How to estimate or determine the strength of a flat roof?

[smp50]

I have a flat roof over a single-storey extension and I'd like to know what kind of weight it would be capable of holding. I know it wasn't designed to bear a load, but clearly it can take something since, for instance, I can walk around on it with no problem and it feels as firm as a floor.

If I wanted to get a good idea what it could support would I need to lift the felt and have a look at its structure? Timber dimensions and so on? I know some people build decks on flat roofs so presumably one can safely figure out how strong they are.

Thanks for any suggestions,

 

[tony1851]

Flat roofs are generally intended to carry about one-half the accepted floor load in a house - in old money, that's around 15 lbs/sq ft (as against 30 lbs/sq ft for a normal domestic floor).

Unless it's in poor condition, it wont collapse if you hold a party up there, but you may get excessive deflection and 'bounce', which could damage the ceiling. Any decking you add to it will reduce the safe load you could put on it.

Also bear in mind that if the felting has leaked at any time, the boarding could be rotten, even if the joists are still OK.

 

[smp50]

Flat roofs are generally intended to carry about one-half the accepted floor load in a house - in old money, that's around 15 lbs/sq ft (as against 30 lbs/sq ft for a normal domestic floor).

Unless it's in poor condition, it wont collapse if you hold a party up there, but you may get excessive deflection and 'bounce', which could damage the ceiling. Any decking you add to it will reduce the safe load you could put on it.

Also bear in mind that if the felting has leaked at any time, the boarding could be rotten, even if the joists are still OK.

Very useful, thanks.