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.

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).