Gina's Observatory

[malc-c]

I'm planning of felting the roof on both sections, but will use a hot torch to melt the tar backing so it adheres to the wood.

[malc-c]

Also, I presume it would need wider roof beams to support that extra weight. 63x38mm studwork has a discount on 20+ and was what I'd planned for the walls and roof

Gina,

I've taken delivery of 100 lengths of Wickes 63mmx 38mm studwork, and on first inspection only had three lengths that had blemishes and only one of those three was split beyond use.... the rest is ideal for what we have in mind for it

[JamesF]

Thank you James I was planning on covering the roof with felt and painting (or maybe varnishing) the walls. I imagine the roof will be much more prone to wet problems, particularly as it will need a join in each section.

I think if you get a good overlap of felt then OSB would be fine for the roof. You probably just need to keep an eye on it and make sure you're quick to fix any damage that occurs.

If you painted OSB walls with something water resistant (it's a bit smelly, but Mole Valley Farmers do a nice line in faux creosote that seems to do a good job and it would probably help keep wasps and other insects out) and gave it a quick re-coat every few years you'd probably be fine. Not sure about varnishing; OSB isn't the most attractive thing to look at I've tried the garden fence type paints that are water-based and to be honest I've not been very impressed with those. They're not particularly expensive and appear to do the job ok at first, but you seem to need to repaint things every year.

MVF might be worth investigating for materials too, if you have one near you (Cullompton, perhaps?). A builder friend of mine reckons for some stuff they're cheaper than the builder's merchants.

James

[Gina]

My "off-the-shelf" 10x6 obsy uses felt-covered, OSB over (fairly complex) 2" timber, roof trusses, spaced every 2'. A ruddy great "main spar" runs the whole length. LOL. There is a "join" since OSB is 8x4'? At the limit of two-man (overhead!) lifting capacity, but still movable with one hand, when on wheels.

I see. I thought it would need some hefty support. And be very heavy. Lifting it would be a bit of a problem.

I sense WELL-LAID felt is fairly fine?

Yes, I would think so. Seems very common.

You might look at this "corrugated bitumenous sheet"(?) though...

I thought of this. We have some sheets but nowhere near enough and the Wickes 3" corrugated is expensive. Homebase sell smaller profile, lighter sheets, will check those out.

[Gina]

I don't rate OSB either. I'm opting to use exterior grade ply for the roof sections, and 18mm for the flooring. I did consider cladding the exterior frame work with 6mm ply, but have managed to source some 7" feather edge boards for a decent price.

Yes, seems ply would be best for the roof. Lighter and easier to construct.

Having said that, our locations seem different, in that I'm in suburbia so want it to look as shed like as possible, and to fit in with the rest of the garden, where you are some what remote, so aesthetics may not be a high priority for you ? - In which case painted or varnished OSB is an option

Yes, our property apart from the bungalow would most likely be described as "rustic" And a crude looking shed is likely to attract less attention though the difference would be hardly discernible except by our immediate neighbours. So cost is the main criterion. The price of OSB v ply depends on how much is wanted and whether the quantity price is obtained. Between 5 & 10 sheets ply is cheaper, marginally.

[Gina]

I'm planning of felting the roof on both sections, but will use a hot torch to melt the tar backing so it adheres to the wood.

That's interesting I've seen that mentioned before but I imagined myself setting everything on fire! I imaging it produces a more lasting result than gluing it on.

I've taken delivery of 100 lengths of Wickes 63mmx 38mm studwork, and on first inspection only had three lengths that had blemishes and only one of those three was split beyond use.... the rest is ideal for what we have in mind for it

That sounds very promising Must say I've bought lots of stuff from Wickes and always found it good. Their delivery is good too.

[Gina]

I think if you get a good overlap of felt then OSB would be fine for the roof. You probably just need to keep an eye on it and make sure you're quick to fix any damage that occurs.

Thanks for that But I think the weight is a considerable con point.

If you painted OSB walls with something water resistant (it's a bit smelly, but Mole Valley Farmers do a nice line in faux creosote that seems to do a good job and it would probably help keep wasps and other insects out) and gave it a quick re-coat every few years you'd probably be fine.

Yes, we've used that on sheds and things before.

I've tried the garden fence type paints that are water-based and to be honest I've not been very impressed with those.

I agree, they don't last.

MVF might be worth investigating for materials too, if you have one near you (Cullompton, perhaps?). A builder friend of mine reckons for some stuff they're cheaper than the builder's merchants.

Yes, been to Cullompton branch of MVF many times. Not for timber though, as I recall. I'll check them out, thank you

[Tom How]

just a minor point, but worth remembering. When putting a floor down, make sure the floor supports are nowhere near the pier / concrete part.

Nothing worse that having the stars jump up and down in the image every time you sneeze.

I have a 6 foot by 6 foot floor made out of plywood that is only supported in the four corners. Being a "suspended" floor, it does bend a little bit, but you can bounce up and down as much as you like without effecting the scope.

[Gina]

What joist size are you going to use for your floor, Malcolm? And what floor boarding?

I'm planning to use chipboard panels for the boarding and may get enough to floor the loft indoors at the same time. (Chipboard flooring 18x600x2400 £8.65 or £6.17 10+ £4.29 per SQM 1off.) I'd think 63x38x2400 studding would be too small unless supported at more than just the ends. Maybe 100x50 (4"x2") which is what's in the loft as ceiling joists. (Wickes 100x47x2400 £6.38) Or 89x38x2400 studding @ £3.75.

[JamesF]

A copy of the building regs can be quite handy when you're not sure on joist sizes and so on. They give the maximum unsupported spans for floor joists of given sizes at different spacings under different floor loads which is probably a good place to start even if you choose to ignore them later

There's a copy of the relevant table here:

Load-Bearing Walls And Safe Floor Joist Spans

For house floors, I believe 400mm spacing is usually used because it fits nicely with chipboard and plasterboard sheeting. In an obsy I guess you choose what fits most nicely around the hole for the pier...

James

[dazza1639]

Gina

89x38 studs are typically the size of stud used in Timber Framed housing prior to the last round of building regulation changes that required increased thermal performance so that required a thicker stud to be used so that more insulation depth could be include in the wall. However, as thermal performance isn't critical 89x38 is more than strong enough. One thing to bear in mind is your stud walls need to provide sufficient racking resitance, this can be achieve by using 9mm thick OSB sheathing boards fixed to one side of the stud wall.

Regards

Darren

[JamesF]

Oh, and whilst I remember, with chipboard flooring my local builders merchant now only sells one type which is water resistant and has tongues or grooves on all four sides, the latter meaning that the ends don't have to join over a joist as long as they're offset, which can be handy. The water resistant stuff is more expensive though, so whilst probably a good idea for the obsy and bathrooms, if you can get the plain non water-resistant variety it may well be cheaper for the loft. A smart person always checks it will fit through the loft hatch first, too (no, I haven't personally made that error, but I do know someone who has

James

[dazza1639]

What joist size are you going to use for your floor, Malcolm? And what floor boarding?

I'm planning to use chipboard panels for the boarding and may get enough to floor the loft indoors at the same time. (Chipboard flooring 18x600x2400 £8.65 or £6.17 10+ £4.29 per SQM 1off.) I'd think 63x38x2400 studding would be too small unless supported at more than just the ends. Maybe 100x50 (4"x2") which is what's in the loft as ceiling joists. (Wickes 100x47x2400 £6.38) Or 89x38x2400 studding @ £3.75.

As for floor joists 47 x 145 pressure treated softwood joists at 400mm centers will span upto about 3.04m for a domestic floor loading situation as stated in the building reg guidance notes.

Regards

Darren

[malc-c]

I'm plumping for 50mm x 150mm (2" x 6") joists at 16" centres, spanned 8' with 18mm ply floor (8' x 4' sheets x 4) - I'm not expecting much bounce once it's all screwed and bolted into place

[Gina]

Gina

89x38 studs are typically the size of stud used in Timber Framed housing prior to the last round of building regulation changes that required increased thermal performance so that required a thicker stud to be used so that more insulation depth could be include in the wall. However, as thermal performance isn't critical 89x38 is more than strong enough. One thing to bear in mind is your stud walls need to provide sufficient racking resitance, this can be achieve by using 9mm thick OSB sheathing boards fixed to one side of the stud wall.

Regards

Darren

Could you explain what "racking resistance" is, please?

[JamesF]

Could you explain what "racking resistance" is, please?

Stopping the frame shearing sideways, basically.

I think you should be fine with your proposed structure as the sheet cladding should give the framing rigidity. If you have a few bits of framing timber left over though then putting a couple of diagonal members in each wall, one facing each way, couldn't do any harm.

James

[Gina]

I'm plumping for 50mm x 150mm (2" x 6") joists at 16" centres, spanned 8' with 18mm ply floor (8' x 4' sheets x 4) - I'm not expecting much bounce once it's all screwed and bolted into place

WOW!! What on earth are you going to put in there??? That's flooring sufficient to take a three piece suite and a lot more!

The flooring I'm thinking of (18mm chipboard, 600x2400) has tongue and groove on all four sides. It will also go through the loft hatch

http://www.wickes.co.uk/tongued-and-grooved-chipboard-flooring/invt/164516/

[Gina]

Stopping the frame shearing sideways, basically.

I think you should be fine with your proposed structure as the sheet cladding should give the framing rigidity. If you have a few bits of framing timber left over though then putting a couple of diagonal members in each wall, one facing each way, couldn't do any harm.

James

Ah right, thank you I see what you mean.

[Gina]

The design is progressing I might just get it done by the time I can afford the bits

[malc-c]

WOW!! What on earth are you going to put in there??? That's flooring sufficient to take a three piece suite and a lot more!

The flooring I'm thinking of (18mm chipboard, 600x2400) has tongue and groove on all four sides. It will also go through the loft hatch

Tongued and Grooved Chipboard Flooring - Chipboard Flooring - Sheet Materials -Building Materials - Wickes

I thought that, but a test with two lengths of studwork supported on a couple of briks still gave a bit of flexing. Whilst it's not critical as the mount will be detached from the floor, I still want to make sure the floor feels secure with a couple of people standing on it.

I too thought about those chipboard flooring packs, but feel that there is more chance of having to replace it after a few years due to dampness. The advantage is that chipboard flooring is cheaper than exterior grade ply

This is what I'm considering http://www.wickes.co.uk/structural-spruce-plywood/invt/110036/

[malc-c]

The flooring I'm thinking of (18mm chipboard, 600x2400) has tongue and groove on all four sides. It will also go through the loft hatch

Tongued and Grooved Chipboard Flooring - Chipboard Flooring - Sheet Materials -Building Materials - Wickes

Gina, from their website

• Type: Chipboard Flooring
  
• Length: 2400mm
  
• Width: 600mm
  
• Thickness: 18mm
  
• Exterior Use: No
  

[Gina]

Gina, from their websiteQuote:

• Type: Chipboard Flooring
  
• Length: 2400mm
  
• Width: 600mm
  
• Thickness: 18mm
  
• Exterior Use: No
  

Yes, I guess our usage could be called "exterior". I guess plywood would be better with the possibility of dampness coming up from below. I'm not sure that it needs to be 18mm thick though - maybe 12mm would be sufficient. OK so the difference is only £20 but all these extra bits mount up.

[astroimpulse]

With all the planning that's going into this project I think it's going to be exceptional.

Watching with interest

And yeh, boy do the little bits add up.

[Macavity]

Mine has waterproof chipboard - Well, it's the greeny-coloured stuff, anyway.

[astroimpulse]

Good stuff that...stuff.

Used it a lot recently for holding back infill while it settles. It's took some serious rain and it's still solid. For an obs it would be perfect.

About £18 for a big sheet when i bought it.