the yesyes observatory - let the planning begin

[yesyes]

I better start replying to the questions before they pile up too much...

Hi Chris

Is there a particular reason for choosing a flat roof over a traditional apex design? I've been pondering on which to choose for my own build. A pent roof may be lower giving better access above the roof once rolled off, but will it have as much strength?

The warm room roof will be flat but the roll-off roof part will be an apex roof. I've seen a few builds like that on here and I like that design. Seems the easiest way. Some of the builds are by malc-c, r3i and of course Gina. My obsy will probably look mostly like r3i's but with the apex roof orientation as in Gina's build and the door on the long side. I will also design in that captive mechanism for the roof as seen in r3i's build here.

[yesyes]

Hi Chris,

Yes that would be fine and it makes the boards easier to handle (2" not as heavy as 4"), however, it would be normal practice to bolt them together using coach bolts and large washers( or bearer plates ).

On 6" wide timber you need 2 rows of bolts... a top row say 2" down from the top and pitched at about 2ft intervals.

A second row 2" from the bottom also spaced at about 2ft intervals but offset by 1ft from the top row... you end up with a sort of zig-zag pattern.

The reason for bolting them like this is to more evenly spread the tensions through both and minimise each board warping in a different direction to it's partner.

Some buildings I have seen glue the 2 boards together and put 1" dia pegs through both boards in much the same pattern.

Hope that all makes sense.

Best Regards.

Thanks for that. I will change the design in Sketchup and use two 6x2 joists next to each other.

Does it have to be coach bolts? Could I also use 80mm wood screws from both sides and maybe glue the joists together with wood glue? Coach bolts are quite expensive...

[yesyes]

HI Chris,

What budget have you got in mind for all the materials i.e completed structure minus the internal fittings etc? The only reason I ask is, I designed my shed and passed the plans to a chap on ebay who made sheds. The wood was tanalised 4x2 with 19mm tongue and groove, shed size 14ft x 8ft penthouse style, with an internal partition and the roof felt was included, it is solid. So the complete shed with all the trimmings plus delivery was £1k...I couldn't have bought the wood for that price. They even offered to erect it...although I declined this as I was still building the pier.

Take a look > http://stargazerslou...ot/page__st__40

I'm hoping for the whole build not to cost much more than £1200-1500 in materials. Timber being by far the most expensive of the whole build.

I was planning on buying the timber myself and cutting it all to length on-site.

But do you happen to have a link to that seller? can't be wrong to get a quote for all the timber cut to size.

[yesyes]

One thing I wish I had done differently was the direction my roof rolls off...my one rolls off to the south which blocks anything low on the horizon, so I have to wait a few hours for it to move around into view. I can push the roof right back and get a better view but the hanging rubber seals to stop water ingress gets pinched when bringing the roof back on after viewing.

At my location I don't really have a choice. The only option I have is to swap the warm room and scope room. I'm going to put the scope room on the south side and warm room north. So the roof will roll off to the north. I have trees and houses all around, so I don't need to worry about fold down flaps and the like. There's still a lot to see at higher altitudes. ;-)

To get a slightly better view to the north I will orient the roll off apex roof as Gina did with the gables above the track, not on the short sides of the building like r3i did. So when the roof is open, the scope will be able to look up over the slope of the roof, not straight at the gable. (I hope that makes any sense, all these terms are new to me.. :-) )

[Lonestar70]

Hi Chris,

80mm screws on their own would not provide sufficient sheer capability... used in conjunction with gluing the boards together would be adequate for your use.

When gluing... clamp the boards together and fit screws whilst clamped.

Coach bolts are quite expensive...

Have you seen the price of waterproof PVA???

Keep Happy.

[Whistlebare]

I'm hoping for the whole build not to cost much more than £1200-1500 in materials. Timber being by far the most expensive of the whole build.

I was planning on buying the timber myself and cutting it all to length on-site.

But do you happen to have a link to that seller? can't be wrong to get a quote for all the timber cut to size.

I've dug through all my receipts and managed to find it.

The company name is : AT SHEDS AND FENCING LTD

TEL: 01384918361

They were based in the Midlands somewhere, Dudley I think.

www.bizdirectory.co.uk/atshedsandfence.html

My 14x8 pent was £1k delivered

I would highly recommend them.

Good luck

[yesyes]

Hi Chris,

80mm screws on their own would not provide sufficient sheer capability... used in conjunction with gluing the boards together would be adequate for your use.

When gluing... clamp the boards together and fit screws whilst clamped.

Have you seen the price of waterproof PVA???

Keep Happy.

Good point. I bought a litre of waterproof PVA for £9.99 at Homebase the other day. No idea how much I would need to glue all these boards. I didn't even know PVA is the same as wood glue..

Turns out coach bolts are not as expensive as I thought. 10 M8x110 including nuts for just over £3. I take it M8 would be strong enough?

[yesyes]

I've dug through all my receipts and managed to find it.

The company name is : AT SHEDS AND FENCING LTD

TEL: 01384918361

They were based in the Midlands somewhere, Dudley I think.

www.bizdirectory.co.uk/atshedsandfence.html

My 14x8 pent was £1k delivered

I would highly recommend them.

Good luck

Thanks for that. I thought you said he was on ebay?

[Lonestar70]

Hi Chris,

M8 would just about be ok... 10mm would be much better.

Don't forget... you also need the large dia washers or square metel plates to bolt up against.

the ones for the head end have a square hole... the nut side has a round hole.

The down side to this method will mean the heads, plates and nuts stick out from the surface which might be a problem for your cladding. At least Screws and Glue will eliminate this problem.

Choice, Choices!!!!

Keep happy.

[yesyes]

Hmmm, I thought the heads are big enough not to need another washer. Would have only used penny washers on the nut side...

I think I do prefer the glue and screw approach... Homebase have a 15% off everything this weekend, so a litre of waterproof PVA will only be £8.50. Now if I knew how much I'll need...

[Gina]

with the apex roof orientation as in Gina's build

Yes, one reason I did it that way was to improve the view over the roof. Also makes sealing the warm room room to ROR easier and a couple of other advantages.

[Astrokev]

The down side to this method will mean the heads, plates and nuts stick out from the surface which might be a problem for your cladding. At least Screws and Glue will eliminate this problem.

Choice, Choices!!!!

Keep happy.

Countersinking the bolt heads would probably avoid this problem. I did this when I built my deck and it proved very successful.

[Astrokev]

Yes, one reason I did it that way was to improve the view over the roof. Also makes sealing the warm room room to ROR easier and a couple of other advantages.

Yes, this is a good approach and one I may also adopt in my design. I'm still arguing with myself over the footprint size for my build, so haven't got round to thinking too much about the ROR as yet .

[Lonestar70]

Hi Chris,

On reasonably smooth surface 1ltr of PVA should be enough for around 64 sq ft of contact surface.

For your 4 outside frame joints you will only have around 25 sq ft.... spread a thin layer over the whole surface using an old straight knife or the back of a hacksaw blade and assemble straight away... it starts to set off pretty quickly.

I would buy a couple of 1ltr bottles at that price as I am sure you will need to glue up lots of other joints etc.

It Keeps well in a coolish place if well sealed.

Keep happy.

[Whistlebare]

Thanks for that. I thought you said he was on ebay?

He was, that's where I found him... I dealt with him outside of eBay, it doesn't look like he advertises on it any longer. Don't think he needs the extra business which comes with eBay ie all the time wasters and people asking to build observatories :-)

[Gina]

I bought a 5litre bottle from Wickes some time ago and it's still going strong

[yesyes]

Hi Chris,

On reasonably smooth surface 1ltr of PVA should be enough for around 64 sq ft of contact surface.

For your 4 outside frame joints you will only have around 25 sq ft.... spread a thin layer over the whole surface using an old straight knife or the back of a hacksaw blade and assemble straight away... it starts to set off pretty quickly.

I would buy a couple of 1ltr bottles at that price as I am sure you will need to glue up lots of other joints etc.

It Keeps well in a coolish place if well sealed.

Keep happy.

ah, thanks. That's a lot less than I thought... ;-)

I still have about half a litre left in the 1 litre tub I bought for the foundation on top of the existing concrete slab (been recommended to pour some PVA between the old and the new concrete).

So I'll just get another litre this weekend....

[yesyes]

I've been working on the Sketchup drawings for the obsy.

I've changed the slope of the roof to the minimum 2% to save on height (I found on several websites that 2% is the minimum recommended for small-ish roofs). That has saved more height than expected, so I raised the wall height by 10cm to get a bit more headroom, especially in the warm room.

I have also replaced the 2x3" (75x50mm) upright next to the front door with another 4x4" (100x100mm) for more stability and security. I'm also considering hinging the front door on the right side instead. Otherwise I'll have 2 doors hanging on the same 4x4 upright that I can't brace properly.

Can anyone spot any major (or even minor) flaws in this design?

Attached is a screenshot and the Sketchup project file (remove the .pdf extension)

yesyes observatory_9.skp.pdf

[yesyes]

What would you recommend to use for the floor and roof sheets? Ply or OSB? And how thick? If ply, does it have to be marine ply (seems quite expensive)?

[malc-c]

OBS is fine for the roof - 12mm.

Floor, I would recommend 18mm external grade ply.

[r3i]

OBS is fine for the roof - 12mm.

Floor, I would recommend 18mm external grade ply.

I used 6mm ply on the roof - it's okay but in hindsight I would go for something a bit thicker so 12mm sounds good to me

[Gina]

I used 11mm OSB on the warm room roof (flat-ish) and 6mm plywood well supported on the RO roof (apex). The roof supported my weight without any problem and I'm no little old lady

[fwm891]

Chris - I used 11mm OSB to clad the inside of my obsy and ali sheeting to the roof. I've added 25mm cellotex insulation to the inside of the wall panels and the underside of the ali roof

[yesyes]

Thanks for the suggestions everyone!

So I'll go for 18mm exterior ply for the floor and 6mm ply or 11mm OSB for the roof (which ever is cheaper).

Has anyone found any issues with the design in my Sketchup drawings? If not, I'll go around a few suppliers tomorrow and get quotes for the timber. Got the day off just for that tomorrow... ;-)

[Bizibilder]

I have had a look at your sketchup drawing and it seems OK - The only slight concern is the very shallow slopes on the roofs?