Load bearing 5*2 1080 centres

[David]

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

[Andrew]

Add a temporary purlin below along the middle of the span and some temporary braced or vertical supports.

[The Natural Philosopher]

yes, they will. One 5x2 is enough to hang yourself off. Over about

6-10ft span.

Your problem will be soley deflection and whether that damages the roof in some way.

It wont break.

[williamwright]

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

Bill

[Brian Gaff (Sofa]

Also may depend on the age of the roof,also on the state of the supports in between the main supports. Brian

[David]

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.

Thinking of it as a sloping floor I would expect somewhere around 400mm centres. At least that is what we have in the loft with 4 * 2 joists.

Cheers

Dave R

[John Rumm]

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!)

[David]

Thanks, John.

So a tentative shuffle might be OK but no Riverdance.

Cheers

Dave R

[John Rumm]

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.

[Andrew]

You are more at risk of a fall, or falling through the (now brittle) plastic covering.