doubling up 2x4 timbers?

[Newbie1981]

does anyone know if you can double up timbers (side by side) to increase the span, i would like to be able to span 3m im working to 2.5m height.
thanks

 

[MikeG.]

What is it you're building, and which bit of it is spanning 3m?

 

[Newbie1981]

Hi Mike, i hoped you would answer.
it would be for roof joists on a garden room, the joists would be 3.6m long but have to span 3m between the front and rear walls, 100mm rear overhang and a 500mm front overhang, it would be a warm roof construction, joists, 18mm osb3, 100mm insulation boards, 11mm osb3 and then a 1.5mm rubber roof

 

[MikeG.]

Why does it need to be 4x2s? That's the job of 7x2s. 6x2s at a pinch.

 

[Newbie1981]

height restrictions, my garden is about a foot and a half higher than my neighbours, so the shed will stand at about 10feet on their side of the fence as it is.
was hoping that i could double up 4x2's to make it less imposing

 

[MikeG.]

Have you got the headroom to have a purlin under the timbers? A 4x2 can only span to a little under 2m.

 

[topchippyles]

height restrictions, my garden is about a foot and a half higher than my neighbours, so the shed will stand at about 10feet on their side of the fence as it is.
was hoping that i could double up 4x2's to make it less imposing

You can span in 4x2 but would need to double up on each joist so your 4x2 becomes 4x4 and and bolt each rafter together.

 

[MikeG.]

height restrictions, my garden is about a foot and a half higher than my neighbours, so the shed will stand at about 10feet on their side of the fence as it is.
was hoping that i could double up 4x2's to make it less imposing

You can span in 4x2 but would need to double up on each joist so your 4x2 becomes 4x4 and and bolt each rafter together.

I'm not saying you're wrong, but no tables of mine support this approach.