The double garage sounds like a worthwhile idea.
PoP
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I worked with a mad chippy who did that. He said you could still unscrew them, which was true ... kindof ... of the few he hadn't bent putting in :-|
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.
There are many issues associated with loft conversions. Building regulation approval is essential, which may involve work in the following areas:
Obviously, there is other work as well, which building control may be less interested in, or alternatively is optional depending on your needs.
Often, you'll find it simply isn't possible to meet the requirements. However, if you can, you'll end up with a proper useful room that will add substantial value to your home. In most areas (ask a surveyor for specifics), the property value increase will exceed the cost of the works.
Christian.
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.
right so it is a lot of work, probably expensive to do properly and i wouldn't gain all that much room. i might be better off building a garage with a room on top perhpas thank you though, it very informative and probably helping me come to a descision what to do best.
sammi
It all depends on what you have to start with, the floor area, available height and the possibility of extending backwards with a dormer.
However, to get them to do the lot, rather than just the structural alterations, is going to cost from around 20K, providing there is no underpinning. Obviously, this can increase markedly if the conversion is large, or dormers are required, or for bathroom fitting etc.
Christian.
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
I floored my mother-in-law's loft a couple of years ago and I've had 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.
Hence a "Birmingham screwdriver" - nickname for a hammer.
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.
Thanks Neil, that's cleared that up for me. Jon.
The tables do indeed bear this out, as does intuition and the formulae 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
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.
yes. i like the idea too. we could have it so that the outside door is parallel to the back door and make a coverd 'passage' with doors to the front and back gardens. i suppose it would cost a fortune though to have a garage like construction without garage door but with windows to the front and back and a side door ? i don't think that is something we could do ourselves.
sammi
that just isn't worth it i think c0nsidering the aomunt of space actually gained. i have space to the side of the house. i could put a garage there and perhaps ask them to put good foundations in should i want to build a room on top of it, and i can't see that costing 20 K, or ?
sammi
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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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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I'm growing the Mr Stumbles stubble right now..... ;)
PoP
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Various wrote etc
I can't understand this obsession with fixing loft boarding to the joists. If you are using 8x4 chipboard cut down and suitably adjusted to meet in the centre of the joist, fixings are a) not required b) a bad idea as it prevents movement and causes ceiling cracks.
As a previous poster said, I've had my loft boards up and down numerous times over the years and IMO fixing them down would have resulted in unnecessary work on many occasions. I personally would never use T & G boards for these reasons. I would add that some of my roof pitches are 17deg and this precludes fixings in a lot of areas as you can't either get them in or out!
Regards Capitol
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