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DIY SIPs
smart51 - 20/5/15 at 06:42 PM

My plans to build a garden room are under way but the 2.5m height limit is causing a bit of hassle. It doesn't leave enough room for internal height, an air gap underneath and insulation. One solution is to use SIPs for the roof, but I can't find a DIY supplier. Is it reasonable to make my own? I'm thinking of hardwood planks for the inner face, glued on 100mm celotex foam insulation and a glued on top layer of some sort. Possibly OSB with a bonded on EPDM membrane. Though I'd like cedar to match the sides.

Even though I probably don't need to, I'd like to meet building regs wherever I can.

snapper - 20/5/15 at 07:26 PM

What is the height discrepancy?
Trust me no one will measure the difference if it 100mm or 2

smart51 - 20/5/15 at 08:08 PM

Max height from existing ground level 2500mm. Min internal height from floor covering to ceiling 2100mm. 120mm of foam insulation to floor and roof. 25mm ground clearance. 130mm left for structure plus internal and external coverings and tolerances.

If I kept it more than 2m from the boundaries, I could make it higher in the middle so long as it is under 2500mm at the eaves.

morcus - 20/5/15 at 08:49 PM

what if the floor was below ground level?

mcerd1 - 20/5/15 at 09:08 PM

or what if the roof structure was steel ?

it should be possible to make a much thinner structure out of steelwork that would leave you more room for insulation etc...

[Edited on 20/5/2015 by mcerd1]

smart51 - 20/5/15 at 09:09 PM

quote:
Originally posted by morcus
what if the floor was below ground level?

That sounds like a pretty good idea, but I don't really want my shed in a pit. I guess having the internal floor slightly above ground level is good, but I'd want to be sure the bottom of the shed will be well drained.

[Edited on 20-5-2015 by smart51]

smart51 - 20/5/15 at 09:23 PM

quote:
Originally posted by mcerd1
or what if the roof structure was steel ?

it should be possible to make a much thinner structure out of steelwork that would leave you more room for insulation etc...

[Edited on 20/5/2015 by mcerd1]


That makes some sense. I'd have do to the calcs but as 63mm timbers are enough, I suspect there's not much to be saved from using steel.

Sam_68 - 20/5/15 at 09:25 PM

Soleplate level needs to be at least 150mm above external ground level on any timber structure, but there are plenty of solutions to that.

Internal height of 2100mm will feel very claustrophobic - you need to be aiming for 2300mm at least.

You can easily make up an analogy of a SIPs panel by means of 'cassettes' formed from rafters skinned with OSB both sides, with Celotex between the rafters (provided it is accurately cut to size, the Celotex can be placed loose, it doesn't need to be bonded to the OSB faces). This will actually be structurally superior to a SIPs panel for a flat roof application: the foam core of a SIPs panel stabilizes the skins against buckling, but does absolutely nothing for bending stiffness, whereas the rafters will contribute to bending stiffness... and roof panels need to sustain bending loads, not compressive (buckling) loads.

With any 'DIY' SIPs panel or flat roof cassette, you need to be conscious of the potential for interstitial condensation, but again there are ways around that.

FWIIW, though, SIPs panels tend to use PUR (polyurethane) foam, not PIR (Polyisocyanurate; Celotex) foam, because the former is structurally superior.

Sam_68 - 20/5/15 at 09:27 PM

quote:
Originally posted by mcerd1
it should be possible to make a much thinner structure out of steelwork that would leave you more room for insulation etc...


And provide you with the finest thermal bridges known to man, thus completely defeating the benefit of the additional insulation.

smart51 - 20/5/15 at 09:40 PM

quote:
Originally posted by Sam_68
Internal height of 2100mm will feel very claustrophobic - you need to be aiming for 2300mm at least.


2100 is the minimum allowed by building regs, which is the only reason I quoted it. it will be a place to sit, not a workshop, so it will probably be fine.

quote:
Originally posted by Sam_68
quote:
Originally posted by mcerd1
it should be possible to make a much thinner structure out of steelwork that would leave you more room for insulation etc...


And provide you with the finest thermal bridges known to man, thus completely defeating the benefit of the additional insulation.


True.

Sam_68 - 20/5/15 at 09:50 PM

quote:
Originally posted by smart51
2100 is the minimum allowed by building regs, which is the only reason I quoted it.


There is no minimum ceiling height in the Building Regulations.

The only headroom requirement in the Regs is above stairs, and that's 2.0m.

smart51 - 21/5/15 at 07:41 AM

After a bit more reading, I've found out that buildings under 15m2 that have no sleeping accomodation are exempt from Part L of the building regs (insulation).

loggyboy - 21/5/15 at 07:47 AM

quote:
Originally posted by smart51
My plans to build a garden room are under way but the 2.5m height limit is causing a bit of hassle. It doesn't leave enough room for internal height, an air gap underneath and insulation. One solution is to use SIPs for the roof, but I can't find a DIY supplier. Is it reasonable to make my own? I'm thinking of hardwood planks for the inner face, glued on 100mm celotex foam insulation and a glued on top layer of some sort. Possibly OSB with a bonded on EPDM membrane. Though I'd like cedar to match the sides.

Even though I probably don't need to, I'd like to meet building regs wherever I can.


Any slope on your garden?
The 2.5m height for Permitted Development is taken from the highest surrounding ground level so can make life a little easier if you have a sloping site.

smart51 - 21/5/15 at 07:53 AM

quote:
Originally posted by loggyboy
Any slope on your garden?
The 2.5m height for Permitted Development is taken from the highest surrounding ground level so can make life a little easier if you have a sloping site.

Yes, as it happens. I don't mind digging down at the high end so long as the low end is "above ground". I was planning to sit the building on pads, so I could put a fall on the dug out ground level to let surface water run off.

Sam_68 - 21/5/15 at 05:21 PM

quote:
Originally posted by smart51
After a bit more reading, I've found out that buildings under 15m2 that have no sleeping accommodation are exempt from Part L of the building regs (insulation).


Buildings under 15m2 that have no sleeping accommodation are exempt Building Regulations; period - not just Part L.

I thought you'd already recognised that from your 'timber frame garden room' thread, but that you'd decided to build to something approximating to B.Regs compliant standards, regardless?

smart51 - 21/5/15 at 05:48 PM

I hadn't twigged that buildings under 15m2 were completely exempt. I wanted to keep the SIP thread separate as it is perhaps an odd idea.

splitrivet - 21/5/15 at 11:27 PM

Scuse my ignorance but WTF is a SIPS.
Cheers,
Bob

Sam_68 - 22/5/15 at 01:33 AM

quote:
Originally posted by splitrivet
Scuse my ignorance but WTF is a SIPS.


Structural Insulation Panels : basically two skins of OSB ('Stirling board' ) bonded either side of a rigid foam insulation core.

Structurally, it acts like the sort of honeycomb panel with aluminium or composite skins that are sometimes used for monocoque race car chassis: the 'core' acts to stabilise the thin skins against buckling (thus making them massively more efficient in resisting compressive loads), whilst spacing them apart so that they have a greater second moment of area (like an I-beam) to make them much more efficient in bending.

...but in a SIP panel, the core also acts as insulation, and the skins also act to give vapour and air resistance to the panel.