Is there actually a proper way of doing splayed reveals on rooflights?

[lineweight]

Maybe I am missing something obvious but the information that eg. Velux and Fakro supply remains rather silent on exactly how splayed reveals at the top & bottom of rooflights are supposed to be formed.

 

They both show & recommend these splays in diagrammatic form:

 

 

Ok, so to achieve these splays, of course you have to set any trimmers back from the opening, as indicated in the Velux diagram.

 

There are some more detailed drawings that include these splays - for example this from Fakro:

 

 

Here, trimmers simply aren't indicated.

 

My basic question is: what is supporting the roof tiles/slates in those portions that project beyond the set-back trimmers?

 

If the roof window width is less than the rafter spacing, then there's no big issue, because the tile battens simply span between the rafters like anywhere else on the roof.

 

But what if the window is wider than the rafter spacing? Then you have the scenario at bottom right here:

 

 

This drawing kind of fudges the placing of the top & bottom trimmers, relative to the roof window frame. But if we assume they are placed to allow splayed reveals, as indicated in my butchered diagram here:

 

 

Then it seems we are asking for either or both of:

(a) tile battens (indicated in blue) to span more than a normal rafter spacing. This can be up to 1.4m for the largest windows, so more than double a 600mm rafter spacing.

(b) if counter-battens are involved (indicated in pink), then they can project beyond the cut-off point of the rafters, but we are essentially asking them to cantilever, and with many geometries this is easily going to be 200-300mm so several tile gauges.

 

Am I worrying about a problem that doesn't exist in practice (battens/counterbattens will cope with this fine, even with big openings)?

 

Or is it that Velux/Fakro don't show what is supposed to happen because actually it creates a slightly dodgy detail that they don't want to explicitly recommend?

 

Is it that actually some kind of reinforcing something has to be added underneath the tile batten, which creates a messy situation as far as insulation/membranes are concerned and they don't want to draw/recommend this?

 

If it's the case that the splayed reveals are only really possible with narrower windows, why don't they just say this?

[ProDave]

Right or wrong, but the only times I have seen it done, the rafters are cut at an angle and the trimmers are fitted at that angle, usually 45 degrees and you then just line it as normal.

 

Slightly larger trimmers are needed.

[Temp]

Perhaps skip to about 8 min..

 

 

[lineweight]

Thank you both. I did actually watch that video earlier today.

 

Both of these approaches though, neither is mentioned (as far as I can see) in any official instructions from rooflight manufacturers.

 

Since I wrote my initial post above, I did actually find this in a Fakro guide. It implies the trimmers are just installed as normal, spaced away from the frame, and tile battens are then expected to span further than the rafter spacing.

 

[Dreadnaught]

I have splayed reveals on my timber frame but on a flat roof (nine roof windows). Happy to share details if that's of any interest. My timber frame is still visible – not plasterboarded yet.

[lineweight]

18 hours ago, Dreadnaught said:

I have splayed reveals on my timber frame but on a flat roof (nine roof windows). Happy to share details if that's of any interest. My timber frame is still visible – not plasterboarded yet.

Thank you, but this question is specifically related to pitched roofs, because it's mainly about what happens to the tile battens and how the weight of a tiled/slated roof is supported; an issue that doesn't really exist with flat roofs.

[Temp]

When you cut out the rafters save the offcuts. Cut them down to form a small "rafter" that fits between the trimmer and the velux frame and supports the tile batten. It may need to be tapered in depth. Get it right and it can also support the plasterboard in the splay.

[lineweight]

20 hours ago, Temp said:

When you cut out the rafters save the offcuts. Cut them down to form a small "rafter" that fits between the trimmer and the velux frame and supports the tile batten. It may need to be tapered in depth. Get it right and it can also support the plasterboard in the splay.

 

Are you suggesting that they actually span between trimmer, and the velux frame? So one end is supported by the velux frame? I don't think this approach is indicated in any installation details from rooflight manufacturers.

 

Anything that is directly supporting both the tile battens and internal lining is, by definition, a thermal bridge.

[Temp]

4 hours ago, lineweight said:

Anything that is directly supporting both the tile battens and internal lining is, by definition, a thermal bridge.

 

Yes but there was a whole rafter there before the opening for the velux was cut 🙂

 

 

[lineweight]

On 18/08/2023 at 20:12, Temp said:

 

Yes but there was a whole rafter there before the opening for the velux was cut 🙂

 

 

True enough, although in the standard configuration the thermal bridge is through the rafters from short edge to short edge, through their longest dimension, rather than their shorter.