ROR Obsy approved by SWMBO - Questions before i start to dig ;-)

[Brutie]

Hi everyone !

I am working on the design of my first obsy. It is mainly based on a design at

http://www.astrosoft.co.uk/Articles/Build%20a%20roll-off%20roof%20observatory.pdf

And the pier will be concrete based on the Sky at Night design using a steel lintel in a tube  (or some shuttering) filled with concrete (cannot find a link for this on the web anymore)

I have looked carefully at many fine build threads  and have a basic plan in my head but I have a few questions about construction of the obsy.

The obsy will be:

* 2.4m x 2.4m with a roll off roof

* Floor will be moisture resistant flooring T&G chipboard on wood joists of tannalised 100mm x 50mm

* Joists will sit on concrete building blocks – 3 blocks per joist – with an air gap below joists

* Walls will be 22mm featheredge wood with 100mm corner posts which will be concreted into the ground.  All wood tannalised.

* Roof to be decided later (see questions below)

* Obsy will house my HEQ5Pro  and 10 inch reflector though I may upgrade to a HEQ6 later

I am a proficient DIYer but  a structure such as this will be a new challenge for me. Main principle I am working to is “Keep it Simple”

So here are my Questions

1  Foundations – Any advice please on how to install the concrete blocks (these are 440mm x 200mm x 100mm). I have only about 200 -300mm of top soil under which is compacted clay and stone. I’m thinking of digging out the soft topsoil and laying the blocks on some concrete on the clay. I might need a second block on top of the first to get  200 -250mm of air gap below the floor and above ground level.

2  Will 100mm x 50mm (or alternatively 75mm x 75mm) joists be suitable bearing in mind that 73% of the length will be directly resting on the concrete blocks ? The 100mm high joists will also give me a bit more floor heigh so they are my first choice.

2  Do I need to have a moisture barrier membrane for the floor ? If so where should it go ie under the chipboard and over the joists OR  under the joists over the blocks  OR just on the ground ?  I will be covering the floor with some kind of carpet in any event.

3  How much air gap below the floor is actually necessary. ? 

4 Walls – most DIY obsies don’t seem to use any moisture barrier on the walls nor any insulation lining or even any internal skin . Is it worth including a moisture barrier membrane on the inside of the walls ?

5 Roof  - Needs to be light but I don’t want cheap plastic corrugated sheet. Any suggestions for alternative materials. I am thinking of a simple wood frame (per the original design article) but instead of the plastic corrugated panels maybe some alternative….but what ? Perhaps a simple boarded roof with normal roofing felt on top (I do have some felt available) . Any other ideas ?  Also is a moisture barrier worthwhile as for the walls ?  Regarding wheels and track I see steel wheels on steel track is often recommended but the wheels are quite pricey. The original article uses more simple wheels and claims no issues with opening & closing  - any comments ?

6 Electrics –  Some wise advice please ! I want to install mains power . My shed will be 6 m from my house. I am thinking of taking a spur off the existing ring main through the wall to the outside via a suitable RCCD . Then the cable will be buried in conduit (or use armoured cable) to the obsy where it will terminate to a switched fused box (possibly with a second RCCD). Thereafter a simple wiring to a few sockets around the obsy . Current consumption will be quite low as only astro gear will be powered plus a laptop eventually . My home is a PME installation. Is there anyone who can advise me how do do an install that is safe and suitable. I have done plenty of wiring in the past and am familiar with electrics generally but I am NOT an electrician.  . This topic seems to be a touchy subject for many people and discussions seem to get a bit heated at times, so I just want some sensible practical advice.

I would appreciate any comments or suggestions on this . I can take this  particular topic to a PM if you prefer J

Thanks in advance for any and all advice / suggestions.

Regards, 

Brutie

[Zakalwe]

It looks like a complicated way of doing the base. How about digging out about 6", compacting some MOT, then 2 inches of sharp sand. Lay some paving slabs on top. Then build a wooden floor just like a shed?

i've heard of a few people having to replace rubber wheels after a while as they develop flat spots and perish. Steel wheels are super smooth and long lasting.

[Brutie]

Hi Zakalwe,

Thanks for that. I hadn't thought of that kind of base, but I do like the sound of it. I will give it some more thought. It certainly fits my requirement of being straightforward.

OK on the wheels , too........I'll have a rethink about that.

Woof woof !

Regards,

Brutie

[Gina]

I used 100mm rubber tyred wheels with roller bearings when I built my obsy but lately have replaced the rubber tyred wheels running on timber with steel wheels running on steel track and the difference is quite fantastic.  I can now move my roof with my little finger!   I too very much recommend steel wheels and track - these are galvanised to stop rusting.  They are designed for outdoor use for sliding gates unlike my rubber tyred wheels, the bearings of which rusted and started making horrible grauching noises!

[wxsatuser]

I nicked the idea of 4 fence posts from that pdf and cast a footing between them.

Then I used C16 95x45mm timber for joists and deck boards as the floor.

Stud work was 70x45mm C16, went over the top with that but it's really strong.

Bought most of my stuff from Wickes as they had offers going during my build.

The outer walls are bitumen sheets and inner is 11mm OSB

The pier base is approx 600mm cubed and the pier is a 10inch drain pipe filled with concrete.

[Zakalwe]

For the roof I think that you are right to steer away from the translucent plastic sheeting. That'd make an effective greenhouse!

I'd go with a wooden apex frame and 8mm plywood. Good quality felt can cover that, or if you are going for a fancier finish you could opt for felt shingles.

There's also plastic tiling to consider, which looks good and would be longer lasting than felt.

There'll be a natural tendency to make the roof as light as possible. I'd advise against that as the roof will get a battering in high winds. A relatively tame wind can generate a lot of lift force, and you don't want the roof ripping itself apart or taking off. Make it solid. The reason why I also recommend steel wheels on steel track is that the will glide beautifully regardless of the weight. Rubber wheels will not work as well when heavily loaded as they naturally will give.

[MCinAZ]

  Steel V-groove wheels riding on angle iron will provide the best load capacity and will allow you to minimize gaps between the roof and the walls as there is no opportunity for lateral motion. Rigid plastic wheels running a channel, similar to Keith Ahren's build, will also work, however open channels tend to accumulate debris over time. I would stay away from rubber-tired (or, I should say, -tyred) wheels as they will develop flat spots. If you build a more conventional roof (OSB, roofing felt and rolled roofing or tiles), it will be heavy. Nothing wrong with that, but if you aren't familiar with framed construction you may want to consult with a carpenter. I noticed a couple of techniques in Keith's PDF are probably acceptable for his lightweight roof but aren't approved practices. One example is the screws used to secure his rafters to the roof runners. You want to avoid having individual screws or nails in shear as an exclusive load-bearing element. Adding a horizontal joist at the roof line to support the rafters and absorb the tension load imposed by the roof is conventional, though in Keith's case it would result in a clearance problem with the telescope. An experienced carpenter could recommend solutions which will withstand any wind or snow loads likely to be encountered.

  Be sure to consider the need to properly secure the roof when closed. As noted above, high winds can generate surprising amounts of lift, and you don't want part of your observatory taking a tour of the neighborhood in a storm.

[kirkster501]

I nicked the idea of 4 fence posts from that pdf and cast a footing between them.

Then I used C16 95x45mm timber for joists and deck boards as the floor.

Stud work was 70x45mm C16, went over the top with that but it's really strong.

Bought most of my stuff from Wickes as they had offers going during my build.

The outer walls are bitumen sheets and inner is 11mm OSB

The pier base is approx 600mm cubed and the pier is a 10inch drain pipe filled with concrete.

My worry with this design is those posts in the ground.  What's the plan if (or rather when) they rot in ten years time?  I suppose worry about that then is a suitable answer but in all seriousness it could be a nightmare to break the obs apart to dig one out.  I had to dig a load of 4x4 fence posts out and it is a PITA and backbreaking work....

[jnb]

The joists on piers approach is how I built my wife's studio but without concreting anything into the ground. In that case it is about 3.5m x 2.5m and the floor is a simple frame of 2.4 joists supported on about 15 concrete block piers. The piers do not need to be too deep into the ground as the weight is sufficiently low that spread across 15 piers nothing is sinking dramatically. IIRC I only went about 30cm down for the footings.

You do not need much of an air gap underneath but the smaller it is the more cautious you need to be about keeping it clear. I kept the gap small so that the overall roof height stayed under 2.5m (to avoid building regs) but I do need to periodically clear the sides to keep the ventilation clear, usually when the gardener has piled stuff up against the back of the studio.

I would put a membrane under the cladding because if the cladding is less than perfect than rain driving against the wall could be pushed into the walls.

If I was building an observatory and not a studio I would probably go with the same design but with the mount attached to a central and deeper foundation which is not connected to the building.

[Tinker1947]

My worry with this design is those posts in the ground.  What's the plan if (or rather when) they rot in ten years time?  I suppose worry about that then is a suitable answer but in all seriousness it could be a nightmare to break the obs apart to dig one out.  I had to dig a load of 4x4 fence posts out and it is a PITA and backbreaking work....

I got round the what if it goes rotten problem, the corner postare are 4' in the ground, the inner one around 2' line up the posts and pore in pea shingle, the posts can breathe there treated the some oil based add by me, if they need to come out suck the shingle out with a hoover and just lift the post out, there really held solidly.....

[kirkster501]

Another technique I have heard is to place a brick at the bottom of the hole then insert the bottom of the post onto said brick and pour concrete around the post as per usual.  The bottom of the post is then able to release water into the brick and hence into the surrounding ground without the post needing to be in constant contact with the soil/clay.  I cannot testify to the success of this method though.

[Tinker1947]

Another technique I have heard is to place a brick at the bottom of the hole then insert the bottom of the post onto said brick and pour concrete around the post as per usual.  The bottom of the post is then able to release water into the brick and hence into the surrounding ground without the post needing to be in constant contact with the soil/clay.  I cannot testify to the success of this method though.

I did see some plans for a Oak Car port and they had 6" or so of 20mm gravel on the bottom for drainage....

[r3i]

Hi Brutie and congratulations on getting planning approval - that's the hardest part of the project completed

Some comments on your questions based upon the experience gained on my own build:

1. Foundations:  I used 10 anchor posts to fix my observatory into the ground.  An advantage of this method is that you don't have to worry about levelling the ground if you have a slope as this is taken care of when you affix the joists onto the upright posts.

2. Joists: 4x2 joists are a common choice and they're what I used.

3. Air gap:  Not sure about what is recommended.  My observatory is on a slight slope so there's a very small gap at one side and a larger one at the other.

4. Walls:  I've got a vapour barrier on mine.  I think it's worth having one in case there are defects in the outer cladding (such as knot holes).  As for an inner skin, some have one, some don't.  I've just gone for a cheap and cheerful hardboard lining which has been fine, though plywood would probably be better if you can accommodate the extra outlay.

5. Roof:  Like many others, I chose EDPM rubber.  More expensive than felt but easier to work with and longer lasting.  As for the wheels, I went for 100mm rubber ones - 2 years on they're still running nicely and I've not experienced any of the problems that others appear to have had with these kind of wheels.  However I've no doubt that the steel v-groove wheels will be a superior choice, though as you say they're more expensive.

6. Electrics:  I have a buried SWA cable between the house and observatory.  At the house end the cable goes into a fused spur at the back of the garage, then from the spur I have a flex with a normal 13A plug on it that plugs into a socket on the house ring main, so it works like a glorified extension cable.  Within the observatory it is terminated into a 2-way garage consumer unit with a built in RCD trip.  The CU has 2 MCBs installed: a 6A for the lighting and a 16A for the sockets.  It goes without saying that if you're at all unsure, it would be best to seek professional advice on the electrics.

Best of luck with the project and keep us informed of progress with plenty of pics. 
 

[laser_jock99]

A couple of points:

Avoid sinking your main uprights into the ground or even into concrete. Treated or not they will eventualy rot and you'll loose the structural integrity.

Try and keep these vital posts dry by using Metpost 100mm Bolt Down Brackets or something similar just to keep the damp at bay. Stand your timbers in a bucket of creosote for 15 minutes prior to fitting as you probably won't be able to reach the bottom part again.

Metposts on concrete plinths

The corrugated plastic sheet roof is a bad idea- mine was destroyed by hail stones shortly after install- luckily it was only over the BBQ deck part of the obsy so no harm done.

[Brutie]

Hi Everyone,

Thanks so much for all the responses. Lots of great suggestions and ideas and I am studying all of them carefully.

Apologies for this delayed acknowledgement but I have been without internet for a month :-(

Final design and construction will begin shortly !

Best wishes to All.

Brutie

[gorann]

I just posted some descriptions of the observatory I recently built and now added some extra pics of the roof (see "My slip-sliding roof observatory"). I built the roof as light as possible (3 x 3 m weighing ca 100 kg) using good materials and plenty of screws to keep the corrugated steel sheets strongly attached to the wooden beam framework. It all supports each other and the roof flexes very little). I am quite confident that winds will not rip it off as I anchor it in the corners with excenter locks. I did not put in any moisture barriers since I wanted the construction as well-ventilated as possible to avoid moisture problems (coming for example from the soil underneath). That is why you can see how light shines through especially around the roof. Instead, I made the other half of the 2.5 x 5 m construction into an insulated and heated control room.

[malc-c]

Comments:

• As mentioned, it's advisable to make the roof sturdy.  Given the gusts of wind we've had over the past few winters that flimsy roof in the pdf would soon be in a garden three doors down.
• Foundation:- incorporate a damp proof membrane in the brick layer.  It will prevent or at least reduce the problem of damp coming through the bricks and damaging the floor in the long term.
• Flooring - Use marine grade external ply.  In the pdf he used lofting boards made from chipboard.  This won't last in the damp dew conditions
• Roof - Agreed with others, don't use clear plastic.  It will need substantial cooling time before you can use the scope without any thermal current issues.
• Frame construction - increase the size of the timbers to take account of the increased weight of the roof
• Use the traditional rubber wheels from screwfix - tried and tested

In terms of foundation, I think I possibly hold the award for having the most solid foundation for a ROR DIY observatory

[Physopto]

Metpost bases are a realy good idea, I've used them before (not for obsy). One point though I would strongly advise against using chipboard flooring no matter what grade. It will always fail in any damp environment. Pay a bit more and use WBP plywood, not the cheap Chinese variety, it is useless. Use a vapour barrier under it. Screwfix sell a good one with a foam layer on the back. ( about 2 or 3mm thick). as someone said all posts treated or not in concrete  or not will fail if in the ground, hence the Metpost idea.

Derek

[Physopto]

Comments:

• As mentioned, it's advisable to make the roof sturdy.  Given the gusts of wind we've had over the past few winters that flimsy roof in the pdf would soon be in a garden three doors down.
• Foundation:- incorporate a damp proof membrane in the brick layer.  It will prevent or at least reduce the problem of damp coming through the bricks and damaging the floor in the long term.
• Flooring - Use marine grade external ply.  In the pdf he used lofting boards made from chipboard.  This won't last in the damp dew conditions
• Roof - Agreed with others, don't use clear plastic.  It will need substantial cooling time before you can use the scope without any thermal current issues.
• Frame construction - increase the size of the timbers to take account of the increased weight of the roof
• Use the traditional rubber wheels from screwfix - tried and tested

In terms of foundation, I think I possibly hold the award for having the most solid foundation for a ROR DIY observatory

Malcolm,

If you haven't got much further I would suggest wrapping the ends of each joist in poly sheet. Also soak the ends with insect treatment. The bricks may over time allow moisture to get at the beam ends. I've stuck in a photo of what I had to do in the house because of damp rot/woodworm problems, once the floor is down etc it is too late and costly then.

Looks a great job.

Derek

[malc-c]

Hi Derek,

the observatory has been up and running since 2011 

The joists were treated with preserve and a short length of DPM placed on the end of each one when placed in the hangers - belt and braces

[gorann]

Comment on the roof: It is not the weight of the roof that keeps it on in high winds, it is how well you screw it together and secure it to the building.

[Physopto]

Oops!

I was a bit late then

Derek