Fitting 550mm wide Velux between 600mm centre joists

[garyo]

Am I asking for trouble trying to fit a Velux with only 5mm wiggle room? The fitting guides say that I should have 40-60mm clearance. Is this to meet thermal regs or so that the fitter has some discretion with where the tiles are cut?

Thanks

 

[Makie]

A 550mm wide window would give 50mm clearance on a 600mm gap not 5mm

 

[catlad]

No they are made for truss roof installation ie. 600 centres. The guide of 40-60mm is there so you can fit the insulation collar where possible, or use insulated
plasterboard.

 

[garyo]

A 550mm wide window would give 50mm clearance on a 600mm gap not 5mm

The joists are at 600 centres and ~45mm thick, so only 5mm unfortunately.

 

[datarebal]

A 550mm wide window would give 50mm clearance on a 600mm gap not 5mm

Gap yes but not at 600 centres

 

[^woody^]

If the Velux is fitted above the rafters, then the space between rafters is only really for the plasterboard

 

[garyo]

If the Velux is fitted above the rafters, then the space between rafters is only really for the plasterboard

I'd wondered if the extra space they recommend is to deal with the potential cold bridge that's formed if plaster is fitted straight to the cheek of the rafter. If I clad the cheeks in, say, 25mm PIR backed plasterboard then in my situation I'd overlap the frame too much.

 

[Makie]

The joists are at 600 centres and ~45mm thick, so only 5mm unfortunately.

Jesus christ I had a nightmare trying to read this last night.... haha

 

[catlad]

The latest generation velux windows sit an inch below the rafters, it is possible to make them sit at rafter level but you would need to order a special flashing kit.

 

[garyo]

Interestung. My rafters are 170mm deep. Would there be any rafter left exposed?

 

[datarebal]

He means the velux does not sit on the rafters but slightly between.
If yours is a standard roof, rafters, batten and not boarded then the underside edges will sit between and not on.. it's how they fit

 

[garyo]

He means the velux does not sit on the rafters but slightly between.
If yours is a standard roof, rafters, batten and not boarded then the underside edges will sit between and not on.. it's how they fit

My impression is that with 65mm (Grovesbury) tiles on 25mm battens, and using 170mm deep rafters, then the top of the Velux would be roughly level with the top of the tiles, and probably wouldn't fully cover the sides of the rafters when viewed underneath?

 

[Mr Chibs]

5mm gap might not give you enough room to square up the velux frame into the rafter.
From memory I left around 30/40mm, in fairness I thought this was too much.

I've recently fitted keylites, and left around 15mm and thought you could get away with less.
They also do recessed flashing kit, so the windows is practically flush with tile.
These fitted between 600c rafters.

 

[^woody^]

The latest generation velux windows sit an inch below the rafters,

What about the other makes?

 

[datarebal]

My impression is that with 65mm (Grovesbury) tiles on 25mm battens, and using 170mm deep rafters, then the top of the Velux would be roughly level with the top of the tiles, and probably wouldn't fully cover the sides of the rafters when viewed underneath?

Not quite getting what you are saying.
The bottom edge of the velux sits within the rafters by about 20-25 mm.
If your trusses are true, then it is just a snug fit if they are pis sex then squaring will be a problem as others have said.