1st DIY project: loft flooring

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After years of living cosily as a tenant - with everything done for me - I'm now getting used to being a homeowner. I went into the loft of my (brand) new house to find a surprising amount of space, so I want to get some chipboard loft flooring from B&Q and lay it.
1) Are joists always a certain space apart from one another, meaning that that one piece of board simply lays across two parallel joists ? 2) How strong is this likely to be ? If I stood and jumped on it a few times would I still be in the loft afterwards or in the bedroom underneath ? 3) Is it simply a matter of whacking a few nails into each board/joist or is there something more complicated (and hence a good reason why a builder would charge a four-figure sum to do the same job) ???
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Not exactly, although modern houses might actually be at a standard spacing, such as 600mm or 450mm.

You should be OK. The joists aren't particularly strong. However, they are sized not because of final snapping load, but to limit flex. By jumping up and down, you are unlikely to end up on the floor below. However, you are quite likely to end up with cracks all over the place. The same may happen if you are tempted to use the loft for heavy storage.
3) Is it simply a matter of whacking a few nails into

Use screws. You can get screws out easily and it doesn't crack the plasterboard skim below.
On a modern house, you may find insulation is installed that rises well above the level of the joists, making boarding quite impractical, especially if the joists are so marginal that the dead load of cross joists would be too great.
Christian.
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Could you expand on that a bit please, what do you mean by marginal and what is "dead load"? By "cross joists" do you mean laying additional 3" x 2" (say) perpendicular to the existing joists and then boarding on top of that?
Thanks, BraileTrail
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Yes. However, the main joists are already pretty weak. Dead load is the weight of the floor itself. Adding the weight of the cross joists might mean there is no spare capacity when finished to put anything on top. This could be solved by attaching the new joists to proper hangers, so they actually provide additional strength to the floor, or bolting much wider joists to the existing ones in line, effectively making them much wider and stronger.
If you go to the building regulations on the web, they give a table of floor strengths depending on span and timber sizes.
http://www.odpm.gov.uk/stellent/groups/odpm_buildreg/documents/page/odpm_breg_600457.pdf http://makeashorterlink.com/?E21262277
Christian.
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You mention that bolting wider joists to the existing ones would give the floor more strength. It has also been mentioned to me that glueing and scewing joists to the top of the existing increases the strength. Which of these 2 methods would give the greater strength, in your opinion?
Cheers, Jon.
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If you can make the joint sufficiently strong you could consider these to be one, larger joist in each case, I should think.
I seem to remember that a thin, tall joist is stronger, relatively, than a wide, short one. The tables Christian posted bear this out.
From Table A1:- If you take 38 x 97 as the 'baseline', with 450mm spacing the dead load is given as 1.69 kN/m2.
Increase the height by ~26% gives us 38 x 122. The load here is 2.39 kN/m2 - an increase of 41%.
Instead, increase the width by ~24% gives us 47 x 97. The load here is 1.91 kN/m2 - an increase of 13%.
Neil
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Indeed, by wide, I meant tall! In a modern house, the height will be probably be determined by the insulation that needs to be covered, rather than strength requirements. It is also probably a good idea to cross lay battens and insulate again to eliminate cold bridging through the joists.
Christian.
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Thanks Neil, that's cleared that up for me. Jon.
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on which the tables are based. You're trying to minimise the bending (both because it makes the ceiling crack, and because supporting weight in the middle is what you're trying to do, and bending the material past its breaking point will make it, um, break). And what gives resistance to bending is less the total cross-sectional area than the height of material you're flexing - think how you can bend a floorboard across its narrow axis versus its wide one, think of how bendy it would be if it were square, try it out for real on some handy lengths of 1x1, 2x1, 2x2 etc. if you have such to hand! You need *some* width to resist buckling and twisting, of course, but to a first approximation a deeper joist will give you much more resistance to bending than a wider one.
Time for the periodic advert: the best pair of books you can read which straddle the 'intuition' and 'engineering formulae' divide, well-written, well-illustrated, and full of anecdotes, are JE Gordon's "Structures: Or, Why Things Don't Fall Down", and his related title "The New Science of Strong Materials, or, Why You Don't Fall Through The Floor". Both still in print in Penguin Science, previously Pelican, and massively informative. They'll tell you why it's worth putting that lip on the bottom of your MDF bookshelf, for one ;-) If buying only one, make it "Structures".
HTH, Stefek
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wrote:

In simple terms strength is proportional to width x depth^2, stiffness (i.e. resistance to deflection) width x depth^3. Thus 38x200 (1.5x8") have the same amount of wood in them as 50x150 (2x6") but have 1.33 times the strength and 1.78 times the stiffness. As long as they stay vertical, and being taller and narrower, strutting they need strutting every 1.2m whilst the 50x150 only need to be held in line at the ends.
In most domestic joist situations deflection is the controlling factor: i.e. if you overload the floor/loft space the joists won't give way, just deflect to an unacceptable degree.
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On Tue, 17 Feb 2004 17:11:02 -0000, "Christian McArdle"

I don't use screws myself - I whack in a couple of light nails to hold the board, then lay the section which is tongue and grooved, then whack another couple of nails in the final board of the section. These boards hold themselves in place - the tongue and groove is rough edged and trying to move a board slightly which is slotted in requires a fair bit of tapping with a hammer!
Cracking the plasterboard skim isn't a problem I've come across, and if the skim is that flakey then just walking on the rafters is going to cause that damage anyhow.
Using screws implies having to drill holes then waste time driving the screws home. For what purpose - to take the boards up again in the future? Why? In the majority of homes the loft space is to hold junk that you don't need to get hold of every day so visits to the loft are very infrequent. It's not like a regular floor where you want to solve a creak problem.
PoP
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wrote:

Or having a decent cordless to drive them straight home - I used Screwfix Goldscrews (not Turbogold) size 6 x 60 (50s would have done) and they went in a treat. Lot easier than hammering and ...

... when you find you've (OK: _I_'ve) laid a board wrong and do need to get it up again it's no more hassle than putting the cordless into reverse.
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to take up boards two or maybe three times since then to add lights and do re-wiring when she decided to move the cooker. It would have been a right roval pain if I'd nailed the boards down. The amount of time and effort required to screw them down is minimal with two cordless drills, one to make the holes and one to screw the screws.
--
Chris Green

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On 18 Feb 2004 11:46:07 GMT, snipped-for-privacy@isbd.co.uk wrote:

Each to his own :)
I don't subscribe to that view though. I put one or two nails in each end of a row of boards - just to hold them in place. It's a piece of cake pulling the boards up - that's what claw hammers were made for.
Whereas screwing involves much more work IMHO.
There is possibly an issue with screws in so far that board expansion could be restricted. I don't think this would be such an issue with nails as both the nails and board would move (a screw won't want to move as it is fixed). Whether one has to worry about that is another matter of course :)
PoP
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www.duraspin.com
I don't follow. Why do you think that that would be? The expansion and contraction is across the board width.
What I did was to lay T&G softwood boards and nail through the tongues where they meet the board. A pneumatic nailer is great for this, and unlike a hammer, it's one thwack rather than numerous blows. This is fast and secure and I suspect less likely to affect the ceiling. I ripped the tongue from every Nth board to provide access and screwed those down.

You'll be doing sharp-intake-of-breath next. :-)

.andy
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wrote:

I'm growing the Mr Stumbles stubble right now..... ;)
PoP
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Going down hill fast then. :-)
The next thing to do is to visit the local trade merchants and introduce yourself to them as Mate. They will recognise this because they all have that as their name anyway.
Then you will become known as the new Mate with the stubble who doesn't work for any other Mate,
I've heard conversations like this in Plumbcenter in Bracknell where they are talking about people not present and got up to 6 different Mates who worked for different companies (not mentioned). The funny thing was that they all knew who they were talking about.
Of course you do have to leave the gold card at home and have an account that you don't settle for months before you are welcome into the inner circle of Mates.
.andy
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wrote:

Plumbcenter in Bracknell? What am I missing.....?

Gold card - don't have one. Refused all offers. And apart from the ongoing expenses that I clear every month I'm pleased to report that I have zero outstanding on any credit cards......
PoP
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Nothing apart from waiting half an hour to get served :-)

.andy
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wrote:

I have to find this Plumbcenter. Can't think how my radar hasn't picked it up as I drive around the town.....I usually make a mental note of places that could be useful to me and I guess this one has evaded my attention.
PoP
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