hansen wrote:
this is not really a DIY question but I'd like to be a bit knowledgable
when I talk to the Council/Architect/structural engineer/builder or
whoever...:
It is DIYable, BTDTGTTS ;-)
I'm looking at the feasibility of converting my loft space into usable
(not really "liveable" but I know the BR doesn't make much of a
difference) space for a study and sadly missing storage, with a
Well habbitable rooms are treated a little differently from non
habitable. Not sure what a study counts as - habitable I would guess.
The width (span) of the area I want to convert is around 2.8m, length
around 7m. There are no modern "purlins/props" that would obstruct a
conversion, however as the roof pitch is around 40 degrees (good
slate/felt/chipboard construction) the max height on one side (towards
the party wall) is only around 2m, i.e. not really suitable as it is.
Oddly there is no minimum height specified for ceiling height in the
regs (there is over stairs - but not rooms). However it you are like me
then building regs become a secondary considderation with being able to
stand up taking first place ;-)
However, the existing loft floor joists (very old wooden beams) are
around 300mm high, and the ceiling height in the room below is around
3.2m. This would, in my opinion, give me the opportunity to do the
following:
Which way do the span the house? (i.e. the 7m or the 2.8m span)
Are there any supporting walls in the middle - or do the have to carry
the full length span.
If they are across the short length then they are well over specced!
- Reduce the ceiling height below to around 3.0m, gaining 20cm ceiling
height in the converted loft from this alone.
Tad messy - but doable.
The questions I have really are the following:
- My previous house in France had glued laminated joists for the attic
floor, around 15cm thick with spans of 4m on one side and 4.5m on the
other. This floor was actually far more stable than my next livingroom
floor, by Barratt in the UK. I believe this was similar to the "Glulam"
products on www.glulam.co.uk but I'm not sure. For a less than 3m span,
and say 40cm between the joists, would this be a suitable solution and
how "thin" could the joists potentially be?
Here is where superbeam will help...
A reasonable floor loading on a joist would be 0.8kN/m as a uniform
load. Since there are lots acting together you can turn on either the
"load sharing", or "4 members acting together" options and this will
increase the statistical strength of the joists and may let you specify
lighter ones.
- I've also seen references to "LVL" which is laminated wood, with
thinner wood layers running vertically instead of horizontally, and
which is supposed to be stronger. However I can't find any examples of
how this is used in a loft conversion.
Sorry, not familier with those.
You may come accross use of "flitch" beams however (steel plate
sandwitched between two joists, all bolted together).
Note: I know that the above are questions for the structural engineer
but I'm looking at "ballpark" figures before spending the first money on
the next-step feasibility calculations. I've tried to use "SuperBeam"
but obviously you need to know well what numbers to feed into it to get
a sensible output ;-)
It might be worth getting an architect to simply look at the layout and
having a feasability chat before going much futher - that way you can
get a feel for how "doable" the whole project is before you actually
commision anyone to do anying.
With regards to the material cost - I don't really care. I believe the
cost of the steel or wood joists would be small compared to the overall
conversion cost anyway. What I *really* want is to gain every centimeter
ceiling height possible.
The whole floor on mine (bigger area) was about 2K all in for wood,
steel, joist hangers etc.
With regards to the roof itself, the rafters that hold the roof are
150mm thick. I believe the building regulations stipulate 50mm
By thick, do you mean deep?
of 0.20 W/mē.K as specified by the BR. However the following two
information sources don't add up:
http://www.fmb.org.uk/publications/m.../july03/30.asp
http://www.scotland.gov.uk/library2/br/brj-11.asp (parts E1/E2)
(Sigh, if "Masterbuilders" and the Building Regulations can't agree what
hope is it for people like me ;-) ? )
No supprise there then ;-)
So the question is really: with 150mm rafters, spaced around 600mm
apart, how much insulation can I stuff inbetween the rafters, what type
of material is the best to use, and how much ceiling space would I lose
apart from the obvious 12.5mm to the plaster board due to any insulation
material that I would have to put *under* the rafters?
If you have 150mm of depth on your rafters then you don't have much to
worry about. Typically 90mm between, and 25mm underneth would be plenty
when using PIR foam (foiled). You may be able to get away with just
insualtion between the rafters, although this allows a certain amount of
cold bridging from the wood itself. Note also that many of the figures
you see are based on a 400mm c/c rafter spacing. 600mm is actually a
little mote efficent do to a bigger insulation to wood ratio.
With regards to fire regulations, I already have a top-hung large Velux
around 60cm above the existing loft floor, which means even if I gain
30-40cm ceiling height it will be less than the max. 110cm in the
It also needs to be within 1.7m of the eves (so somone working from a
ladder could reach someone at the window).
building regulations. The new staircase would be linked with the
existing staircase, looking like a "natural" continuation of the
staircase from the ground floor. What confuses me a bit is the BR that
appear to stipulate a 30min fire resistance, and the BR example of 9.5mm
plasterboard plus 60mm mineral wool plus 12.5mm chipboard that might
achieve this. My ex 2002-built Barratt house did not incorporate the
60mm mineral wool between the floors, only the plasterboard and wonky
chipboard, so I wonder how Barratt homes stand up compared to the BR ;-)
Knock the "compared to the BR" bit off the end of that last sentance and
you have the more pertinent question! ;-)
The regs get more complex in this respect when you add a third storey
(or above). Fire regs rquire 30 min protection to the new floor (but no
changes made to the existing ones). As you say 100mm rockwool/isowoll
plus 12.5mm PB will do the trick. You would need firedoors on the new
storey on the habitable rooms, and any doors in the rest of the house
that open onto a potential escape route would need self closers added.
You also will need mains powered interlinked smoke detectors on each
floor.
... My thought would have been to make the loft an open gallery area to
the stairway, so would the 30min fire resistance be irrelevant, or would
it force me to scrap the "open gallery" idea???
Don't know... sounds susspect.
Anyway that's it for now I guess, any feedback to any of the above
would be immensely appreciated...!
HTH
--
Cheers,
John.
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