New Lean to Roof

[ben9090]

Hi

Ive just had my new extensions roof tiled 1 week ago.

Its a lean to roof consisting of 6x2 C16 rafters at 450 centres fixed to a 6x2 bearer. The span is around 3m on the floor or 3.3m measuring taking the pitch into account (the span tables suggest 6x2 is OK upto around 3.6m clear span.) Ive added noggings accross the middle to prevent any twist.

Ive noticed that the rafters have settled so when you put a straight timber across the length of the rafters there is a 8-10mm gap at one end.

Its very slight, not noticable to the eye so do you think this is normal/acceptable or do you think I need to look at at installing a purlin or further support? Id rather not due to the aesthetics but obvioulsy I want it to be sound.

What Ive installed appears more then sufficient and building contol havent had any issues but I just want to make sure its done right before we install the insulation and plaster board it.

Thanks for looking

Thanks

 

[pinenot]

It's not unusual insomuch, and not a structural worry, hence BC's attitude. Are they exposed and therefore bothering you or are you more at ease now?...pinenot

 

[ben9090]

Hi
Thanks for the reply, I'm a bit more at ease. I looked again today and its barely 5mm.
The bearer defiantly hasn't shifted and isn't going anywhere and the 6x2 c16 at 450cc should definitely be sufficient for a 3m room.
I'm going to get building control out again just to make sure.

 

[tony1851]

All joists deflect when loaded; 5mm on a 3000mm span is well within the normally accepted limit for a roof.

 

[^woody^]

I'd say 10mm is too much if there will be a ceiling on the underside to crack

 

[tony1851]

I'd say 10mm is too much if there will be a ceiling on the underside to crack

Eurocode 5 (timber) recommends a maximum permitted deflection of span/250 for roofs carrying a plaster ceiling. For 3.2m span, this is over 12mm. Also, the rafters are not horizontal so the component of the load acting normal to the joists will be less.

IMO, OP won't have any issues.

 

[ben9090]

I'd say 10mm is too much if there will be a ceiling on the underside to crack

This ceiling had now taken the load and has flexed a little before the ceiling is in place. Once it's plastered then there shouldn't be any more give.

Building control are visiting tomorrow.

 

[^woody^]

It's timber, it will move some more seasonally

 

[tony1851]

It's timber, it will move some more seasonally

Scraping the barrel

 

[RonnyRaygun]

I'd say 10mm is too much if there will be a ceiling on the underside to crack

Eurocode 5 (timber) recommends a maximum permitted deflection of span/250 for roofs carrying a plaster ceiling. For 3.2m span, this is over 12mm. Also, the rafters are not horizontal so the component of the load acting normal to the joists will be less.

Also, most of the dead load deflection will already be in place before the ceiling is plastered (everything but the skim), so doesn't need to be considered in relation to cracking of the plaster.
And will the roof ever see the design live load of approx 60kg/m2? Not likely...and even if it did, how much deflection would that cause?

 

[^woody^]

Scraping the barrel

Well, if you think it a good idea to design rafters for a plastered pitched roof, using span tables for an unplasterd pitched roof then perhaps its your customers who will be scraping the barrel?

 

[tony1851]

Scraping the barrel

Well, if you think it a good idea to design rafters for a plastered pitched roof, using span tables for an unplasterd pitched roof then perhaps its your customers who will be scraping the barrel?

OK - some figures;

Area of roof supported by each rafter = 3.2x0.45 = 1.44m^2

Dead load 1.0 kn/m^2 live load 0.75kn/m^2

total load on each rafter 1.44x1.75 = 2.5kN.

bending and shear deflection approx 11mm (not writing the formulae, but take my word for it!)

This does not take into account the reduction in load due to slope, and also the reverse-moment effect of the overhang beyond the wall plate.

QED