Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

OSB or Ply ?


Astrokev

Recommended Posts

Yet another question to help me complete my obsy build!

I'm ready to buy sheeting for the roof, and can't decide which would be best - OSB or Ply?

The roof will be covered with EPDM rubber, so I'm leaning towards OSB since the risk of water getting to the wood should be close to zero. This will be the cheaper option, which is a primary consideration as the obsy costs seem to be going exponential right now! On the other hand, ply has a much smoother finish, which may be better for EPDM adhesion, and I find easier to work with.

The other question is grade and thickness. If I go for ply, 9mm is less expensive and I think should be strong enough compared to 12mm.

I'd be interested in views from others to help me decide.

Many thanks

Kev

 

Link to comment
Share on other sites

  • Replies 29
  • Created
  • Last Reply

Perhaps you're committed to the rubber for waterproofing but, having tried most of the proprietory options, I've come down firmly on the side of corrugated steel. It bolts down hard with the bolt holes proof against tearing, the overlapping joints are foolproof and it lasts indefinitely.

However, if using a waterproof sheeting the OSB will be fine if you never get a failure in it. If it does fail you'll be glad you used ply.

Olly

PS I crossed posts with tooth_dr there. 'The felt came off my roof again...' It's come off for me, too, and synthetic corrugated panels have torn. Steel, steel, steel!

Link to comment
Share on other sites

Nice bit of triple wall polycarbonate would do the trick, waterproof, lighter than plywood / OSB and you can get all the bits for trimming the edges so it looks pretty :)

Dave

Link to comment
Share on other sites

Our observatory had an OSB floor, not a roof question, but it's worth saying humidity from the ground condensed on the underside, and ruined it. Someone could have pierced it with a kick. So we made a new floor with marine-grade plywood three years ago, which resists well. I still don't trust wood in humid places on the long run, so Dave's idea of polycarbonate does away with all rot and corrosion issues.

Link to comment
Share on other sites

1 hour ago, ollypenrice said:

Perhaps you're committed to the rubber for waterproofing but, having tried most of the proprietory options, I've come down firmly on the side of corrugated steel.

Thanks Olly.

Steel certainly does the job as you say, and looks good in your set-up in the Med. Unfortunately, I don't think it would suit the garden environment from an aesthetic point of view.

Yeah, you've hit on the risk (and my reticence) to use OSB. Great when it's dry, but get water anywhere near it and it can deteriorate badly. I know this from experience with a small run-off hut I built years ago for a small EQ mount. I know others have used it with, presumably, some success.

Link to comment
Share on other sites

1 hour ago, Davey-T said:

Nice bit of triple wall polycarbonate would do the trick, waterproof, lighter than plywood / OSB and you can get all the bits for trimming the edges so it looks pretty :)

Dave

Thanks Dave. Certainly waterproof and light, but the relatively poor security aspect would worry me. Also, wouldn't it act as a greenhouse, making the obsy too hot?

Link to comment
Share on other sites

40 minutes ago, Ben the Ignorant said:

Our observatory had an OSB floor, not a roof question, but it's worth saying humidity from the ground condensed on the underside, and ruined it. Someone could have pierced it with a kick. So we made a new floor with marine-grade plywood three years ago, which resists well. I still don't trust wood in humid places on the long run, so Dave's idea of polycarbonate does away with all rot and corrosion issues.

Thanks Ben.

Yes, I plan to use ply for the floor, with breathable membrane underneath to help control rising moisture vapour.

Link to comment
Share on other sites

30 minutes ago, Astrokev said:

Thanks Dave. Certainly waterproof and light, but the relatively poor security aspect would worry me. Also, wouldn't it act as a greenhouse, making the obsy too hot?

No less secure than the door I doubt, maybe a bit hot on about 7 days a year in the UK, I've got triple wall bronze Polycarbonate on my conservatory roof and it does a good job of keeping the heat and UV rays out.

If I ever build another obs'y I shall make the roll off roof with an aluminium frame and polycarbonate.

Dave

Link to comment
Share on other sites

8 hours ago, Davey-T said:

No less secure than the door I doubt, maybe a bit hot on about 7 days a year in the UK, I've got triple wall bronze Polycarbonate on my conservatory roof and it does a good job of keeping the heat and UV rays out.

If I ever build another obs'y I shall make the roll off roof with an aluminium frame and polycarbonate.

Dave

I like the triple wall polycarb approach, I plan to use it in my build.  Curious how I would go about accessing the middle of it (on a large roof), that is; assuming one can't walk or crawl on it?

The ally roof frame is an interesting idea, not worked with it very much, how would you build it?

Tony.

Link to comment
Share on other sites

10 minutes ago, nightvision said:

I like the triple wall polycarb approach, I plan to use it in my build.  Curious how I would go about accessing the middle of it (on a large roof), that is; assuming one can't walk or crawl on it?

The ally roof frame is an interesting idea, not worked with it very much, how would you build it?

Tony.

It's pretty strong, you can use something like scaffold boards across the supporting frame.

You can get knock together joints for square aluminium tube.

Dave

Link to comment
Share on other sites

Either will work fine, Kev.  I would say if you go with OSB, make sure it is OSB3, and would go with 12mm.  If you go the ply route then you could go with 9mm, but would need 400mm spacings between joists as the EPDM is actually pretty heavy, and if it has a covering of snow on it............You can imagine the rest.

Don't forget that you need an air gap under the ply or OSB as EPDM isn't breathable, so if you don't leave an air gap you'll end up with condensation issues.

I went 9mm ply on the small warm room part, and 12mm on the larger rolling part.  You won't need marine grade ply.

Link to comment
Share on other sites

I have used OSB3 on two shed roofs and an observatory and that was using felt + bitchumen with no issues. I cant imagine having issues with EPDM and OSB3.

Edit...although allot depends on having effective run off.

Link to comment
Share on other sites

4 minutes ago, Adam J said:

I have used OSB3 on two shed roofs and an observatory and that was using felt + bitchumen with no issues. I cant imagine having issues with EPDM and OSB3.

Edit...although allot depends on having effective run off.

Agreed, OSB3 will work absolutely fine. EPDM is completely waterproof and suitable for flat roof applications so run off shouldn't be an issue. 

I think a lot of roofing companies now recommend OSB3 over ply for EPDM as it has better adhesion properties so is less likely to bubble. 

Link to comment
Share on other sites

22 hours ago, RayD said:

Agreed, OSB3 will work absolutely fine. EPDM is completely waterproof and suitable for flat roof applications so run off shouldn't be an issue. 

I think a lot of roofing companies now recommend OSB3 over ply for EPDM as it has better adhesion properties so is less likely to bubble. 

Thanks Ray. Looks like it may be more down to price then, although point noted about EPDM adhesion on OSB3.

I need to check out prices and then get off the fence!

Link to comment
Share on other sites

On ‎27‎/‎05‎/‎2018 at 00:04, RayD said:

Either will work fine, Kev.  I would say if you go with OSB, make sure it is OSB3, and would go with 12mm.  If you go the ply route then you could go with 9mm, but would need 400mm spacings between joists as the EPDM is actually pretty heavy, and if it has a covering of snow on it............You can imagine the rest.

Don't forget that you need an air gap under the ply or OSB as EPDM isn't breathable, so if you don't leave an air gap you'll end up with condensation issues.

I went 9mm ply on the small warm room part, and 12mm on the larger rolling part.  You won't need marine grade ply.

Just thinking on about the condensation risk.

The scope room will be left without any internal ceiling, so the board will be fully exposed to ambient air within the room. I assume this will be OK, but I may paint the inside surface of the board with either decorative paint, or a protective treatment of some kind.

For the warm room, things are a little more complicated. My design really leave room for an air gap below the internal surface of the board. My plan was to put insulation between the ceiling joists, in contact with the board, and then put a think ply internal ceiling to hold the insulation in place. The ceiling isn't really high enough to allow me to add a gap.

Is there likely to be condensation issues if the OSB (or ply) is sandwiched between EPDM and insulation (either foam board or rockwool type)?

Thanks

Link to comment
Share on other sites

2 minutes ago, Astrokev said:

Just thinking on about the condensation risk.

The scope room will be left without any internal ceiling, so the board will be fully exposed to ambient air within the room. I assume this will be OK, but I may paint the inside surface of the board with either decorative paint, or a protective treatment of some kind.

For the warm room, things are a little more complicated. My design really leave room for an air gap below the internal surface of the board. My plan was to put insulation between the ceiling joists, in contact with the board, and then put a think ply internal ceiling to hold the insulation in place. The ceiling isn't really high enough to allow me to add a gap.

Is there likely to be condensation issues if the OSB (or ply) is sandwiched between EPDM and insulation (either foam board or rockwool type)?

Thanks

The warm room will be fine, Kev.  If you have nothing underneath it you won't have an issue.

With the scope room, depending on the type of insulation you are having, this could be an issue.  If it is standard loft type fibreglass insulation then it will probably be fine as it isn't that dense.  However, if it is foil backed like Celotex or Kingspan or similar then you will definitely need to leave a gap between the insulation and the roof board.  It only needs to be small, just enough for air to circulate, so perhaps consider slightly thinner insulation such as 25mm instead of 50mm?

I showed my Godson the benefits of insulation yesterday by having him feel a section of the roof which is insulated and covered, and then a part with none, so just the board then EPDM on the outside.  He was shocked as the uninsulated bit was actually hot to touch, but the insulated part cool.  He even noted how cool it was in the warm room (odd phrase).

 

Link to comment
Share on other sites

13 hours ago, Astrokev said:

Thanks Ray. Looks like it may be more down to price then, although point noted about EPDM adhesion on OSB3.

I need to check out prices and then get off the fence!

Yes once you start buying multiple sheets it can make a big difference, Kev.  I think, from memory, for a sheet of 9mm it was about £7 cheaper for the OSB3 to hardwood faced ply which, if you are buying 20 odd sheets, makes a big difference.  I lined mine with ply just because I wanted the smother finish, but obviously lining isn't really necessary, just a nice to have. 

Link to comment
Share on other sites

2 minutes ago, RayD said:

The warm room will be fine, Kev.  If you have nothing underneath it you won't have an issue.

With the scope room, depending on the type of insulation you are having, this could be an issue.  If it is standard loft type fibreglass insulation then it will probably be fine as it isn't that dense.  However, if it is foil backed like Celotex or Kingspan or similar then you will definitely need to leave a gap between the insulation and the roof board.  It only needs to be small, just enough for air to circulate, so perhaps consider slightly thinner insulation such as 25mm instead of 50mm?

I showed my Godson the benefits of insulation yesterday by having him feel a section of the roof which is insulated and covered, and then a part with none, so just the board then EPDM on the outside.  He was shocked as the uninsulated bit was actually hot to touch, but the insulated part cool.  He even noted how cool it was in the warm room (odd phrase).

 

Thanks Ray. So for the warm room, I can get away with using fibreglass insulation in contact with the roof board, but Celotex would need a gap. Looking at the dimensions again, I could probably use 50mm Celotex (or similar) insulation which would leave a nominal 18mm space as I've used smallish gauge joists for the roof. I can easily vent the edges of the space to outside. I think (hope!) this should work.

 

Capture.JPG

Link to comment
Share on other sites

Just now, Astrokev said:

Thanks Ray. So for the warm room, I can get away with using fibreglass insulation in contact with the roof board, but Celotex would need a gap. Looking at the dimensions again, I could probably use 50mm Celotex (or similar) insulation which would leave a nominal 18mm space as I've used smallish gauge joists for the roof. I can easily vent the edges of the space to outside. I think (hope!) this should work.

 

Capture.JPG

Yes Celotex will definitely need a gap, and 18mm is absolutely fine.  You need to make sure this has flow of course, so as you rightly say venting the ends via a soffit or meshed gap or similar is ideal.  What you have in your diagram here is essentially identical to mine, and I can confirm that works perfectly.

I would think the fibreglass insulation will be fine in the warm room, but make sure it isn't the dense fibrous type, just a normal rockwool type one which you can 'fluff up' if that makes sense?  Basically all you are looking at doing is providing an air circulation on the inside as the EPDM is completely sealed and non-breathable, unlike felt for example, which isn't glued so allows the timber to breathe externally.

Link to comment
Share on other sites

The reason it is so important to have the gap behind the foil covered products like Celotex is that the foil acts as a built in vapour barrier, and is why you don't need a separate one if you use it.  This means then that the moisture trapped between the roof board and the insulation when the outside is sealed by EPDM has absolutely nowhere to go, so will start rotting the timbers.  Rockwool type insulation doesn't form a vapour barrier so should let things breathe fine.

Link to comment
Share on other sites

53 minutes ago, RayD said:

Yes Celotex will definitely need a gap, and 18mm is absolutely fine.  You need to make sure this has flow of course, so as you rightly say venting the ends via a soffit or meshed gap or similar is ideal.  What you have in your diagram here is essentially identical to mine, and I can confirm that works perfectly.

I would think the fibreglass insulation will be fine in the warm room, but make sure it isn't the dense fibrous type, just a normal rockwool type one which you can 'fluff up' if that makes sense?  Basically all you are looking at doing is providing an air circulation on the inside as the EPDM is completely sealed and non-breathable, unlike felt for example, which isn't glued so allows the timber to breathe externally.

Thanks Ray, you've been a great help ?

Link to comment
Share on other sites

Just to add to the discussion, for the second Obs I built, I also went for an EPDM solution covering an OSB3 roof. It's worked great for 3 years now, and I actually like the look of it as well.  I also considered marine ply, but given the EPDM is waterproof, I think that would have been overkill.  Installation was easy, just make sure you have enough to wrap around edges sufficiently.

Link to comment
Share on other sites

2 hours ago, snowy911 said:

Just to add to the discussion, for the second Obs I built, I also went for an EPDM solution covering an OSB3 roof. It's worked great for 3 years now, and I actually like the look of it as well.  I also considered marine ply, but given the EPDM is waterproof, I think that would have been overkill.  Installation was easy, just make sure you have enough to wrap around edges sufficiently.

Thanks snowy911. Good to hear it worked well for you and gives me confidence to press ahead.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.