in your workshop which makes any difference with a suspended floor or a big step up.....it's what goes in and out that counts.

So, firstly, the worker. Are you OK with a big step, and are you sure you'll always be OK? Secondly, if furniture making is your thing, and you've got to manhandle a big Welsh dresser out, for instance, then you might struggle with steps. Thirdly, if you are constantly bringing big lumps of wood in, as I do, then steps would be a difficulty.

I do all of that Mike and it's never made any difference. Whenever I've had a biggish bit of furniture to move out, my neighbour Dave has always been very helpful and we always take care coming down the step...but it's not the only one. As the 'shop is on a slope, there are a further three to be negotiated before we get to the back door of the house, so the workshop step is neither here nor there - Rob

 

[the bear]

When my planer was delivered by Axminster the driver definitely couldn't do it on his own with a sack barrow. Took him, my neighbour and me and we were all sweating at the end of it. Not particularly difficult access either. What I would say about the Axi driver was he was super helpful and wouldn't dream of dropping it kerbside and running.

I think it depends on your personal circumstances and physical ability, both of which change, rather than suspended floor and step can work for all and vice versa.

I have to move it again in a few weeks into its new home, not looking forward to that.

Mark

 

[AJE78]

Cheers all for the comments and discussions, it is all very much personal choice. Its definitely nothing to do with the suspended floor not being solid or strong enough. Its solid and really superb. It's the being ok with getting stuff in and out. I've realised I've got big problems getting stuff in and out easily on my own. Either because I just can't do it on my own or because it's just awkward on my own and with a bit of re-jig I can fix both these things.

Mark - I hope the planer move goes ok. I take my hat of to people that move stuff for a living. Its a skill to be able to shift these things. The Sedgwick I purchased was deliverd off a tail lift and then power palleted to the back gate. It made it look so easy! Then I took over....

On the subject of volumetric mixes then.....I've had a price from the local contractor here (Bob - cheers for your suggestion I've made a note of that firm). They were really quick to come back and the good news is they're able to mix the small amount I need. I'd be grateful for opinions on whether it sounds like a fair price.

Up to 1 cubic metre of Gen1 (this is the minimum volume they do)
£192 including VAT delivered to site
Includes 30 mins time allowance to transfer the mix, over this charged at £100 per hour or part of

Does this seem about the going rate for this?

Does it seem feasible that me and one other (probably will get a builder to do it - although the more I look at the area to fill it does seem quite small) would get this all wheel barrowed the short distance through the gate and into the workshop? By short distance I guess 20m run from where the mixer would park up to workshop door.

Cheers

Andy

 

[AJB Temple]

Price is not crazy for a small volume. Bulk bag of ballast plus requisite cement would yield about 0.6 cubic metres and round our way cost about £85 with free trade delivery from TP or similar. So a ready mix of 1 cu m where a lot of the work is done for you is convenient. Especially if they supply a couple of barrows.

Assume about 15 builders barrowloads in a cubic metre, but avoiding slop and to make ease of getting it up a plank ramp, maybe a few more.

 

[billybuntus]

Build a ramp that can be slotted in when you need it? Seems like a lot of work otherwise.