max span 2.5" timber?

That depends on the beam spacing and how the load is distributed. Also how much bending is going to be acceptable.

If you use 400mm centres, and a working on a design load of around 100kg / m^2 (which is a uniform load on the joist of about 0.8 kN/m), and you have them sharing the load, then you top limit would be about 1.6m.

(that's setting the limit based on the maximum permitted long term stress on the timbers in the building regs - not the permitted bending (which we probably care less about in this application). It also assumes they are made from something approaching C16 timber).

They would sag a bit at that load, but not spectacularly.

Yup, and that would make a substantial difference. That would get you to

One hopes he realizes the 5" must be the height of the beam, not the width.

which is far removed from what they're IRL capable of.

yes, screw every 6"

Separate the 2 pieces by little offcuts every so often and you can go further still.

NT

That won't help much. Why not use blocks?

Bill

That would be stacked "vertically" not "horizontally" of course.

take weigt of couple of people, not putting a dance floor up there ;-)

make a 2.5 x 5 " beam?

If you have a decent router and a table, you could get some lengths of 9 mm OSB about 5 or 6 inches wide and make some I beams. Maybe ?.

You'd get a lot out of the wood that way, but wouldn't you need to test to destruction?

NT

Less than I thought..sometimes it`s better to ask first.

It ain`t B&Q banana wood thats for sure, there are some lenghts that are unplaned at 2.75, planed is exactly 2.5" square.

Quantity stacked outdoors where it has turned a fetching shade of sliver grey but dosent want to rot or warp. Was used as barge boards stacked in vertical I beams for big coal bunkers.

That sounds more useable, could I move to 600mm centres with doubled up joists and shorter span? 2.2- 2.5 probably about as deep as need to go.

engineered joists mentioned below.

Thanks, very much apreciated

Get into the world of engineered joists, which look like a very efficient use of material, and very DIYable

But bit like SIPs, Structural Insulated Panels, without certification, 45 quid a panel, with 125 quid.

cuttng plotters , make a racket from roll feed rather than transmtted sund like mill.

Its an insurance issue if they fail at some point in future, but think may see a use as roof trussing in single level exterior sheds, need to reach some snow loading to hit failure.

I've been considering a similar problem for re-roofing an existing workshop. Flat roof with minimal fall covered in box section steel over an ancient felted timber structure now crumbling.

The timber rafters have sagged badly.

Like the OP I have some stored materials which could provide a cheap way out. In my case scantling cut Oak around 80mm square. I have in mind creating *pozi* joists by screwing lengths of WBP 12mm ply to the sides. Say 1m x 300mm with 300mm gaps for service runs.

The stumbling block is my lack of structural engineering knowledge:-(

The span required is around 5.5m and the spacing could be 1.5 or 3 or

Yup at 2.5m you would be ok on 600mm

Basically two sections of timber stacked glued and screwed along the length. (you could skip the screws if you make a decent glue joint).