Birdsmouth on raised tie trusses

[peterb20]

Does anybody know an on-site technique for positioning birdsmouths on raised tie trusses?

I know the theory behind it all and could work it all out using the span, etc but i just cant see all those 1st fix framers working it all out everytime - surely there must be a practical alternative (similar to the technique of cutting and marking a pattern for a cut roof with timber 2/3 the thickness of the rafter)

Peterb

 

[RedHerring2]

I had a related query sometime ago:
//www.diynot.com/forums/viewtopic.php?p=1118985#1118985

I birds-mouthed them individually, eventually, IIRC.

 

[peterb20]

although useful, it doesnt really help with my original question. I need an on-site technique rather than a theoretical technique for the actual birdsmouthing!

 

[noseall]

You need to measure across the plates and make sure that at no time do the birds mouth plumb cuts exceed this measurement. However, it should be said that if you do go over or under slightly, make sure that they are all identical.

When cutting the b.m.'s it is handy to have a piece of timber that is exactly the same distance as the plates are apart. This will give you a physical measure, that you can drop in between the b.m.'s to check that the cuts are accurate.

It is possible to mock-up a pattern rafter with a birds-mouth cut in it, but make sure that when you lay it on the truss, you check against the important parameters before pencil marking, i.e. the bottom (joist) chord and the central rafter peak, are aligned.

Not all grouped trusses are identical.

 

[peterb20]

brilliant, all good advice! But as for the distance down the rafter once i know the distance between wall plates? is it just measure each rafter from the ridge so the two distances are equal, adjusting if neccessary?

 

[noseall]

If it is a stand alone roof, i.e. you are not aligning or marrying into another, then initially use the bottom chord (ceiling joist chord) as a measuring point as this will determine the ceiling height.

As for actually centering the birdsmouths - then yes, measure down from the peak each side of the rafter chord. It won't hurt to check though that they are also equidistant from the bottom chord as this is what creates a level ceiling. (providing the plates are level of course!)

As i said, once you have cut the first truss, there is nowt stopping you templating up a dummy rafter which you could lay upon all the rest of the trusses.

 

[noseall]

P.S. don't forget to fit glide shoes instead of regular truss clips.

You are also supposed to take the diagonal bracings right down to (and fix to) the wall plate. This means that it actually interferes with the raked ceiling.

Some bco's will accept a plywood gusset/shearwall instead. This does not effect the insulation detail which is likely to be 100mm between with 50mm underdraw and a 50mm air-gap above.

 

[peterb20]

fantastic. thanks alot, youve been a great help!