Correct Flat Roof Joist Sizes [Drawing]

GateBeam · 13 Sep 2019

From this drawing, what are the correct joist sizes for my flat roof.

Some facts;

Joist Span 4.8m
Gradient 1 in 80
Joist size 170 x 45 [Regularised Size / Finished Size]
Spacing @ 450 c.c

C24 SPAN TABLE

According to this span table
A Dead Load [kN/m2] below 0.25 can allow a span of 3.83m ,
So would this mean my joist sizes are under the specified standard?
What would be the Imposed Dead Load for a Flat roof?

Thank you.

^woody^ · 14 Sep 2019

Your span looks like 3700mm.

What's going on with those pointless insulated soffits?

noseall · 14 Sep 2019

^woody^ said:
What's going on with those pointless insulated soffits?

It's a continuation of the thermal break from the top of the masonry to the underside of the warm deck. What would you suggest they use - hope?

noseall · 14 Sep 2019

^woody^ said:
Your span looks like 3700mm.

Correct.

GateBeam · 14 Sep 2019

^woody^ said:
What's going on with those pointless insulated soffits?

noseall said:
It's a continuation of the thermal break from the top of the masonry to the underside of the warm deck. What would you suggest they use - hope?

As @noseall said, I wish to insulate the back of the fascia board and soffit to prevent cold spots and condensation.

^woody^ said:
Your span looks like 3700mm.

noseall said:
Correct.

Ah, I see, I was clearly measuring it wrong. Thank you for the correction.

So this means that these 170x45 joist are fine to use?
Is 0.25 kN/m2 the Imposed Dead Load for a Flat roof? Do I have to factor in snow load?

^woody^ · 14 Sep 2019

noseall said:
It's a continuation of the thermal break from the top of the masonry to the underside of the warm deck. What would you suggest they use - hope?

Yes they could do.

Or more traditionally, just take the wall insulation up to meet the roof insulation.

noseall · 14 Sep 2019

^woody^ said:
Or more traditionally, just take the wall insulation up to meet the roof insulation.

Oh, and just call it by a different name. Gotcha.

tony1851 · 14 Sep 2019

@GateBeam; Your dead load will be higher than 0.25kN/sq m - you would probably be in the 0.25-0.5 column, which is the one most used for flat roofs.
That figure refers to dead load only (ie the weight of the structure). The live load is normally taken as 0.75kN/sq m, (unless you are in an area of higher than average snowfall) but the load/span tables usually allow for that in their figures.

GateBeam · 14 Sep 2019

@tony1851

Thanks for the very useful info. I'm from Sheffield so no high snowfall I believe.
But using your info, I will be using;
C24 170x45 joists at 400mm spacing which allows for 3.81m span. With my span being 3.7m

Thanks again.

GateBeam · 14 Sep 2019

^woody^ said:
Yes they could do.

Or more traditionally, just take the wall insulation up to meet the roof insulation.

Something like this?

Flat Roof Extension Drawing 14-09-19-Layout2.png

catlad · 14 Sep 2019

The first one was better.

GateBeam · 14 Sep 2019

catlad said:
The first one was better.

Right, i'll stick with that. Thanks.

^woody^ · 14 Sep 2019

GateBeam said:
Something like this?

View attachment 171354

Yes. No need for all that nonsense in the soffit and doing jack.

noseall · 15 Sep 2019

^woody^ said:
Yes. No need for all that nonsense in the soffit and doing jack.

noseall · 15 Sep 2019

catlad said:
The first one was better.

Agree. Woodys pride won't allow him to back peddle.

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Correct Flat Roof Joist Sizes [Drawing]

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