Need some sanity checking on slightly unusual shed floor build

[Croolis]

Hello folks. New member, hope to be returning for the conversations as I'm building a small workshop shed thing, to do some mid life crisis hobby carpentry in .

Have bought a cheap metal shed. 3000mm x 2500mm. I shall be putting together the metal square that forms the outer floor dimensions of the thing today, so I have something definite to work with for my drawings. But for now let's assume it's 3000mm x 2500mm and will sit nicely on a 2 inch upright board.

I provide drawings below. The floor I'm making is unusual in that the cheap metal shed is basically made for short people who aren't going to be walking around in it much. It's a gable roof, not a pent one. The highest point will be about 10mm above my head. Over by the walls it'll be way lower. I'm 5'8" and hope to use it as a light use workshop.

I'm not experienced. To raise the roof, I am making the outer rim joists from 2x10 C24 boards, and then hanging C16 2x4s at the bottom of the 2x10s. My joist spacing will be 363mm from centre. After adding 18mm OSB for floor boards that will get me a raise of 132mm. I very much wish I could get more, a lot of effort for not too much gain. I do understand that this will make a step when going into the shed and that the door is still the same size . I choose 2x4s rather than 2x6s because I'm trying to get as much height as possible. I will have span support accordingly, see below.

I am renting. I can't make a concrete base, that's not going to happen, I need to be able to deconstruct if neccessary. So it's paving slabs with gravel underneath and concrete blocks on top (see drawing). I shall put 9 of these down. I have five bigger paving slabs at 610mm and the rest are more standard ~410mm. So the bigger ones will go at the four corners and one in the dead centre of the floor, then smaller ones in teh other locations.

These will support three 2x4 C16 span support beams (skids?). Looking at the table linked below, I seem to be within tolerance by using 2x4 C16 for skids and floor joists, unless I'm being ignorant about something. There are no noggins in the drawing I provide (lazy), but I will be using them.

https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?load=1.5&class=C16

Besides pouring concrete bases and building walls out of concrete blocks, I'd love to think of a way to raise the roof further. 2x10s is all I can get for the money I have, even if I could find anything bigger (not so far). I think simply making a second outer rim joist square to just sit on top of the first isn't going to work? Won't sit flush I expect?

Any thoughts? Replies much appreciated.

 

[Croolis]

How about putting the second outer rim joist square just sitting on top, but then just having some very deep flashing coming down from the top of the upper rim joist square so it covers the join between the two? That way, any gaps between the two caused by not sitting flush would be covered up. Not airtight, but I'm not insulating the thing anyway.

 

[MikeJhn]

Concrete fence posts laid on the ground, one or two on top of each other or concrete gravel boards, joist hangers off them, build them up as high as you need.

 

[Inspector]

Build it like it is a wall. Plate top and bottom with studs (North American terms) between and clad with OSB/Plywood. Treated wood and plywood will make it last longer. You can make it any height you want and or are allowed to, plus it can be insulated.

Pete

 

[Agent_zed]

Build it like it is a wall. Plate top and bottom with studs (North American terms) between and clad with OSB/Plywood. Treated wood and plywood will make it last longer. You can make it any height you want and or are allowed to, plus it can be insulated.

Pete

this is what I'd do. probably only need 2x2 timber as the shed is barely going to weigh anything if its the type of metal shed I think it is. Might be cheaper to use feather board on the outside rather than osb/plywood as it quite expensive here.

Random idea - you could even try something a bit different and use felt support trays for the cladding https://www.roofingsuperstore.co.uk...m=4575755078947165&utm_content=Full Inventory if you turn it upright so you short edge sticks outwards it would move water away from the base also, or you could cut it off. I bought some last year for about £3.50 each from my local builder merchant.

 

[Croolis]

Build it like it is a wall. Plate top and bottom with studs (North American terms) between and clad with OSB/Plywood. Treated wood and plywood will make it last longer. You can make it any height you want and or are allowed to, plus it can be insulated.

Pete

this is what I'd do. probably only need 2x2 timber as the shed is barely going to weigh anything if its the type of metal shed I think it is. Might be cheaper to use feather board on the outside rather than osb/plywood as it quite expensive here.

Yes - I'd come to this conclusion myself a couple hours ago - why not make it with short little studs and then, as you say, can go as high as I like, plus I get to put the floor OSB on top of the outer rim joists, instead of trying to fit them inside (I was looking forward to gaps in the floor at the edges and was just going to swallow it).

I had already considered using corrugated bitumen for the cladding, but that stuff you link to there is dead cheap. Cheers for the find .

I've already been working on the drawing - there's only one drawback to all this: if I go slab, concrete block on top, span joist from 2x4, floor joists from 2x4 plus 18mm OSB, then mini stud wall to desirable height... I'm up to bottom of doorway 600mm off the ground urrrgh. I guess I could lose the blocks and just go with slab, saving 100mm. Still 500mm....

 

[Croolis]

Yes it is the £300 flimsy shed type with the sliding door . The height difference between roof apex and where it meets the side walls (eaves?) is 230mm. It would be nice to get the side walls to my 5'8" so I don't have to worry about banging my head if I go near the sides. This means raising it by about 220mm at least as I think, inside an unaltered shed, my head will be just shy of touching the apex roof beam (sry I don't knwo the technical terms).

 

[Jameshow]

I

Yes it is the £300 flimsy shed type with the sliding door . The height difference between roof apex and where it meets the side walls (eaves?) is 230mm. It would be nice to get the side walls to my 5'8" so I don't have to worry about banging my head if I go near the sides. This means raising it by about 220mm at least as I think, inside an unaltered shed, my head will be just shy of touching the apex roof beam (sry I don't knwo the technical terms).

D stand some 2x225mm timber on edge on top of your base. If you use long screws it's not going to fall over!

 

[Croolis]

stand some 2x225mm timber on edge on top of your base. If you use long screws it's not going to fall over!

Dimwit question: how would I screw a piece of timber like that into the base? Sort of diagonally down from the outside into the base? I've got an impact driver. What about cupping or whatever it's called. Can I truly get a 2x225mm to sit flush on a "flat" surface? As I say, I'm inexperienced.

 

[Bingy man]

Dimwit question: how would I screw a piece of timber like that into the base? Sort of diagonally down from the outside into the base? I've got an impact driver. What about cupping or whatever it's called. Can I truly get a 2x225mm to sit flush on a "flat" surface? As I say, I'm inexperienced.

Plenty of long screws available that won’t break the bank. I use the ones that have a torx head as these are less likely to strip . Multiple ways to achieve this . You could fix a 3x2 to the paving slabs using concrete screws- they just need a pilot hole as per the manufacturer instructions.


Concrete Screws 7.5 X 100 - box of 100 - eFixings.com
£6.92

Then fix your main 2 x 225 to the 3x2 using something like this

TimbaScrew TX Timber Screws Wafer Thread-Cutting 6.7mm x 150mm 50 Pack (9458J)
£20.99£24
+£5.00 delivery

Both these types of screw are available in multiple sizes and the 250 mm one don’t usually need a pilot hole but it doesn’t hurt to do this . A decent impact driver will soon sort these out ..

 

[Silken]

So if I understand, what you're looking at doing is building a timber floor using 2 x 4 timbers, then building some dwarf walls out of timber 2 x 10s and then erecting your metal on top of that.

To gain hight I would build floor first 2 x 4 etc, Try if you can keep timber up of the ground, a ring of bricks on a sand bed to get a level then damp proof membrane (DPM) then 2 x 4 timber floor.

then build a ring beam of stud walls, say 600mm or 2 foot tall using 2 x 3 or 50 x 75mm sawn timber, clad externally in OBS and wrap breathable membrane wrapping over and around the stud. Link
I would then clad it in feather-edge boarding from ground up covering from floor to just below top of dwarf walls.

Next I would build metal shed (all the best with this part they tend to be a pain) on top of the dwarf walls.
Once built put one more top row of feather-edge boarding to close the gap between metal shed and dwarf walls. You will need to seal around the top edge of the boards to prevent water ingress. There are a few sealants on the market from a few £ and up here a couple,
Sealant 1
Sealant 2

You will need to extend to door to the metal shed, you could do this in a few ways, One way way would be to leave the doors as they are and instead build a section of wall and hinge it to the floor with bolts on the inside to hold it in place when closed.

Once your happy and shed is enclosed from the weather lay the floor, 18mm OSB or plywood.

All the best with your build.

EDIT. I would not use this (the outer rim joists from 2x10 C24 boards) to gain hight. You can use 2 x 3 or 2 x 4 inch sawn to gain hight.

 

[Croolis]

Plenty of long screws available that won’t break the bank. I use the ones that have a torx head as these are less likely to strip . Multiple ways to achieve this . You could fix a 3x2 to the paving slabs using concrete screws- they just need a pilot hole as per the manufacturer instructions.


Concrete Screws 7.5 X 100 - box of 100 - eFixings.com
£6.92

Then fix your main 2 x 225 to the 3x2 using something like this

TimbaScrew TX Timber Screws Wafer Thread-Cutting 6.7mm x 150mm 50 Pack (9458J)
£20.99£24
+£5.00 delivery

Both these types of screw are available in multiple sizes and the 250 mm one don’t usually need a pilot hole but it doesn’t hurt to do this . A decent impact driver will soon sort these out ..

Thanks for reply. What I'm concerned about is, if I make the shed base using 3x2 or whatever, and stick the OSB 18mm on it, so I have my floor built, will a chunkier board like a 2x9 (that's a 225mm, right?) sit flush to the board when screwed down?

I can imagine if I took, say, a 2x2 and it was a bit cupped or warped or something, then screwing it down onto the OSB would sort that out. Should I receive a 2x9 which isn't quite straight, will screwing it down straighten it out?

Thx for help

 

[Bingy man]

Thanks for reply. What I'm concerned about is, if I make the shed base using 3x2 or whatever, and stick the OSB 18mm on it, so I have my floor built, will a chunkier board like a 2x9 (that's a 225mm, right?) sit flush to the board when screwed down?

I can imagine if I took, say, a 2x2 and it was a bit cupped or warped or something, then screwing it down onto the OSB would sort that out. Should I receive a 2x9 which isn't quite straight, will screwing it down straighten it out?

Thx for help

Ok this is where the initial 3x2 //2x2 // is secured to the slabs - are we talking about lightweight decorative slabs or something more substantial like the old grey council 2x2 or 3x2 slabs you will have no problem fixing Eg a 3x2 to said slabs . Obviously 2x9 is much heavier and it would depend on how much of a gap you have . But imo if you secure it each end and let’s say you have a 1/2 “ or 3/4 “ gap between the bottom of your beam and the top of the 3x2 then you should be able to close that gap even if you have to put a screw in the centre and another say a meter either side of the centre fixing and gradually close the gap . However it will come down to which has the greatest resistance- the 2x9 pulling flush to the 3x2 or the 2x9 pulling the 3x2 and the slab up . End of the day if your delivered timber is no straight and true then reject it . Explain that you need the timber to be straight and free from twists ( within reason of course) hope you can follow what I’m saying as I’ve probably had too many single malts by now ..good luck with your project.

 

[Croolis]

That's very helpful, thx . Enjoy your single malt.

 

[okeydokey]

Just a quick thought while I'm on a train.
Unless I'm missing the point or havent read the op enough fence posts 4x4 are treated and a cheap way of gaining height with shed on top - could be stacked. Perhaps a look at Wickes (other brands are available) decorative timber garden sleeper and try googling Railway Sleeper. You would have a step over the frame into the shed onto paving slabs or whatever

 

[Bingy man]

I think you could use pretty much anything you can get at the best price . Breeze blocks will last forever and can often be purchased for a few ££ it’s amazing how many folk have left over after building work just lying around. Reclaimed railroad sleepers again have already stood the test of time and have been impregnated with creosote etc . A trip to a local reclamation yard can often yield quality materials for reasonable prices.

 

[Croolis]

Thanks all very much for replies. I'm going with the clad dwarf wall idea, have ordered the timber so I'm committed. Doubtless spent too much money but TBH this has been in the running for months and I'm sick of thinking about it, just want to get it done.

That said, here is the new drawing for what I'm going to do, and I appreciate any thoughts on what you see and how to improve the design, within the boundaries of the timber I've ordered, which is all 3.6m lengths, so loads of offcuts as the shed is 3m x 2.5m. Wood is 2x4s and 2x2s for the dwarf stud wall. I'm sure that 2x2 horizontal piece on all those studs can support what is a light metal shed).

In the picture I've extruded the OSB floor a little bit on either side so you can get an idea how that's going to sit. I shall be putting some flashing strips between the dwarf wall base and the top of the OSB, hanging down, to protect the OSB ends from water ingress from the shiplap cladding I shall put over the dward wall stud.

My main 2 questions I still have in my head are:

1. The concrete slabs you see there that the span joists will sit on. I'll put some DPM between the slab and the span joists, but are we concerned about water pooling on the slab? I don't have any blocks in the plan now because I want to keep the height down. Does anyone have any suggestions on this?
2. The outer two span joists - could I/should I bring them in bit so the left and right sides of the structure overhang them a bit? I have no idea really why I should do this, beyond getting them away from the weather a little bit, but I've seen couple of shed floor designs online that do this. Bearing in mind that my metal shed is going to be sitting on top of everything we see.

Thanks very much for your help in all this. Looks like a good forum to come to when I'm making stuff in the shed .

 

[Inspector]

I think it is overly complicated. I would have made the 4 stud walls with the bottom 2x4 laying flat on the pavers. Another 2x4 down the middle on the flat too. Joists laid on top of the bottom plates and fastened to the sides of the studs. Shed on top of the walls. Membrane under the 2x4s that touch the pavers. I'm not to fond of 2x2 material because it is too easy to split when fastening so I would have used 2x4 throughout. I would use sheet goods on the stub walls as there isn't a lot of surface to cover and it is one of the best ways to stop racking etc. Siding of choice over top. What I suggest would use less material and give you the most headroom but if you are in a flood prone area water would cover the floor the quickest. Your method will work but adds a little extra material. Either way you will be happy once finished and using it.

Pete

 

[pip1954]

Tuck the slab in and clad down to just above the floor leaving air gap at bottom,crack on mate looks good just a shame about metal shed part good luck.
Philuk

 

[Croolis]

Tuck the slab in and clad down to just above the floor leaving air gap at bottom,crack on mate looks good just a shame about metal shed part good luck.
Philuk

Cheers for that . When you say floor, do you mean ground or OSB? As I say, I shall put some flashing pieces over the ends of the OSB, keep out moisture water ingress from cladding.