Firring question

[Adrianlondon]

Hi all

I am building an extension and the width of my extension is 6.5m (the width of my house). The length will be 6m. The joists will be going width ways (6.5m long).

My plans state I must have a drop which is at least 1 in 80. So, by my calculation I need a drop of 75mm (1/80*6*1000)

I have been led to believe that it is better to get a piece of wood which is lets say 100mm high at one end so the other end will be 25mm. This way it is easier to nail into the joists.

What I'd like to know is the following...........

1) Does each furring have to be 6 meters long? (I'm assuming it does)
2) Should I make sure that if the drop needs to be 75mm then I should have one end being 100mm and the other end 25mm?
3) What wood should I use?
4) I'm assuming a good quality nail gun will be the best way of holding the furrings in place.

All answers are very much appreciated.

 

[freddiemercurystwin]

6m is a pretty hefty span for timber joists or are they supported in the middle? Has your plan drawer sized them?

 

[tony1851]

It's even longer - 6.5m; has to be supported somewhere?

 

[Adrianlondon]

The length of the timber is 6.5m This is the width of the extension. I double checked with the architect. The timber will be 75 x 220mm C16 roof joists at 400cc From what I can tell, that means that the gap between each joist will be 400mm from the centre of each joist.

100x50mm wall plate

I can only go by what the architect has put on the plans.

They will not be supported in the middle.

 

[freddiemercurystwin]

Span between inner skins will actually be about 5.9m tony I'd guess, which just about gets him into using the span tables, assuming the lowest dead load.

 

[tony1851]

The length of the timber is 6.5m This is the width of the extension. I double checked with the architect. The timber will be 75 x 220mm C16 roof joists at 400cc From what I can tell, that means that the gap between each joist will be 400mm from the centre of each joist.

The tables from a 1992 edition of the Approved Document A show that, at spacing of 400, with a dead load of between 0.5 and 0.75 and with grade C24 timber, the max. span is 5.97m. For C16 grade it is 5.76m.

Perhaps space the joists a little closer?

 

[freddiemercurystwin]

@ Adrianlondon, the 400 centres means from centre to centre not the gap between each joist.

At 6.5m your fall is 81mm so I would aim for 100mm fall to be safe so would have said 150mm joist cut down to 50mm at one end though I'm not sure you'll get that length in one.

 

[freddiemercurystwin]

edit

 

[Adrianlondon]

@ Adrianlondon, the 400 centres means from centre to centre not the gap between each joist.

At 6.5m your fall is 81mm so I would aim for 100mm fall to be safe so would have said 150mm joist cut down to 50mm at one end though I'm not sure you'll get that length in one.

Sorry, that's what I meant. From the centre of the joist to the centre of the next joist. Sorry for not explaining myself very well.

Regarding the firrings, it would not be 6.5m only 6m

The joists will be going along the width of the house (which is 6.5m). The firrings will be going from the house towards the bottom of the garden which is going to be 6m.

 

[freddiemercurystwin]

Ahh OK, hence your 75mm.

 

[catlad]

In 99 per cent of construction what you are doing does not exist so think about what you are doing.

 

[theoldun]

In 99 per cent of construction what you are doing does not exist so think about what you are doing.

Catlad is right. You are working ar*se about face.
6500 width gives 5900 span.
75x220 C16 at 400 centres with an imposed load (snow load) of 0.75KN/m2 and a dead load of 0.50 KNm2 with access only for maintenance and repair will span 6170. So your joists will comply.
If you are doing a warm deck, joists will need to be 6600 long, if a cold deck then they will need to be 6900 long to allow for cross ventilation through the soffit.
It will take you about 14 days at least to source these lengths
NOTE. Firrings do not run at 90 degrees to the joists. Firrings run the length of the joist. If you wish to make your fall at 90 degree to the joists, then you use reducing rips planted on top of joists.
i.e. joist against house wall 75mm deep rip on top of joist, next joist say 70mm deep on top of joist, next joist say 65mm deep ditto. You work your depth of rips out by dividing number of joists into total fall or stick a blocik on first and last joist and run a line over the top and measure down to each joist.
Regards oldun

 

[Adrianlondon]

In 99 per cent of construction what you are doing does not exist so think about what you are doing.

Catlad is right. You are working ar*se about face.
6500 width gives 5900 span.
75x220 C16 at 400 centres with an imposed load (snow load) of 0.75KN/m2 and a dead load of 0.50 KNm2 with access only for maintenance and repair will span 6170. So your joists will comply.
If you are doing a warm deck, joists will need to be 6600 long, if a cold deck then they will need to be 6900 long to allow for cross ventilation through the soffit.
It will take you about 14 days at least to source these lengths
NOTE. Firrings do not run at 90 degrees to the joists. Firrings run the length of the joist. If you wish to make your fall at 90 degree to the joists, then you use reducing rips planted on top of joists.
i.e. joist against house wall 75mm deep rip on top of joist, next joist say 70mm deep on top of joist, next joist say 65mm deep ditto. You work your depth of rips out by dividing number of joists into total fall or stick a blocik on first and last joist and run a line over the top and measure down to each joist.
Regards oldun

The problem with only posting certain information is that people who reply to my questions don't know the full story. This is my fault BUT in my defense I sometimes don't realise what info you need.

The roof is going to be a warm flat roof. I'd like to post the plans on here but I've no idea how.