Hi Knowledgeable Ones,
Can anyone point me at building reg's type span tables or similar for
steel purlins?

We're interested in a house that SWMBO wants to convert the loft in
(should we get it), and wooden purlin tables only go to about 4m spans, we'd
be looking at 6m (the current wooden 8x2" ones are braced to the tops of
interior walls at about 2m spacings, and right where the means of escape
windows would have to go, so there will need to be two purlins under each
face of the roof, one above and one below the windows...) It's a trad' cut
roof, so without the braces there's an usable space of about 25' (to the
eaves/wall plates) by 17' between end walls, enough for our New Big Bedroom
And En-suite, and a maximum floor joist span of 12'6", so within limits for
the span tables I've seen... there's even room for the staircase, we reckon
)

The house is brick + block, with a cast concrete floor, 2 storeys and
has probably used its permitted development rights when the previous owners
had the garage converted into a couple of habitable rooms and a further
bedroom with en-suite built above - the area above this extension is another
8'6 wide (nice short joist span, phew!) for another loft room, again 25' to
the wall plates but with an end wall suitable for a means of escape window,
so no issues with moving purlins for roof windows like the main area.

The Kitchen's too small for her too (and me, I like cooking!) so we'd
want to knock through the (load bearing) wall into the current dining room,
with an Irish Jay spanning about 12'. I presume we'll have to have a pair of
piers to support the ends - any idea on the sizing for these?

Thanks in advance,
Dave H.
(The engineer formerly known as Homeless)

Hey the group is DIY, not qualified structural engineers! Get a copy of
Architects Pocket Book (Charlotte Baden-Powell) - it has tables for
safe loads on steelwork up to 8 meters (and formulae for timber beams).
But given the seriousness of your plan, I think you'll be paying a
structural engineer to get past BC.

wrote:

Hey the group is DIY, not qualified structural engineers! Get a copy of
Architects Pocket Book (Charlotte Baden-Powell) - it has tables for
safe loads on steelwork up to 8 meters (and formulae for timber beams).
But given the seriousness of your plan, I think you'll be paying a
structural engineer to get past BC.

Too true. It wouldn't have ben so bad if he'd cross posted it to
alt.building.construction or some such. Stupid omission rectified BTW.

British homes are regulated AFAIR to take the weight of a large man
with a big bag of tools walking across the roof in a blizzard.

The weight or load on a pitched roof in an F12 is considerable as the
slope is an aerfoil. The rafters are let into the wall plate and any
purlins and fixed with whopping big nails cross stiched.

The wall plate is a 4 x 3 that spreads all this loading finally onto
the masonry. Being wood its harmonics tend to deaden vibration. Long
(and long unsuported spans, especially) can have amazing resonance
problems and steel is a pretty good sounding mechanism.

That's all I know.

That and "Google is your friend."

AIU this chaps problem, the purlins cross horizontally through the area
where he wants to put velux windows. So he's considering replacing the
entire length of the purlin with 2 new steel purlins above and below
that line. However I don't understand the bit about the purlins being
braced to the walls at 2m spacings. I can't picture this at all. Are
there no roof trusses? Are the purlins supported at the gable ends
only? How come there's internal walls every 2m under the line of the
purlin?

The objective seemed to be means of escape windows (are the gable ends
not suitable?). However if there's sufficient space above the purlin
for a velux window, a built in step or steps up to the window may be
acceptable if the bottom edge of the window would otherwise be too high
for means of escape (1 meter max?).

Dave seems to be looking on basing his soultion entirely on the tables
in the approved documents. Other (timber based) solutions are possible,
but require engineers calculations to be submitted to BC.

The house is brick + block, with a cast concrete floor, 2 storeys and
has probably used its permitted development rights when the previous
owners
had the garage converted into a couple of habitable rooms and a further
bedroom with en-suite built above -

Check with planning. The garage conversion might not have used any PD rights
at all if it didn't increase built volume. Even if it involved extending the
roof, there may be some left.

Christian.

"Dave H." wrote in message
...
Hi Knowledgeable Ones,
Can anyone point me at building reg's type span tables or similar for
steel purlins?

We're interested in a house that SWMBO wants to convert the loft in
(should we get it), and wooden purlin tables only go to about 4m spans,
we'd be looking at 6m (the current wooden 8x2" ones are braced to the tops
of interior walls at about 2m spacings, and right where the means of
escape windows would have to go, so there will need to be two purlins
under each face of the roof, one above and one below the windows...) It's
a trad' cut roof, so without the braces there's an usable space of about
25' (to the eaves/wall plates) by 17' between end walls, enough for our
New Big Bedroom And En-suite, and a maximum floor joist span of 12'6", so
within limits for the span tables I've seen... there's even room for the
staircase, we reckon )

The house is brick + block, with a cast concrete floor, 2 storeys and
has probably used its permitted development rights when the previous
owners had the garage converted into a couple of habitable rooms and a
further bedroom with en-suite built above - the area above this extension
is another 8'6 wide (nice short joist span, phew!) for another loft room,
again 25' to the wall plates but with an end wall suitable for a means of
escape window, so no issues with moving purlins for roof windows like the
main area.

The Kitchen's too small for her too (and me, I like cooking!) so we'd
want to knock through the (load bearing) wall into the current dining
room, with an Irish Jay spanning about 12'. I presume we'll have to have a
pair of piers to support the ends - any idea on the sizing for these?

Thanks in advance,
Dave H.
(The engineer formerly known as Homeless)


Can't help you with your prob but just for your info your 'Irish Jay' is a
Rolled Steel Joist or RSJ.

"Slurp" wrote ...




Can't help you with your prob but just for your info your 'Irish Jay' is a
Rolled Steel Joist or RSJ.
Now, I'd never have guessed... hence the ;o) in the header!

Dave H.
(The engineer formerly known as Homeless)

"Weatherlawyer" wrote in message
oups.com...

wrote:

Hey the group is DIY, not qualified structural engineers! Get a copy of
Architects Pocket Book (Charlotte Baden-Powell) - it has tables for
safe loads on steelwork up to 8 meters (and formulae for timber beams).
But given the seriousness of your plan, I think you'll be paying a
structural engineer to get past BC.

Thanks for the recommendation re the book, agreed a structural engineer will
have to look over it, buut having seen what a colleague at work got from his
S. Eng., there's actually not a lot to the calculations needed to meet the
B. Reg'S!

Too true. It wouldn't have ben so bad if he'd cross posted it to
alt.building.construction or some such. Stupid omission rectified BTW.

Thanks - I couldn't see the a.b.c group on the server I use, will seek it
via google groups (aargh).

British homes are regulated AFAIR to take the weight of a large man
with a big bag of tools walking across the roof in a blizzard.

The weight or load on a pitched roof in an F12 is considerable as the
slope is an aerfoil. The rafters are let into the wall plate and any
purlins and fixed with whopping big nails cross stiched.

Known and understood, there are tables for the wind load expected "once in
50 years", "once in 100 years" as part of the notes to the reg's, I'm aware
that the dead load has to be taken as a starting point, not a maximum, with
a scale factor to calculate the actual dynamic loads/metre-squared over the
roof area.

As well as the rafters being attached to the wall plate, there are
structural requirements for dealing with the horizontal loads transmitted to
the walls, usually dealt with by tying the walls to the floor and ceiling
joists via Sturdy Brackets, as you say, often skew-nailed, although bolts
with proper wood fixing washers are better than nails for the
joist/purlin/rafter joints (more expensive though, hence rarely used and
often misused).

The wall plate is a 4 x 3 that spreads all this loading finally onto
the masonry. Being wood its harmonics tend to deaden vibration. Long
(and long unsuported spans, especially) can have amazing resonance
problems and steel is a pretty good sounding mechanism.

Hmm, that must be why no one makes musical intruments out of wood... ;o)

That's all I know.

That and "Google is your friend."


Thanks for the comments,

Dave H.
(The engineer formerly known as Homeless)

wrote in message
ups.com...
AIU this chaps problem, the purlins cross horizontally through the area
where he wants to put velux windows. So he's considering replacing the
entire length of the purlin with 2 new steel purlins above and below
that line. However I don't understand the bit about the purlins being
braced to the walls at 2m spacings. I can't picture this at all. Are
there no roof trusses? Are the purlins supported at the gable ends
only? How come there's internal walls every 2m under the line of the
purlin?

The purlins are supported at the gable ends, yes, as their replacements
would be. There's an internal wall 6' or so from one gable (the stairwell
and hallway, running from front to back of the house) and a 45-degree brace
runs from a wall plate on top to each purlin; the other braces run from the
purlins to an oversized ceiling joist spanning the house front-to-back and
bearing on an internal wall in the middle (with 2 off 12'6" spans). There
are no horizontal links crossing the loft between the two purlins, other
than the brick gable ends.
This is a traditional cut roof, not a modern fink-truss roof with lots of
triangulation dividing it into 4'-sided triangles (which probably wouldn't
have enough headroom to be worth converting in the first place!).
Does that give you a clearer idea of the roof structure?

The objective seemed to be means of escape windows (are the gable ends
not suitable?). However if there's sufficient space above the purlin
for a velux window, a built in step or steps up to the window may be
acceptable if the bottom edge of the window would otherwise be too high
for means of escape (1 meter max?).

Unfortunately, that wouldn't meet the reg's - there's a very strict maximum
distance of 1500mm from the edge of the roof (including guttering) to the
bottom edge of the means of escape, and if the bottom of the window's (for
instance) 1200mm above the wall plate (allowing 200mm for floor joists
etc.), the roofing projects (typically) 300mm from the wall and the roof has
enough pitch (eg 45 degrees) to make the space usable for a conversion, the
bottom edge of the window is going to be around 2000mm from the edge...
BCO's are *very* hot on means of escape, and so would I be!

Dave seems to be looking on basing his soultion entirely on the tables
in the approved documents. Other (timber based) solutions are possible,
but require engineers calculations to be submitted to BC.



Thanks for your interest,
Dave H.
(The engineer formerly known as Homeless)

Thanks for your clarifications Dave - I see exactly what you're getting
at now. I had forgotten the distance to the roof edge requirement on
escape windows.Complete agreement also on some structural engineer
calculations being within the scope of the non-structural engineer
(having paid my bill for the engineer calculations on my chapel
conversion - I'm revising it myself - and quite honestly can do rather
better than that which paid £250 for).