Not enough noggins?

[Fitzroy]

So I'm about ready to lay the wood floor in the new shed and I've found it's not flat, grrrr! The floor frame is 6"x2", 2.9m span, on 18" centres with 18mm Osb, the foundation blocks under the frame are 4' apart and align with the edges of the Osb sheets. So each sheet edge is fully supported but the two intermediate joists are not, they are hung on joist hangers.

Each and every board is 7mm lower in the middle than at the edges, so the whole floor 6m wide dips 5 times across its width. The only thing I think I didn't understand was the need for lots of noggins, so the frame only has them where the edges of the Osb meet. Was this my error?

I've bought a 5mm thick wood fibre underlay board, thoughts on going double thickness with the underlay board down the centre of each low section? Flooring is 3/4" engineered oak T&G that I'll be screwing down.

F.

 

[skipdiver]

You need more pads under the joists in the centre. 2.9 span on 6"x2" is pushing it, especially on a building that will house heavy equipment.

 

[Bm101]

No builder but I'd agree with Steve. I'd also nog it like brick stretcher bond. That way your strengthening against both downward and lateral forces. Know it's not what you want to hear but with a bit of luck and some searching you might find padstones of the right dimensions that you can slip underneath then nog it. Worth doing it now because sag will only get worse over time. It'll be well worth the time and expense in the long run. Cheap to rectify now in relative terms to 3 years down the line. Good luck.
Cheers
Chris

 

[RobinBHM]

6 x 2 has a span of 2.8M at 400 centres for domestic load, so you arent far out.

I would suggest measuring the supported joists along their length and the non supported ones and compare both -but making sure you are not standing on the joists at the time.

If the the unsupported ones have a dip of 7mm then they arent sagging due to a load, they must be bowed in the length for some reason.

Has the base settled a bit? Perhaps the perimeter blocks have settled downwards and the mid way ones have stayed making the supported joists stick up a bit. Its all covered by DPM so I cant tell if they are sitting on concrete or bedded on soil / hardcore etc.

 

[skipdiver]

He has 450 centres though and that puts it right on the edge. The fact it is a workshop puts it over the edge with it's imposed load IMO. None of this is relevant though as there is no weight on them yet, so something else is going on. Did you fit them crown up?

I would have put another joist right down the middle and cut the cross joists in half. This would have acted like a trimmer and been a lot more solid. The problem will get worse in time and be a bit bouncy in use, so you will need to beef it up somehow.

 

[RobinBHM]

He has 450 centres though and that puts it right on the edge. The fact it is a workshop puts it over the edge with it's imposed load IMO. None of this is relevant though as there is no weight on them yet, so something else is going on. Did you fit them crown up?

I would have put another joist right down the middle and cut the cross joists in half. This would have acted like a trimmer and been a lot more solid. The problem will get worse in time and be a bit bouncy in use, so you will need to beef it up somehow.

Yes indeed I should have pointed out its on the edge of being sufficient, I was clumsily trying to point out it doesnt explain what is going on at the moment.

 

[Lons]

Exactly what Steve said, not enough support imo and certainly should have had noggins which while stiffening the structure have the main job of preventing twist in the joists, in fact twist can affect the heightas clearly the boards are then resting on a corner rather than flat surface.

If you did a "proper" job and screwed down the boards it's easy enough to rectify, a bit more difficult if nailed with ringshanks though.

Bob

What's stopping any of the blocks settling btw? Are they on a decent foundation or concrete base?

 

[Fitzroy]

Stopping the blocks settling:- Not a lot but as much as I could. Tree preservation root zone in conservation area prevented digging down more than 10cm over small areas. Pits dug to 10cm deep, 10cm larger all round than blocks, filled with type one hardcore and rammed to fully compacted. Concrete blocks sat on top, dpm, then frame.

OSB nailed with ringshanks, about the only thing I did not screw together. Not a chance of lifting.

As you've all said this is an unloaded situation. I wonder if I cocked up the install of the joist hangers and where there is no block under the cross joist I've managed to set the joists lower fractionally. I set the joists crown up I think (was a year ago now though).

They are 16" centres sorry not 18", was lying in bed thinking about it and realised 18"x3 is not 4'. And c24 graded rather than c16.

I may be able to slide/wedge/hammer some more support blocks under the centre of the unsupported joists. If not it'll be a learning experience. I liked the central trimmer idea, for next time!

Thanks for all the thoughts.

F.

 

[skipdiver]

There is always the option of just using the space and forgetting that the floor is not quite level and maybe a little on the bouncy side. You still have a nice workshop to play around in and it's not like the floor is going to collapse on you. I've seen far worse.

 

[Lons]

Yep I'd leave it be now then if it was mine and just accept it.
If you could only go down 100mm then you're on the back foot to start with as will definitely move a bit especially if tree roots are below. Depends on the type of ground you have but if clay then it will heave and shrink with water table changes anyway.

Personally I wouldn't attempt to wedge under blocks as there's every chance of the blocks breaking under the load over time as it is unlikely you can wedge evenly over the whole of the block, they are quite brittle if not fully supported. Only way is to wedge up the timber on top of the blocksbut guess that's likely not an option now.

As Steve said, it's a workshop and discrepancy is minimal so forget about it and enjoy the space, more important thaings in life.

Bob

 

[Paul200]

Hi Fitz

I had exactly the same problem when I built the base for my workshop. I bodge-solved it by running a purlin(?) below the offending timbers held in place with joist hangers. As a retro-fit bodge it actually worked really well.

You can just make out the purlin at mid point below the end timber. Every other cross member was unsupported and was sagging - the purlin levelled everything up and made it stronger too. I just wondered if you had the room and were able to slide timber underneath and do the same? Just a thought.

Cheers

Paul

 

[Chrispy]

If it were me I think I would cut a hatch in the OSB over the middle of each joist so that I could fit some support blocks, yes the ground will still heave but at least it would reduce the floor bouncing.

 

[Phil Pascoe]

This is one of those jobs I'd rather look at afterwards and think I might have spent thirty quid too much than look at and wish I'd spent another thirty quid and done a better job. My father was a builder who knew a bit - he and his partner at the time were the biggest house builders in Devon and Cornwall - and he always advised spending your money above the top ceiling and below the bottom floor. Everything else can be altered afterwards. He was right.

 

[ColeyS1]

Take the floor up, punch the nails through. Take a day to do it, but you'll be glad you did.

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Edit - did I just read you were laying an oak floor on it anyway ? Take it up, do it now before the oak goes down.

 

[rafezetter]

I have to agree with what's been said to rectify it now, I'm not a builder (spent 12 years living with one though and chatting about most of his extension works), and if you ever got a hankering for some heavy type machinery down the road you'll be in trouble.

Get the flooring up and fix it - central beam (Dave says split 3 ways is better) and noggin the carp out of it taking possible heavy weight load placement into consideration.

If it were me I'd move the whole frame out of the way and put down patio slabs under the blocks too, spread the load as wide as you can manage; 10mm of rammed core "foundation" might as well be nothing according to "Dave the builder" - as it has no cohesive movement defense - the central part under the block can move independently of the surrounding area and unless you've done a thorough test of the ground, and understand what it may do with the tree roots etc then overbuilding it is the ONLY option if you want it to last.

He says at the very least put slabs under the back half of the base where it's likely most of the built in storage and machines will be. Three (two base one central) and have the block in the centre if you can stretch to the extra cost.

Once you've started the walls etc there's no going back, it's now or never.

 

[Lons]

I wasn't intending to comment again but the builder in me has won, so:

As a shed it's ok as it wouldn't matter much if it moved - and it will - but for a workshop you'll never be happy as clearly you already find the 7mm discrepancy irritating.
The blocks are almost certain to move further as soon as the significant load of walls and roof are fitted and that's without workbenches, storage tools and the workman himself.

Bearing that in mind if you want to do the job properly and construct a workshop that will last you really should remove the whole floor structure and start again. I'd get half a dozen mates and just lift the whole thing off in one piece, I'd then shutter the base and pour a 100mm reinforced concrete slab and refit the floor on DPC, ideally on pressure treated 100 x 25mm or 100 x 50mm lengths of timber, easily available from builders merchants, which will create an air gap and help prevent rot.

OK, not what you want to hear and going to cost a few quid but will last a long time and unlikely to move unless the ground suffers significant heave or shrinkage and you'll have a workshop you'll be proud of rather than an annoying bodge.

As said, you can't change the founds which are the most important part of the structure once it's built.

cheers
Bob

EDIT:
Just noticed the second pic. If you've already fitted the walls my above comments are too late, unless you have enough mates to lift it, or a crane! #-o

 

[ColeyS1]

I think the walls and roof are already on. When he mentions laying the floor, I think he means laying an additional oak floor

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[Lons]

I think the walls and roof are already on. When he mentions laying the floor, I think he means laying an additional oak floor

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Yeah, noticed just after posting #-o

 

[Fitzroy]

Yup all in place. A concrete slab would have been the first choice but I live in a conservation area, so had to apply for planning permission, and the location for the building is under a set of protected trees so minimal allowable ground works. Hence the pier block foundations. It all rather is what it is, question was really asked from a learning viewpoint should I ever do it again.

The building is a glorified shed, yes I'll use it for my projects and it'll have some equipment in it, but not tonnes and tonnes of the stuff. I Plan to open a few slots up in the floor and see if I can insert some intermediate blocks for support mid span. I'll then fix the final floor boards over the Osb and history will be the judge. The whole thing is built on the cheap, but that's all I wanted to spend, you pay your money and make your choices.

F.

 

[ColeyS1]

Set a skill saw for depth, cut a 600mm rip right through the middle and try and reuse the OSB. Worst case scenario is you'll need a couple new sheets.
This threads sounding all doom and gloom- reality is, it'll take a few hours to add the additional support/noggins. This really is nothing more than a minor hiccup.Once your oak floors down and it's fully loaded with tools it'd be much more of an issue.

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