On 12/04/2021 19:31, David wrote:
On Mon, 12 Apr 2021 17:16:53 +0100, John Rumm wrote:

On 10/04/2021 13:33, David wrote:
On Fri, 09 Apr 2021 17:22:04 +0100, williamwright wrote:

On 09/04/2021 13:22, David wrote:
I'm trying to work out what weight can be safely put on timbers
supporting a shallow sloping roof.

I potentially need to get up on the roof to service the plastic
covering and also the flashings at some points.

My embryo plan is to construct some crawling boards which won't slip
(essential) and will carry my weight (around 168 pounds) and some
tackle.

Possibly 2 or 3 boards so I can work my way along from board to
board.

First I need to know if the timbers will support my weight, though.

Cheers


Dave R



Do you mean 5" x 2" spanning a metre? You could park a car on that.

Bill

Sorry, not enough information.
Posted in a hurry to remind myself then haven't got back until now.

Rafters made from 2 * 5 timber.
Centres (gap between centre of two rafters) 1080 mm.
Length of rafter (not mentioned) roughly 2.3 metres.

Aha, yup that makes it much easier :-)

I did a quick model in superbeam on a C16 47x125mm timber, assuming
worst case load of 1kN at centre span (i.e. a point load of 100kg),
single beam acting alone, and specified a medium term load (when we are
only talking about short term loading really), and lastly horizontal
(which creates a higher load than on a sloping timber where some
component of the force will be longitudinal)

End result is, total deflection would be just over 5mm, which would even
meet building regs for a floor!

(although it would fail to meet permitted deflection for building regs
(6.9mm) if I climbed up on it!)

Thanks, John.

So a tentative shuffle might be OK but no Riverdance.


I suspect you could jump up and down on it!

Keep in mind that the building regs limits are well below the point of
failure - they are concerned with creating floors that don't sag to much
so that they would damage decorations in the room below, or feel too
bouncy when walked on. For the purposes of sticking a few scaffold
boards (or a sheet of ply) across a roof for safe access, you have
plenty of reserve capacity available. Remember I was modelling more than
your entire weight concentrated on just the centre of a single joist -
not spread over several.


--
Cheers,

John.

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