Timber Dimensions

[arfurrrr]

Can anyone help me with the dimensions for some timber roof joists .

I am building a rear extension extending 4mtrs ,

The overall span is 4mtrs , including 100 brick 100 cavity and 100 brick .

The council said use 8 x 3 timbers , the 8" has caused a problem with the roof pitch .

Any suggestions ???

Also I have a 3mtr door opening and looking at catnic the lintels have a 220 height on the spline which is also causing me a problem , what other options have we got .

TIA .

 

[tony1851]

You don't say what the slope of the roof is, or what tiles you are using (which affects the loading).
The inspector has probably taken the joist size from the TRADA tables, which are conservative.
If pitch is a problem (eg you are up against a 1st floor window cill), it is often possible to use shallower joists, but at closer centres.
Also, you seem to need to reduce the height at the eaves. On a 3m span supporting just a roof, a 6 x 3 1/2 UB (on the inner skin) with a 4 x 2 timber plate bolted on top would do.

 

[arfurrrr]

Thanks


the roof pitch with 8 x 3 is 14 degrees , the tiles are marley modern .

any ideas on the catnic lintel alternative

 

[tony1851]

If you are using Marley Moderns, the minimum pitch is 17.5 deg. - they are not guaranteed below this pitch; whether or not the inspector picks up on this is another matter.
So far as I know, there are no Marleys which go below 17.5deg. (they did one called Malvern which went down to 15deg. but I think that was recently dicontinued).
Redland Regent tiles go down to 12.5deg.
Are you installing a Velux? min. pitch needs to be 15 deg for these.

For your joists, I think you could go down to 7", but spaced a little closer than the usual 400. Have you considered using C24 grade timber instead of the normal C16? It is a little stronger and may help in reducing the section size.

If you are really tight for space over the doors, a 6" x 3 1/2" steel beam will do, with a 4" x 2" plate bolted on top to receive the rafters.
This would be shallower than a 220 catnic plus w/plate.

 

[noseall]

Marley Wessex will deal with 15 degrees.

Why not have a flattie?