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Polycarbonate rooflight modification

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mc2fool
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Polycarbonate rooflight modification

#421970

Postby mc2fool » June 24th, 2021, 4:46 pm

The small block of (collectively enfranchised and self managed) flats I live in has a skylight at the top of the stairwell, which opens to give access to the (flat) roof. The previous skylight was a single skin dome that had turned very brown over its 40 years, and had developed some cracks and was also, as a result, not very well fitting on its frame, and would not very often but sometimes let some rain in if it was windy and slanted at a particular angle.

So, a couple of years back when we were having the roof re-felted we asked the roofer to also replace the dome. Well, of course it wasn't a standard size, and having a custom dome made was going to cost £lots, and changing the size of the hole to fit a standard size skylight (including upstand) was going to cost even £more.

However, as the existing metal frame for the dome was only a few inches smaller than one standard sized dome, it was decided to extend the existing frame by a few inches all round by bolting some 3/4" wooden planks to it and then bolting a standard sized dome to that, all for just a few hundred £.

Photos of the end result are at https://i.imgur.com/fnvJnGx.jpg and https://i.imgur.com/1DuNmXe.jpg, and if that's not clear the cross section looks like this (wood in brown, original metal frame in black, upstand in dark green, not to scale, and of course it actually fits on the upstand tightly):

Image

All well and good ... except that on certain days in winter we now get masses of condensation on the inside of the dome, which drips heavily down the stairwell -- and I mean masses: once I tried to sponge it off and half filled a washing up bowl!. We'd only put in a single skin dome 'cos that's what was there before, and we didn't have the condensation problem previously (although that may be 'cos it was cracked and badly fitting), and in any case as it was just an unheated stairwell we weren't worried about heat loss.

So, trying to figure a reasonable way of dealing with that (without the cost of throwing away the current dome and replacing it with a double or triple skinned one) and am thinking of adding a second "skin" myself in the form of a celled polycarbonate roofing sheet, e.g. https://www.roofingmegastore.co.uk/poly ... therm.html.

My first thought is to attach it to the inside of the existing structure, below the dome, like so (I can easily add some flanges for it to rest on and then silicone it all tight).

Image

That would be easiest, but then one of the problems with the wooden plank extended frame is that it's pretty heavy and makes opening the skylight quite difficult, so I thought, well, what if I just throw away the planks, get a slightly bigger polycarbonate sheet and attach it to the frame and then the dome to the sheet, so replacing the planks with a single sheet that covers the whole area, like so:

Image

Now, the thing is I've not worked with these polycarbonate roofing sheets before (or any polycarbonate material for that matter). All the (sellers) blurb about them says they're pretty much indestructible, but I wonder about the possibility of a very high wind ripping the dome off of the sheet, or the sheet off of the frame, or some other problem that my inexperience with the material fails to foresee ... :?

Anyone got an relevant experience and would like to comment, please?

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Re: Polycarbonate rooflight modification

#421984

Postby ReformedCharacter » June 24th, 2021, 5:43 pm

No experience with polycarbonate sheeting but plenty with condensation. I'd be inclined to think a bit more about the condensation. If you didn't have a problem before then it may because your previous dome was 'cracked and badly fitting' and provided sufficient ventilation to prevent significant condensation. It might be worth considering whether you can add some ventilation back. Would it be possible to modify the sealing edges, at the opposite side to the hinge, so that it does not quite close but leaves an air gap and allows some air movement? It should be easy enough to test without major works.

RC

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Re: Polycarbonate rooflight modification

#421992

Postby Itsallaguess » June 24th, 2021, 5:58 pm

ReformedCharacter wrote:
I'd be inclined to think a bit more about the condensation.

If you didn't have a problem before then it may because your previous dome was 'cracked and badly fitting' and provided sufficient ventilation to prevent significant condensation. It might be worth considering whether you can add some ventilation back.

Would it be possible to modify the sealing edges, at the opposite side to the hinge, so that it does not quite close but leaves an air gap and allows some air movement? It should be easy enough to test without major works.


Another option that might not involve the sealing edges might be to introduce some ventilation holes using the wooden planks, with the idea that if anything doesn't quite work out, then there's a replicable solution to go back to that looks relatively cheap to reintroduce, or the holes could just be sealed back up with a flexible white sealant...

There looks to be a lot of unused 'meat' on those planks, in between what looks like just 8 main fixings all the way around, so I was wondering if a number of horizontal and vertical holes could be drilled, to be aligned for each pair of holes, which would then act to provide some ventilation holes 'through' some of that plank meat, in a way that stayed waterproof throughout, with them being fully under the outer domed area.

Here's what I was thinking, with the vertical/horizontal drilled ventilation holes highlighted in yellow -

Image

Four of those on each side, away from the actual fixing areas, might well provide enough long-term ventilation to solve the issue, but you could try with a couple and work your way up, and monitor any results...

The benefit if that approach seems to be that it would be relatively simple to carry out, and also be almost fully reversible with the introduction of a flexible sealant to the holes if the idea doesn't actually work....

Anyhow - just some idle thoughts from looking at the pictures....

Cheers,

Itsallaguess

mc2fool
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Re: Polycarbonate rooflight modification

#422008

Postby mc2fool » June 24th, 2021, 6:37 pm

Thanks for the thoughts guy, but we've tried both leaving the top floor stairwell window (top hinged) open, and propping up the skylight so there was around a one and a half inch gap at the "front" (opposite the hinge) side, but to no avail -- and got the fright of my life one night when it was like that and there was a massive bang at 3am 'cos it had got really windy and the wind had lifted the not fully closed skylight and dropped it back!

The thing is it only happens a few days in winter, so it's problematic to try-and-test solutions. It's pretty much always the high pressure crisp blue sky days when it's warm during the day and cold at night. The building, including the stairwell, is brick and concrete and so on those sorts of days it retains a lot of heat, and hence high relative humidity I guess....

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Re: Polycarbonate rooflight modification

#422011

Postby DrFfybes » June 24th, 2021, 6:49 pm

If venting doesn't solve the problem then perhaps the cause has changed.

Perhaps the clear dome allows more light in, heating the stairwell more.

I wonder if these "heavy condensation" days coincide with "bright and sunny do some washing and start it off in the tumble drier" days.

Paul

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Re: Polycarbonate rooflight modification

#422024

Postby Itsallaguess » June 24th, 2021, 7:27 pm

mc2fool wrote:
My first thought is to attach it to the inside of the existing structure, below the dome, like so (I can easily add some flanges for it to rest on and then silicone it all tight).

Image

That would be easiest, but then one of the problems with the wooden plank extended frame is that it's pretty heavy and makes opening the skylight quite difficult, so I thought, well, what if I just throw away the planks, get a slightly bigger polycarbonate sheet and attach it to the frame and then the dome to the sheet, so replacing the planks with a single sheet that covers the whole area, like so:

Image

Now, the thing is I've not worked with these polycarbonate roofing sheets before (or any polycarbonate material for that matter). All the (sellers) blurb about them says they're pretty much indestructible, but I wonder about the possibility of a very high wind ripping the dome off of the sheet, or the sheet off of the frame, or some other problem that my inexperience with the material fails to foresee ... :?


If ventilation hasn't helped already, then it looks like the above approach might be worth considering for the cost involved, but I wonder if there's a third option that sits somewhere in-between the two shown above, that would both remove much of the weight of the wooden planks, and also help to introduce a broader section of polycarbonate into the solution at the same time, rather than a smaller section that just sits inside the existing planks...

What if you modified the existing planks, and cut them back so that they have smaller, individual wooden sections around each of the fixing areas, and then back-filled right up to where the existing plank areas used to be with the polycarbonate sheeting?

You might need to introduce some additional smaller fixing points for the smaller wooden sections onto the metal flanged area, to stop any likelihood of each section simply pivoting on a single frame-fixing, but that looks feasible from the photographs, and would provide some good support, whilst still maintaining a good, solid fixture into the done itself at each of those points, avoiding the use of the polycarbonate in doing any of the heavy support tasks...

It would mean a bit more detail on the polycarbonate sheet cutting, but it would maintain the strength of the wooden supports in each of the fixing areas, whilst minimising weight as much as possible at the same time, perhaps...

Cheers,

Itsallaguess

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Re: Polycarbonate rooflight modification

#422032

Postby mc2fool » June 24th, 2021, 8:05 pm

DrFfybes wrote:Perhaps the clear dome allows more light in, heating the stairwell more.

I wonder if these "heavy condensation" days coincide with "bright and sunny do some washing and start it off in the tumble drier" days.

Oh there's no doubt the new dome lets more light in, massively so. I'm not sure what the previous dome was made of; it was some kind of plastic-like material with wire mesh embedded (Georgian glass style), and had turned almost totally dark brown. It's been like that, and cracked and poorly fitting, since I arrived in 2003. :D It's been on the "to be fixed" list since even before then, but as it was only rarely a problem it just got procrastinated endlessly....

Everyone has washer-dryers, which have condensers. Although most, like me, use a drying rack over the bath rather than the dryer most of the time. :) There's no outside line so laundry isn't a weather dependent activity....

Itsallaguess wrote:What if you modified the existing planks, and cut them back so that they have smaller, individual wooden sections around each of the fixing areas, and then back-filled right up to where the existing plank areas used to be with the polycarbonate sheeting?

:? Ummm ... errr ... watching TV now, so I'll have a head scratch tomorrow to see if I can visualise out what you mean there! 8-)

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Re: Polycarbonate rooflight modification

#422042

Postby Itsallaguess » June 24th, 2021, 9:00 pm

mc2fool wrote:
Itsallaguess wrote:
What if you modified the existing planks, and cut them back so that they have smaller, individual wooden sections around each of the fixing areas, and then back-filled right up to where the existing plank areas used to be with the polycarbonate sheeting?


I'll have a head scratch tomorrow to see if I can visualise out what you mean there! 8-)


Poorly worded on my part, clearly, so hopefully this equally poorly drawn image might help!

Image

You'd like to get rid of some of the plank weight, but you also seem a little concerned in using the whole polycarbonate sheet as a mechanical fixing for the dome, which is fully understandable, so I was wondering if there was a half-way-house solution, where you cut back the planks to form a number of wooden 'pads', shown above as the red areas (with additional pad fixings into the metal framing shown as yellow dots, to help secure the pads to the frame..), and where the wider polycarbonate sheeting can then extend to the 'full plank width' where the plank areas have been cut out, shown above by the purple lines....

I've only shown the top three fixing areas with the above photo, but it would be utilised in all areas where a dome-fixing is found, which is a total of eight in number I think, from the photos?

Some fixings can be drilled through the top of the metal frame to hold the polycarbonate sheet in place, similar to those shown in your original diagrams for the plank fixings (so between the red pad areas in the above photo, for instance..), and the polycarbonate sheet wouldn't then be forming any part of the weight-bearing structure, but as much weight as possible is also taken out of the original planked areas, which seems to satisfy both requirements whilst removing the potential mechanical risk of using the sheet as a mechanical fixing, whilst only really introducing a bit of fiddly cutting to size, and a bit of additional sealant needed where the sheet then meets the pads, depending on how tight you think this thing will all need sealing up...

Anyhow - just an idea to throw into the mix...

Cheers,

Itsallaguess

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Re: Polycarbonate rooflight modification

#422300

Postby mc2fool » June 25th, 2021, 2:15 pm

Itsallaguess wrote:Poorly worded on my part, clearly, so hopefully this equally poorly drawn image might help!
:
You'd like to get rid of some of the plank weight, but you also seem a little concerned in using the whole polycarbonate sheet as a mechanical fixing for the dome, which is fully understandable, so I was wondering if there was a half-way-house solution

Aha! I see! And many thanks for the time you put in to illustrate and explain it. :D

The fixings are more numerous and complicated than is easily evident. There are in fact only 5 just dome-wood nuts'n'bolts but there's also 9 dome-wood-frame nuts'n'bolts (i.e. that go through all three), those 14 making up the capped fixings evident from above. Then there's also 3 frame-wood nuts'n'bolts and 4 frame-wood screws.

Now that doesn't invalidate your idea, but what would make it difficult is that the frame is very hard iron. The roofers blunted a metal drill bit without hardly making a mark when trying to drill a hole in an "optimal" position, and I had much the same when I tried with mine. In the end all bolts and screws were put through already existing holes in the frame, which were pretty much in fairly good positions anyway.

I think the real concern I'd have about the idea is that the polycarb sheet would end up with lots of crenelations... which would give lots of opportunities for me to screw up! :lol: A slip of the saw, etc.

My concern about the sheet as a fixing for the dome is really out of ignorance, and I just don't know if it's justified or not. :?

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Re: Polycarbonate rooflight modification

#422328

Postby Itsallaguess » June 25th, 2021, 3:25 pm

mc2fool wrote:
My concern about the sheet as a fixing for the dome is really out of ignorance, and I just don't know if it's justified or not.


The only thing I'd suggest there then, with regard to the third diagram on your opening post, would be to perhaps try to insert some sort of flush, or even ever so slightly proud 'tubing-inserts' around any of the fixings that went through the polycarbonate sheet, preferably made of metal - that way the bolts and nuts would be clamping against those flush tubing inserts, rather than clamping down on the polycarbonate sheet over long periods...

This is what I'm meaning by that, with the red areas indicating where it might be useful to reinforce the clamping arrangement with appropriately sized metal tubing -

Image

Some oversized washers might then be required as well, to make sure the various washers would clamp over the outsides of the metal tubes, but I think it would offer some real confidence by the fact that you'd then not be relying on the long-term stability of the polycarbonate sheet in providing the meat of the upper dome fixing arrangement, as it would hopefully all be 'metal to metal' with that type of clamping solution...

Cheers,

Itsallaguess

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Re: Polycarbonate rooflight modification

#422349

Postby mc2fool » June 25th, 2021, 3:58 pm

Itsallaguess wrote:The only thing I'd suggest there then, with regard to the third diagram on your opening post, would be to perhaps try to insert some sort of flush, or even ever so slightly proud 'tubing-inserts' around any of the fixings that went through the polycarbonate sheet, preferably made of metal - that way the bolts and nuts would be clamping against those flush tubing inserts, rather than clamping down on the polycarbonate sheet over long periods...

This is what I'm meaning by that, with the red areas indicating where it might be useful to reinforce the clamping arrangement with appropriately sized metal tubing -

Image

Some oversized washers might then be required as well, to make sure the various washers would clamp over the outsides of the metal tubes, but I think it would offer some real confidence by the fact that you'd then not be relying on the long-term stability of the polycarbonate sheet in providing the meat of the upper dome fixing arrangement, as it would hopefully all be 'metal to metal' with that type of clamping solution...

That's a good idea! I think it might be even better if I put a (single) thin sheet of metal both under and on top of both fixings, in effect joining them together. Or even bend it round the outside of the sheet, so that it starts below the sheet to the left of the frame fixing in the diagram, has the bolts for both fixings go through it below the sheet, then bent around the end and coming back to have the bolts for both fixings go through it above the sheet. Hmmm .... :)

And actually, thinking about there being only 5 just dome to now-sheet fixings and 9 dome-sheet-frame ones I'm probably being over concerned as the dome will still, at those 9 points, be fixed to the frame, with the sheet just sandwiched between them....

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Re: Polycarbonate rooflight modification

#422352

Postby Itsallaguess » June 25th, 2021, 4:11 pm

mc2fool wrote:
Itsallaguess wrote:
The only thing I'd suggest there then, with regard to the third diagram on your opening post, would be to perhaps try to insert some sort of flush, or even ever so slightly proud 'tubing-inserts' around any of the fixings that went through the polycarbonate sheet, preferably made of metal - that way the bolts and nuts would be clamping against those flush tubing inserts, rather than clamping down on the polycarbonate sheet over long periods...

This is what I'm meaning by that, with the red areas indicating where it might be useful to reinforce the clamping arrangement with appropriately sized metal tubing -

Image

Some oversized washers might then be required as well, to make sure the various washers would clamp over the outsides of the metal tubes, but I think it would offer some real confidence by the fact that you'd then not be relying on the long-term stability of the polycarbonate sheet in providing the meat of the upper dome fixing arrangement, as it would hopefully all be 'metal to metal' with that type of clamping solution...


That's a good idea! I think it might be even better if I put a (single) thin sheet of metal both under and on top of both fixings, in effect joining them together. Or even bend it round the outside of the sheet, so that it starts below the sheet to the left of the frame fixing in the diagram, has the bolts for both fixings go through it below the sheet, then bent around the end and coming back to have the bolts for both fixings go through it above the sheet. Hmmm .... :)

And actually, thinking about there being only 5 just dome to now-sheet fixings and 9 dome-sheet-frame ones I'm probably being over concerned as the dome will still, at those 9 points, be fixed to the frame, with the sheet just sandwiched between them....


Yep - some sort of 'fall-back' retaining pieces, linking both the inner and outer bolts would be a good idea, and provide a lot of confidence with that solution I think.

Screwfix do a range of relatively cheap 'builders-band' stuff, with some handy holes already available for picking up...

Image Image Image

Sources -

https://www.screwfix.com/p/ced-all-round-band-12mm-x-10m/18298

https://www.screwfix.com/p/sabrefix-builders-band-galvanised-dx275-9-6m-x-20mm/38619

https://www.screwfix.com/p/sabrefix-builders-band-stainless-steel-9-6m-x-20mm/59863

That middle galvanised option looks like a good one for your 'wrap-around' idea, and just £8 for a 9.6m roll....

Cheers,

Itsallaguess

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Re: Polycarbonate rooflight modification

#422416

Postby jaizan » June 25th, 2021, 10:10 pm

Think I would aim for a double glazed sealed unit skylight to fix the condensation and other problems. Just angle it to ensure the rain runs off.

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Re: Polycarbonate rooflight modification

#422425

Postby mc2fool » June 25th, 2021, 11:04 pm

jaizan wrote:Think I would aim for a double glazed sealed unit skylight to fix the condensation and other problems. Just angle it to ensure the rain runs off.

Requiring professional roofers round for two or three days to remove the current upstand, change the size of the hole to suit a standard sized skylight unit, and build a new upstand and re-felt and re-lead it in for a cost of several thousand pounds. Guess you overlooked the second paragraph of my OP. ;)

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Re: Polycarbonate rooflight modification

#423531

Postby mc2fool » June 29th, 2021, 5:29 pm

Itsallaguess wrote:The only thing I'd suggest there then, with regard to the third diagram on your opening post, would be to perhaps try to insert some sort of flush, or even ever so slightly proud 'tubing-inserts' around any of the fixings that went through the polycarbonate sheet, preferably made of metal - that way the bolts and nuts would be clamping against those flush tubing inserts, rather than clamping down on the polycarbonate sheet over long periods...

Small update, it looks like 'tubing-inserts' are going to be essential as, "When drilling fixing holes an expansion allowance of 3mm around all fixings is required. Do not fix or clamp sheets too tighly - leave room for thermal movement. " https://www.arielplastics.com/en-gb/pro ... xwall.aspx (under guidance).

So, it looks like, for the fixings that go through all three of frame, polycarbonate sheet and dome, it'll have to be something like:

Image
Where the red is the slightly proud tubing insert you describe, going through a 6mm-wider-than-itself hole, with a washer on top, and the yellow is low modulus (stretchy) sealant. That way the polycarb sheet is kinda "floating", being actually attached by stretchy sealant and so free to expand and contract with the temperature.

Ok, so where to get the 'tubing-inserts'? They seem to be called either spacers or collars (neither of which is particularly useful when googling 'cos of all the other kinds of spacers & collars that show up!), and the few places I've found any (mostly ebay) seem to have only a very limited range of sizes.

Ideally I'm after ones that'll take an M5 bolt through them and be a tad over 25mm high, being the preferred thickness of polycarb (and as their height will need to be pretty consistent, and their tops and bottom not at all angled, I suspect that getting a long tube and chopping it up with the angle grinder will produce an awful lot of too-short/too-longs/just-wrongs! :( )

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Re: Polycarbonate rooflight modification

#423876

Postby Itsallaguess » July 1st, 2021, 7:50 am

mc2fool wrote:
Ideally I'm after [inserts] that'll take an M5 bolt through them and be a tad over 25mm high, being the preferred thickness of polycarb - and as their height will need to be pretty consistent, and their tops and bottom not at all angled, I suspect that getting a long tube and chopping it up with the angle grinder will produce an awful lot of too-short/too-longs/just-wrongs!


I don't think you'll need to be getting the angle grinder out to make these inserts from an appropriately sized length of piping, and I think a good metal hacksaw blade and a medium-sized flat metal file, along with a good vice to hold it whilst cutting and filing would be as much as you'll need to get going with this. If you get the inner-diameter and outer-diameter right, and can find a length of pipe to suit, I'd give that a go and see how you get on - I think you'll surprise yourself...

I wouldn't be trying to achieve NASA-levels of material flatness with your chopped ends here, and a bit of over-measuring and then careful filing down will do the job quite adequately...

Cheers,

Itsallaguess

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Re: Polycarbonate rooflight modification

#424162

Postby richlist » July 2nd, 2021, 10:27 am

Be aware that they may rust if they're made of steel ?
Last thing you want are rusty streaks appearing at a later date.

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Re: Polycarbonate rooflight modification

#424251

Postby Mike88 » July 2nd, 2021, 2:16 pm

I can only speak about polycarbonate from personal experience. For what its worth I live on the end of a cliff where winds of 50 and 60mph are commonplace. There has been no evidence of the polycarbonate sheets on my conservatory moving in the high winds and there is absolutely no condensation. The sheets I used are double skinned with a gap in the middle and are held in place with aluminium glazing bars.

In this particular case could the glazing bars be fitted over the top of the rooflight with the existing sheeting remaining in place?

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Re: Polycarbonate rooflight modification

#424278

Postby mc2fool » July 2nd, 2021, 4:29 pm

richlist wrote:Be aware that they may rust if they're made of steel ?
Last thing you want are rusty streaks appearing at a later date.

Or verdigris streaks if they're made of copper or brass. :D I'll go see if the local B&Q/Homebase/Wickes has any suitably sized aluminium pipe tomorrow (or maybe stainless steel, but that would need the angle grinder...).

Mike88 wrote:I can only speak about polycarbonate from personal experience. For what its worth I live on the end of a cliff where winds of 50 and 60mph are commonplace. There has been no evidence of the polycarbonate sheets on my conservatory moving in the high winds and there is absolutely no condensation. The sheets I used are double skinned with a gap in the middle and are held in place with aluminium glazing bars.

In this particular case could the glazing bars be fitted over the top of the rooflight with the existing sheeting remaining in place?

Hi Mike. Not sure what you mean by "over the top of the rooflight", as it's a dome, and there is no existing sheeting, just a (single skinned) dome. See https://i.imgur.com/1DuNmXe.jpg and https://i.imgur.com/fnvJnGx.jpg.

In any case, there's no need for any glazing bars at all, as there's only going to be a single polycarbonate sheet added, below the dome.

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Re: Polycarbonate rooflight modification

#424280

Postby jfgw » July 2nd, 2021, 5:22 pm

"Standoff" might be a better search term. There are plenty on eBay.

Most are metric, 25mm and 30mm lengths are available, e.g.,

https://www.ebay.co.uk/itm/332366408497

You could add a stainless washer to a 25mm one if you want "just over 25mm".

M6 ones are available in 1" in stainless,

https://www.ebay.co.uk/itm/291169910104

M5 aluminium, pick a colour,

https://www.ebay.co.uk/itm/272688110078


Julian F. G. W.


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