Hi,

My roof trusses are made from 3" x 2" and there is a 4.5m span between the
wallplates, with just one internal tie from the apex to the mid point of the
joist. It all looks so flimsy that I'm quite nervous about storing anything
up there.

I had originall toyed with the idea of putting proper floor joists in so
that I can use it for storage, and then should I wish to do a full loft
conversion at a later date, the floor would already be up to the job. For
various reasons, I am moving away from that idea, but would still like to
beef up the floor by putting joists in raised a little further off the
wallplate for clearance.

Anyway, I;ve done all the calcs, and to satisfy the regs, over a span of
4.5m I'll need to use 2.5" x 9" on a 600mm pitch, or 2" x 8" on a 450mm
pitch for this floor to be upto loft conversion standard. Thats a lot of
expensive wood, and as I'll now only be using it for storage (admittedly
there will be books, and some heavyish stuff), how would I quantify the
risks of putting in 2" x 6" on a 450mm pitch, for instance. It is clearly
not up to everyday domestic use, but i am guessing that it will be sturdier
and a better solution thatn just laying boards on the 3" x 2" trusses. What
I can't grasp is the effect of the tie in the truss that runs verttically
from the apex to the centre of the horzontal. Does this effectively halve
the span of joist? If so, is 3" x 2" over 2.25m any better/worse than 2" x
6" over 4.5m.

In anycase, not laying boards on the trusses appeals as I am less likely to
crack the ceiling below while walking around, moving boxes etc.

Any thoughts?

Cheers

Chris

Your roof apears to be a traditional rafter construction and not a
trussed roof. Is this so?

No, they really are trusses - just thethat they are the flimsyest ones in
history.

A trussed roof has a typical W arrangement all the way across the loft,
a traditional roof is much more open with fewer timber struts.

The trusses are 600mm apart, and there is a totoal of 19 of them

each truss has just 4 sections, each is made from 3" x 2"

- The horizontal that the ceiling below is fixed to
- A pitched beam that goes from either end of the horizontal up to
the apex (i.e. the ones that the tiles roof tiles and felt are fixed to
- A vertical that goes from the apex to the mid point of the
horizontal

Also do you mean spacing instead of pitch?

By pitch i meant spacing (as in on a screw thread, or teeth on a sprocket) -
but i can see how that would be a reserved word when talking about a roof
:-)

The vertical internal tie sounds like a hanger, and can have the effect
of reducing the effective span of the joists, but may impractical or
not effective if for a loft floor - only for a ceiling. These hangers
would normally be on every other rafter/joist or connected to a binder
which runs on top and at 90 deg to the joists. How many do you have?

I am guessing that they are only there to help prevent the horizontal
section of the truss from sagging.

Are you using the span tables for a loft floor, or are you treating it
as a ceiling - which would seem more appropriate and would give thinner
timber sections.

I was using span tables for a floor. In many ways, I guess the actual
trussed and roof construction is irrelvant - I do tend to witter.

A more succint question would be :

I want to put joists between my wallplates (slightly raised, so they clear
the plasterboard of the ceiling below).

Using the span tables for a building a floor in a loft conversion, it seems
that i need 2" x 8" joists on a 450mm spacing between wallplates with a
clear 4.5m span to give a floor strong enough so as to use the mew "room"
for normal domestic purposes (i.e. loft conversion).

I now want to construct a floor, but I'm not going to be using it as an
everyday room, so it is not a proper loft conversion, not inhabitable, but
it is strong enough for storage and to walk around on.

With that in mind, if i were to use (cheaper) 2" x 6" beams on a 450mm
spacing spanning 4.5m, am I still going to have a good solid floor thats
just not quite up to spec for a real loft conversion, or would the resulting
floor be weakened so much by using 6" deep joists (instead of the 8" ones
i'd have to use for a real loft conversion) that it's not really suitable
for storage?

I am guessing that the 8" beams that meet the regs are over-engineered even
to take heavy use. Assuming that the strength of the resulting floor doesnt
diminish exponentially with the reducing depth of the joist, will a 6" beam
still give a good surdy floor? I.e. for storage, can I get away with 6"
joists.

Nuts, i have rambled again :-(

Thanks

Chris

Chris Styles wrote:
Time for a few pictures I believe.

If you haven't got a digital camera the cheapest way to do it is run a
cable up the access hatch and take soem lights with you then set your
computer up there and play around with a cheap web cam.

You can get a couple of halogens and a web cam for about £20 I should
think. You can post them on any space your ISP provides or sign up to
hotmail and join or make up your own MSN group with it.

Roof trusses have a number of different shapes depending on the spans
and roof pitches:
http://www.diydoctor.org.uk/projects/roof_trusses.htm

Their spacing or distance apart is called "at x centres", usually 600
millimetres. The end one might be any spacing under that. The spacing
is to allow for boarding the ceiling and so that roof lights are
standard.

Your problem, if you haven't thought about it, is how you are going to
get the extra timbers into the roof space. If you have a good, well
lit, look around up there you will see that not only are the trusses a
bloody awkward nuisance but they are stitched together with 4 x 1s.

If you are serious, you will have to raise the slates or tiles on the
roof, open the felt and introduce a beam each 600mm or whatever spaces
you decide on. You need to avoid damaging electric and plumbing
services up there and avoid dropping the end of it onto a ceiling.

Then you need to drag a shed load of floor boards up there and steeple
chase them into place. For the cost of the scaffolding involved you
would be able to buy a nice shed to store your stuff in. Or rent a
garage or something.

What exactly were you not thinking very clearly of?

6 * 2 = 1.20/m or 5.40/ 4.5m length. 8*2=1.60/m or 7.20/ 4.5 m length
an additional 1.80 per length * 19 length's = 34.20. Are you that
pushed that to do the job right you won't spend the extra 34.20?? Any
thoughts on how you would get them up there??

OKay,

Thanks for the followups (Weatherlawyer and legin)

- I'll go back to the the timber yard and ask to recheck the prices I got
from them, because they were quoting 2"x8" as more than twice the cost of
2"x6" - which makes the difference between a relatviely inexpensive job to
an expensive one.

- The trusses I have are "King Post"

- There are a few 4" x 1" planks nailed across the horizontals of the
trusses near where the king post join. I'd figured that these can be removed
as there were originally intended to hold it all the trusses in position
while the trusses werre being fixed to the wall plates. If this isnt the
case, and they can;t be removed then I'm knackered. But if that were the
case it wouldnt be possible to do a loft conversion at all, which doesnt
seem right.

- Getting the 2"x8" joists into the lofts isnt too much of a problem. I have
already taken one up there via the loft hatch as a test.

- The roof is a steep pitch, and so at the outside edge of the wallplate
there is a clear 12" gap between the wallplate and the felt, so an 8" joist
on a extra 1" wallplate (for clearance) will fit okay

- If the cost is really that insignficant between the two joist sections,
i'll go for the 8" joists, no questions.

Cheers

Chris

I should have mentioned that those prices are the trade prices that I
pay but are plus VAT so allow a little extra for the Vat. Depending on
where you are the prices do vary but those prices should be available
pretty much widely across the midlands.

Chris Styles wrote:

- The trusses I have are "King Post" [composite trusses]

- There are a few 4" x 1" planks nailed across the horizontals of the
trusses near where the king post join. I'd figured that these can be removed
as there were originally intended to hold it all the trusses in position
while the trusses werre being fixed to the wall plates. If this isnt the
case, and they can't be removed then I'm knackered. But if that were the
case it wouldnt be possible to do a loft conversion at all, which doesnt
seem right.

These trusses are designed to give a lot of rigidity to the whole roof
when the 4 x 1s are in place. Not only are there the spacers you
mentioned but there are triangulated ones up near the tiles.

What did you want to store up there? They will take a water tank if set
in the right place and spread out ove rthe required number of trusses.
Can you put spreaders over an internal wall? You should be able to see
where any stud walls are from the attic space. Lay some planking across
those and you should be able to store some boxes up there.

There is likely to be some space between the walls' headers and the
trusses. There is nothing to stop you packing those gaps. They won't be
more than an inch at most. Probably 1/2" -just enough to tilt the frame
into place. (and make packing awkward. Use paired wedges -and keep them
off any drywalling or you'll press it off.)

A drawing might help for further advice. It will be less room but a lot
cheaper to do it like that. Putting stuff on unsupported trusses will
cause them to fail eventually. (Of course you can walk across them
occasionally, for maintainance.)

I had originall toyed with the idea of putting proper floor joists in so
that I can use it for storage, and then should I wish to do a full loft
conversion at a later date, the floor would already be up to the job. For
various reasons, I am moving away from that idea, but would still like to
beef up the floor by putting joists in raised a little further off the
wallplate for clearance.

If you are storing books, you should really use at least the thickness
recommended for a habitable room.

You might get away with 6" x 2" if the flooring is made part of the
structure. This requires it to be 18mm plywood glued liberally to the joists
with PVA. Disadvantage is access to electrics etc underneath and isn't
exactly conducive to a trussed installation.

I'm using a similar construction in my loft conversion, which allows 5"
joists instead of 7", saving me loads of headroom. A similar construction is
used on the (flat) roof. They are firred down as well to give a level
ceiling, but sloping roof. At their thinnest, they are 3", despite an almost
4m span, and apparently could have been shallower structurally. All designed
by a structural engineer and approved by BCO.

If you have a trussed roof, you could install 6" for light storage now and
then PVA down a plywood surface when you decide to convert, removing the
trusses. Check with a structural engineer first that this will be OK. I
believe they call it a diaphragm floor. Commonly applied in commercial
properties, but surprisingly rare in domestic, despite the height advantage
for loft conversion.

Christian.