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Condensation in Roll Off Roof Sheds

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Grant Privett's picture
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Condensation in Roll Off Roof Sheds

A few years ago I purchased a Skyshed POD. Nice design and easy to erect/use - though the quality of the finishing was a bit off here and there. The downside was the condensation. The inside seemed perpetually damp with water dripping from inside of the dome at times  - though, in mitigation, I did have it on a wooden platform which was less than ideal. I could have run a dehumidifier in there, but that seemed a fairly desperate remedy.

So, now I have moved house and sold the POD I am looking at either a glass fibre dome or a wooden roll off shed - perhaps similar to those made by the UK Home Observatory people. The prices are remarkably similar (or a lot lower if I build my own shed - £1k compared with £3-4k). However, I wondered what the experience is of people with wooden construction ROR sheds? Are they drier than glass fibre domes - if rather less elegant? Fibre is of course much longer lasting, so there is a balance to be struck in costs.

My gut feeling is that if I use tanalised wood, seal well and insulate, things can be fairly benign. But your thoughts on this would be very welcome.

doley68's picture
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Hello Grant.

Hello Grant.

When I moved into my house some years back, there was an existing shed in the garden. I converted it into a runoff roof observatory. I made a roof from 16mm poly carbonate sheets on a simple wooden frame. It slides on PTFE runners. In the six years it has been in use, I have never had any condensation whatsoever on any surface. It's well ventilated around the roofline which makes the difference I believe. A big advantage is that during the day it's full of natural light. The whole project probably cost a few hundred quid including replacing some rotten shiplap.

Hope this helps.

Chris.

Grant Privett's picture
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Thats sneaky!

Its a long time since I read the Arditti book, but I don't recall seeing a ROR made of polycarbonate before. Nice. Is  the sheet screwed on to the frame or glued? A material that light weight: I wonder if it could be hinged?

I have been wondering about recycled plastic for the roof to minimise the impact of weathering, but its pretty heavy stuff.

Thanks for that.

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Grant,

Grant,

I still have a 13' x x12' roll of roof observatory with such a roof.  I used well painted hardboard sheets nailed to timber for the walls.  It's still standing and was used for over 20 years and fell into some disrepair as a result of long standing serious illness.

I did find that during the summer the interior got very warm and equipment took some time to cool down.  Condensation was never a problem.

I now have 2.7 metre dome on top of a wooden construction for which the dome and metal work (including telescope) suffer from condensation for which I use de-humidifiers that solve the problem.

Regards,

Eric

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The polycarbonate is screwed

The polycarbonate is screwed to the frame using standard fittings from B&Q which include a rubber doughnut to seal it from water. Very cheap. The roof is 9' x11' and in total weighs about 20kg. Probably to much to hinge but easy to slide by hand without the complication of wheels & tracks etc.

I'm also keen not to lose to much garden, and my upright roof supports are removable. I have steel square section tubes set into the grass and the supports slot in when required and stored in the shed when not...easy.

Size wise I would say that 7' square would be very tight. For the set up you describe I'd say 8x10 for comfort and 8x8 as absolute minimum.

Never had an issue with creepy crawlies. Maybe the daylight puts them off.

Grant Privett's picture
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Creepy crawlies

I hear that leaving LED night lights on in a shed discourages bats from roosting too!

Stewart Moore's picture
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Eun Off Roof

Grant, I've had a wooden run off roof observatory for around 12 years.  The walls are tongue and groove and the roof is ply covered in roofing felt.  It runs on steel angle rails with B&Q plastic wheels.  The roof is ventilated around the roof line.  It has been totally water proof with no condensation whatsoever - an advantage of wood - and the telescope tube never has condensation on it.  The only problem is that it is so dry it gets taken over by spiders.  Ladybirds also like to over winter in it.  Another advantage of wood is that it's easy to screw boards etc. to it and fix electric cables.

Grant Privett's picture
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Spiders

Ah, that sounds familiar. A wooden shed I have occasionally used in the past does that too. Everytime you go there you have to spend time clearing webs from the lights and mains sockets.

It was the dryness of that which made me wonder why I put up with the POD's condensation problem.

I'm thinking 7ftx7ft with a pier mounted NEQ6 and probably a Celestron 11" RASA (can't afford a 14" RASA and mount upgrade) plus something like an SX694 CCD. Am not sure 7ft is big enough, but am not keen to throw away garden - the run off frame area is pretty much dead space in the garden. Also , I want to keep the thing as low profile as possible so it doesnt take over the view from the house - yeah, I'm not making life easy for myself.

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Minimum size 8ftx8ft?

Has been suggested here that wooden observatories are - at least while maintained - fairly benign environments, but that 8ftx8ft would represent a minimum size. Does that fit in with everyone elses views? I have found a 9ftx9ft shed usable and the circular Skyshed POD quite tight with its 2.4m radius.

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Wooden Observatories

My observatory is 9ft 6in x 9ft 6in and I certainly wouldn't want it any smaller.  If I was building again I'd make it 10ft x 10ft.  But then I'm a visual observer who needs to walk around the telescope and climb up step ladders etc. I'm pleased I made it very substantial. It is bolted to the base by studs cast into the concrete - essential when you live close to the windy east coast. It also has a damp proof membrane to protect the wood

Grant Privett's picture
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Shed size

9'6" x 9'6" (3m?) sounds about right I think given the size of that scope - or you would be hanging over the walls. I've worked comfortably with a scope and small foldup table in a 9ftx9ft. I am wondering if 8ftx8ft is just a bit too snug. But if I can set the thing up for remote use (30ft away in the lounge) would even 7ftx7ft be enough? 

Separate note. Is that a MkIV Fullerscopes mount? They were hefty.

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Condensation in Roll Off Roof Sheds

Grant,

My plastic roll off shed, described here: https://britastro.org/node/11048

is pretty much condensation free; the plastic walls stay dry, although the plinth gets very damp due to its sheer thermal mass.

All my wooden sheds attracted gigantic spiders, some the size of small dogs.......... (OK, a slight exaggeration, maybe)

The plastic shed attracts snails....they even scale the plinth and sit on the C14 dovetail sometimes! But the spiders stay well away from the plastic..... Of course, my sheds are complete roll-off systems, not run-off roof systems.

Martin

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Plastic Shed

Thats surprising - one of our plastic garden stores does occasionally get condensation. You would think the plastic would have the same problem as the POD. Perhaps its borderline. I think I would take the precaution of lining to some extent - if it doesnt make the whole thing too heavy.

Yeah, some spiders must be very keen astronomers, they gravitate to optics. Rather like the family of earwigs that once took possession of my Fullerscopes MkIV mount....

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Sheds.

Hi Grant, I posted my recent attempt in a separate thread a while back. A good quality 8x9 internal dimension tongue + groove to replace a Skyshed POD.  £460.  The pent roof is the standard shed covered with 'non-breathable' rubber roofing membrane - 1/20 the weight of felt and has withstood several storms, snow, ice, etc.  If it does deteriorate in some way I reckon it'll take all of 15 minutes to renew but it hasn't shown any such signs.  Completely waterproof..  The roll of membrane cost £18 for 45 metres from Eurocell - lots left!  I'd tried the plastic corrugated roofing (B&Q) in a fold out config in pre-POD days but had difficulty maintaining seals.

As the guys above say, because the roof is slightly raised (in my case B&Q angle iron runners and lots of simple castors) there's great airflow compared to the POD.  Zero consensation and I've never used my dehumidifier.  The shed stands on tantalized bearers on a concrete slab - so good airflow underneath.  I lined the internal walls with "Jablite" foam, again from B&Q and covered with the membrane for a sealed finish.  The result is good airflow but well insulated.

I offset my pier (with CGEM and C925) and there's still room for a CPC800 with wedge plus a partitioned corner for the PCs.  A bit of a squeeze but workable.

The only downside is a neighbours "security" light, that was masked by the POD is now (just) visible and needs a bit of board wedged on one end when it triggers.

The supports for the roof fold in-out to keep her-indoors happy.

Hope this helps.

Tony

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Sounding good

It is sounding like a decent shed frame of at least 8ftx8ft dimension, with a clear plastic pent roof (possibly underlaid with aluminium foil) and thermally lined,  preferably with a drop down side, is a hopeful way forward. I can always look at whats available commercially off the shelf and do some hacking.

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Sounds like a good plan...

Will you post your progress Grant?

T

Grant Privett's picture
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Progress

If anyone is likely to be interested, but  nothing will happen for a few months as we moved house in December and I have the new one to sort out (plasterer due tomorrow, new boiler next month, kitchen to rebuild etc) - theres always stuff needs doing. Thats the real world. :)

I have promised to post a first impression on the Celestron 11" RASA I sometimes use. Tonight would have been perfect, but my poorly wife needs tea, sympathy and general looking after. Sundays forecast looks hopeful too.

 

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Hi Grant,

Hi Grant,

I essentially have two wooden sheds attached together with a doorway between them, only one roof comes off though and this is hinged. The roof has a light weight wooden frame with light corrugated plastic on the top.  It is easy to open and I can be up and ready in only a few minutes!

I have to say I don't get a lot of problem with condensation in there, and I myself prefer it to a dome.  You can see all of the sky (so you can spot any meteors) and of course you don't have to keep turning the dome!  The floor is concrete and it houses my 8 inch reflector- an arrangement I have had for over a decade and which I find very satisfactory!

Best wishes,

-Paul

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Civilised

Sounds very nice. Have wondered about corrugated as an alternative to the stuff used for conservatory roofs. Ensures good airflow of course.

I think the USAF observatory on Haleakala has it sorted. The roof opens and the circular walls are lowered to expose the whole sky. Don't think I could make that work in our garden though. :)

https://en.wikipedia.org/wiki/Air_Force_Maui_Optical_and_Supercomputing_...