Help with rafter cut @ rsj ridge

[Jony5]

Made a mistake with placing rsj 178x100 slightly too high (one end is built into an external wall, the top of the rsj being 230mm below a second storey window cill)

this is for an single story roof (span 3.2m, 38deg pitch beam centre 1.6m)

rafter spec was 150x50@450 centres with 100mm insulation between, but with tiles takes the mid point too high

is there any way i can still create a warm roof with having to lower my beam to get the ridge tile low enough ???

 

[tony1851]

A pic or two here would be useful.

 

[Jony5]

A pic or two here would be useful.

sorry, i did try and am still trying

 

[Jony5]

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• Jony5
• 13 Mar 2014
• 1

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• Jony5
• 13 Mar 2014
• 1

 

[tony1851]

Plan A.

Assuming it's an 8x4 steel (?) bolt 7x2 timbers into each side through the web, and fix the rafters to those.

 

[theoldun]

Without pics or drawing, and from your description, will assume roof is coupled with vaulted ceiling Assume vaulted due to steel beam and not ndge board. New beam built into existing rear wall of house and into new rear gable.
Total run over all plates3200 = half run overall plate and centre of beam 1600.
You have rattled your beam in to high and now find that you can not get your roof, ridge tile and saddle back flashing in under window cill. Common mistake.
Is this correct? If so come back and tell us what height existing cill is above new wall plate level.
Couple of other points. 100mm insulation between rafters doses not comply with L1B 2010 0.18W/m2K. With Celotex you require either 165mm between or 100mm between and 35mm under.. Suggest you go 400mm rafter centres instead of 450mm. Will save you a lot of money on insulation .
Regards oldun















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[Jony5]

correct

Vaulted ceiling
New beam built into existing rear wall of house and into new rear gable.
Total run over all plates3200 = half run overall plate and centre of beam 1600.
beam in too high and now find that you can not get your roof, ridge tile and saddle back flashing in under window cill. Common mistake.

Dimensions with attached drawing.

Original design 60celotex between (4" rafters)/60mm above @ 400 centres

Consideration 100celotex between (6" rafters) between/20mm airgap/ecofoil@ 450 centres



Any suggestions considered, design considerations:

apex as high as possible (greatest pitch of roof) window considering
U-value














.[/quote]

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• Jony5
• 20 Mar 2014
• 1

 

[catlad]

Why are you seating the rafters on the steel? tony has given you an option that lowers the ridge height.

 

[noseall]

Why are you seating the rafters on the steel? tony has given you an option that lowers the ridge height.

Some people will just not listen.
Perhaps he has Tony on 'ignore'?

 

[theoldun]

Why are you seating the rafters on the steel? tony has given you an option that lowers the ridge height.

Possibly, he does not agree with Tony. Possibly he knows the correct way to do a birds beak detail for a ridge beam. Possibly he has taken the advice from Tony. Who knows?

Mr Poster
Sorry for not coming back to you. Been very busy last three days. If you have sorted your problem out, then all well and good, if not then come back and will help you out.
Regards oldun

 

[Jony5]

Why are you seating the rafters on the steel? tony has given you an option that lowers the ridge height.

Some people will just not listen.
Perhaps he has Tony on 'ignore'?

Hi all

apologies for the delay and maybe for some confusion

the original sketches were a poor attempt to convey the situation.
the more detailed sketch is my original (poor) design (which is too high)

1) if i put 2" x 7" in the web of the I beam and attach the rafters to these, the apex heigh reduces so that the top of the ridge tile is approx 15mm below the window cill. Will this work or do i need to slide the rafters slightly "down" the 2" x 7" 's ??

2) never worked with beams nor insulation over the rafters ... does the cross section look ok (space as in previous post)

3) 120mm std screws ok to fix cross counter batons?

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• Jony5
• 23 Mar 2014
• 1

 

[tony1851]

If space is that tight, why do you appear to be doing a warm roof, with the insulation above the rafters?

 

[stuart45]

You need special fixings to do the counter batons.

 

[Jony5]

If space is that tight, why do you appear to be doing a warm roof, with the insulation above the rafters?

(Poor) design was before the realisation of height constraint
Vaulted roof, trying to maximise "internal" space.

60mm between 100mm rafters, and 60mm above @400 centres

Neighbour has just completed project with 100mm between 150mm with bubble foil under rafters @450centres.


Looking for someone to make a sensible suggestion for an alternative (to meet u-vlaue). fyi i already have the 50mmX100mm timber, but am willing to replace with 150mmX50mm if there is a working solution.

 

[theoldun]

Will see what we can do. Just hope you understand..
A vaulted roof should always be birds beaked over the ridge beam for two reasons.
1. To enable top tile batten to go up high enough to bed your ridge over the 75 or 100mm lap gauge you are using on roof tiles
2. To assist the load and tension from the ridge beam to go down the rafters to the wall
Your detail to the ridge beam is perfect, however it can be modified.
Will work on the pitch you state of 38degrees. Half run of 1600 to centre of beam = rise of 1250 and hypotenuse (pitch line) = 2030. Remember the pitch line is not the top of rafter but is the pitch line (hypotenuse) two thirds down from top of rafter at 90 degrees to give the depth of heel cut on bottom birds mouth
38 pitch will give a top plumb cut of 52 degrees, approx 126mm long. Heel cut of 52 degrees 44mm deep. Sole cut of 38 degrees 50mm long
47x100 C16 will span 2280 and will support a dead load of 1.0 KN/m2

Back to your rise you require 1250 plus the top apex of rafter above birds beak which will be 82mm plus tile batten, tile, ridge and saddle flashing say total rise of 1632 to underside of cill. Have played around with it all ways, but no can do. Bite the bullet drop the steel and alter pitch. One other thing rise is always from top of wall plate and not from underneath as you have shown.

Insulation. 60mm celotex above joists does not comply with L1B 2010. There are three ways to comply with L1B 2010. 18W/m2K,and we list the easiest one in your instance.
Vapour permeable roofing felt, 100mm celotex GA4000 between the joists and 50mm PL4000 under joists. Fix the PL4000 with Insofast fixings
Regards oldun, /