Hi all, I’m currently building a garden room and have a question regarding the flat roof I plan on installing. 
The room is 5.3x5.3 outside walls and internal walls are 4.7x4.7  
I’d like to know if 8x2 joists @400 centres will suffice over this distance, installing an rsj is not an option for me due to doors and window lintel sizes and positioning. 
Many thanks 

No, you would need to install 47 x 220mm C24’s at 400mm c/c.

 

What are the walls constructed of?

 

What is the overall height snd how close to a boundary are you?

Hi and thanks for the reply. The overall height will be 2.5m once the 220 joists and roof deck and coverings are installed. 1 metre away from the boundary. 
So just to confirm I’d be looking at 2x9 for the joists? 
I’ve had someone else tell me that I should be using 3x9, not sure if that overkill, that said I’d rather have it as strong as possible. 
many thanks 

Using C16 timber, 47 x 220mm (roughly 2x9) will suffice for the clear span you will have and will allow you to be more efficient with your insulation. "3x9" is indeed overkill.

 

If internal height is an issue, then moving to 63 x 195mm C16 will give you an extra "inch" but will reduce insulation performance.

 

You could also use C24 timber, allowing 47 x 195mm or 63 x 170mm.

 

See https://www.timberbeamcalculator.co.uk/en-gb/span-table/flat-roof-joists?load=0.75&class=C16 for C16 and https://www.timberbeamcalculator.co.uk/en-gb/span-table/flat-roof-joists?load=0.75&class=C24 for C24 timber. 

2 hours ago, garrymartin said:

Using C16 timber, 47 x 220mm

Quite concerning how different the tables you’ve referred to differ to the standard Trada tables as the latter don’t confirm your size.

3 hours ago, Harry13 said:

to confirm I’d be looking at 2x9 for the joists?

Yes, but play it safe using C24’s and not C16’s.

 

Is it being constructed with masonry walls?

6 minutes ago, DevilDamo said:

Quite concerning how different the tables you’ve referred to differ to the standard Trada tables as the latter don’t confirm your size.

Do you have a link to the Trada tables you're looking at? Perhaps the parameters are different? I've assumed access for maintenance and repair only. Are you also looking at the "clear span" of approximately 4.7m?

 

These two links are Eurocode 5 compliant spans (as per Trada);

 

https://www.taylormaxwell.co.uk/uploads/files/D.100.01-01_C16-Span-Tables_Design_Span-Tables.pdf

https://www.taylormaxwell.co.uk/uploads/files/D.100.02-01_C24-Span-Tables_Design_Span-Tables.pdf

 

Hi, the walls have been made with concrete blocks and there are 2 leafs with a cavity. External wall measures 5.3m on the outside edge and the internal leaf is 4.7m on the inside. Thanks

3 hours ago, garrymartin said:

Do you have a link to the Trada tables

Not a link but I have them saved. None of the C16 tables justify a 4.7m clear span with 47x220mm @ 400c/c

20 minutes ago, DevilDamo said:

Not a link but I have them saved. None of the C16 tables justify a 4.7m clear span with 47x220mm @ 400c/c

I'm lost??? Is there perhaps something I'm misunderstanding in the tables?

 

The first table clearly shows a 5.00m clear span for a 47x220mm C16 @ 400 centres (Table 7.1) and the second table shows a 4.89m clear span for a 47x220mm C16 @ 400 centres (Table 7.3)

Incorrect dead load. You need to take the middle section of each table.

22 minutes ago, DevilDamo said:

Incorrect dead load. You need to take the middle section of each table.

I suppose we should have asked what the roof make-up would be.

 

Pretty sure when I did the calculations for my roof, the dead load was less than 0.5 kn/m2 for 18mm plywood, EDPM, internal 12.5mm plasterboard, skim, and insulation.

 

Do you have something that would give me some advice on when to use 0.5, 0.75, and 1.0 ratings? Is there any specific roof make-ups to dead load comparisons?

 

 

I'm clearly doing something wrong or fundamentally misunderstanding the calculations. I've just repeated my roof calculations as follows;

 

12.5mm plaster + skim = 0.2 kn/m2

XPS insulation = 0.04 kn/m2

18mm plywood = 0.1 kn/m2

1.52mm EPDM = 0.02 kn/m2

 

The total for that is 0.36 kn/m2 so well below 0.5 kg/m2.

 

@DevilDamo Am I doing something wrong or misunderstanding some part of the calculations? Or is it just we didn't clarify what the proposed roof make-up was?

Edited August 10, 20242 yr by garrymartin
noted as 0.36 kg/m2 and should have been 0.36 kn/m2

I was initially comparing your figures to mine… both using a dead load of 0.75 and that is what/why I said they were different. 

1 minute ago, DevilDamo said:

I was initially comparing your figures to mine… both using a dead load of 0.75 and that is what/why I said they were different. 

I think we've inadvertently confused the hell out of @Harry13

 

Harry, just make sure to use C24 timber! 🤣

https://www.buildingcontrol-ni.com/assets/pdf/TechnicalBookletD2012.pdf

5 minutes ago, ETC said:

https://www.buildingcontrol-ni.com/assets/pdf/TechnicalBookletD2012.pdf

Not sure that helps loads - it's an old document and for Northern Ireland...

2 minutes ago, garrymartin said:

Not sure that helps loads - it's an old document and for Northern Ireland...

It’s a current document and physics is still physics irrespective of where you are in the UK.

Edited August 10, 20242 yr by ETC

3 minutes ago, ETC said:

It’s a current document and physics is still physics irrespective of where you are in the UK.

🤣 Fair comment.

 

Though the tables don't have the same numbers in them... 😉 

Just now, garrymartin said:

🤣 Fair comment.

 

Though the tables don't have the same numbers in them... 😉 

Of course they don’t - they’re for different imposed loads which vary for location.

11 hours ago, ETC said:

physics is still physics irrespective of where you are in the UK

The force of gravity, and therefore time, changes depending on altitude and ground density.

 

All the granite in Cornwall makes it 20 years behind the times.

Edited August 11, 20242 yr by SteamyTea