Joist span for flat roof

[DIYdar]

Hi all,

I have recently completed a block built summerhouse/workshop in the garden. My intention is to have a flat roof with a slight overhang at the front edge.
The building size is approx 6.5m wide by 4.5m front to back.
My question revolves around the correct joist size for the roof. As far as i can see there is no readily available timber size suitable for what will probably end up being a 5.5/6m overall length. I have looked at ‘sistering’ 152 x 47 with something like a 600mm overlap and bolted using toothed washers, but can’t seem to find any advice on whether this is acceptable?
Should probably mention the building is nearing its height limit so anything that could be done to keep the height of the roof down would be appreciated. Thanks Darren

 

[tony1851]

152 x 47 on that span will deflect too much.

 

[DIYdar]

Thanks for the reply Tony. Does this apply still if the joists are overlapped and bolted though, as I suppose im thinking the actual span of a “single” joist is only about 2.5/3m then reinforced through the overlap? Or am I missing the point? Cheers

 

[Notch7]

Would your span not be 4.5m?

Joists for that span would normally be around 9"......

If you want 6" joists you need a steel midway.

You def cant put a joint in the timber. Although I think you can get 6 metre joists in 6 x 2.

Warm roof?

 

[DIYdar]

Thanks Notch.
Yes wall plate to wall plate would be around 4.5m, the 6m figure comes from the total length of timbers with overhang front and back.

The intended construction is cold roof and to be honest, bearing in mind its only a workshop/summer house my aim is to try and keep costs (and height!) down. I had seen various references to “sistered” joists and a colleague mentioned that a structural engineer had ok’d it for a project he was doing so was hoping it might work for me...

 

[Notch7]

Thanks Notch.
Yes wall plate to wall plate would be around 4.5m, the 6m figure comes from the total length of timbers with overhang front and back.

The intended construction is cold roof and to be honest, bearing in mind its only a workshop/summer house my aim is to try and keep costs (and height!) down. I had seen various references to “sistered” joists and a colleague mentioned that a structural engineer had ok’d it for a project he was doing so was hoping it might work for me...

Sistered joists usually means full length for stiffening purposes. Not to strengthen a butt joint.

Your cheapest option is to use deep enough joists in a single length.

The old rule of thumb was length in feet / 2 then add 2" so in your case 15'0/2 = 7.5 +2 =9.5

Span tables give 8 x 2 = 4.3 metre span at 400 centres

Maybe you could get away with 7 x 2 and have a bit of a sag. Maybe double up 3 of the joists to improve stiffness. Or even add a flitch plate to them.

Dont forget you need to allow a bit of strength for snow loading. I can tell you 8 inches of snow weighs a lot

 

[tony1851]

Technically there's nothing wrong with using 6x2 for a flat roof of clear span 4.5m - it won't collapse. What it will do is deflect to a considerable amount, particularly under snow load (see Notch above).
But excessive deflection is not really a structural issue - it's what's known as a serviceability issue, which is of less importance. For example; the deflection may be noticeable inside - could you live with that? (over time, the deflection will get more noticeable as timber 'creeps' under long-term load); it may crack a plasterboard ceiling - but do you need to have a ceiling in a workshop?; or it may cause ponding of water on the roof - a good, durable finish could alleviate that.
If you have to use 6x2, why not slope it down by have one side 6" lower than the other - that would probably prevent any ponding of water, and don't bother with a ceiling.

 

[DIYdar]

Thanks everyone for your replies. Certainly a bit to think about. I think i may just bite the bullet and up the sizes to 8x2 or even 9x2, there seems to be some fairly local suppliers who supply these sizes in the long lengths i need and at least this way it should give a little bit more rigidity for any future use. Cheers Darren

 

[catlad]

Don't spoil ship for happenny worth of tar!