Help completing a Garden Room

[efunc]

Hello again,

well, it's 6 months on and I'm going to take steps to resolve this train wreck so I'm reviving my original thread. Everything has been left in its original state through the winter and my current plan is to dismantle what has been built, and to rebuild it properly using as much of the existing material as can be salvaged, and order whatever else is needed to complete. I really need some of you more experienced builders to help me with the job spec so that I can show it to my new crew and ensure the job is done correctly this time. This time I want to ensure I have decent solid foundations and that the groundwork is done properly before proceeding to the construction. I would also like to build a brick plinth for the walls with a damp proof course on top before installing the sole plate and timber frame on top. I would like to produce a fairly comprehensive list of all the depths, dimensions and materials required for the foundation, plinth and walls. Is there a basic outline I can use or can anyone start me off? It's only an outhouse really so I don't think I need to dig too deep. So, what, 300mm MOT, 100mm Kingspan, 50mm screed for the floor? 3 courses of brick for the plinth? Diagrams or photos would be handy. geraldthehamster linked to a photo of this type of construction previously which swayed my decision.

I will use the same wall construction as before since I don't think there was anything fundamentally wrong here. 2x3 timber frame clad in OCB board on the exterior, wrapped in breathable membrane, secured by battens and then clad in vertical larchwood cladding. Interior will be rockwool insulation, breathable membrane, plasterboard and plastered to finish.

Roof will be flat cold roof, due to lack of height at the back wall. A warm roof would be preferable but it would no doubt need another 10-20cm and the height at the back wall is only about 2.6m, so hardly anything left to accommodate the roof flashing.

I would be grateful for any guidance on the above or useful links since I would like to produce the job spec today. Thank you!

 

[efunc]

So, I've gone ahead and jotted down some notes to present to the builder, but I'm not sure if this is sufficient in itself. Also, I'm unclear on some of the technical details on the foundation. Have I got the basic structure right? Is the compacted hardcore even necessary or do they just use a bed of gravel for drainage?

Scope of project: To dismantle and rebuild a garden room, install lighting and connect electricity to mains supply.

After dismantling existing structure the groundwork and supporting garden walls will need to be made good.

SITE & FOUNDATIONS:
Ground to be excavated to 400mm, followed by 150mm of Type 1 MOT, 1200 gauge DPM membrane, 100mm of concrete, 100mm of Kingspan PIR Insulation (K103 phenolic insulation?), 75mm of screed to produce a level floor for tiling.

Garden Room base to be built with a brick plinth of three courses with a damp proof course on top before installing the sole plate and timber frame on top.

On the far right perimeter the garden wall will need to be raised to the height of the roof to support the overhang on the right side (about 4 courses). Also some re-pointing necessary on the rear garden wall.

On completion the Armoured Cable trench will need to be re-filled with concrete and levelled.

BUILDING:
Dimensions - 3.3m x 2.6m with 2.5m overhang to the right for storage. One centrally positioned set of french doors in anthracite grey 150cm x 200cm, and 4 glass side panels of 50cm x 200cm, 2 on the front wall, one of the right and one on the left.

The wall construction of the new Garden Room will be the same as the existing structure. 2x3 timber frame clad in 18mm OSB board on the exterior, followed by suitable house wrap, secured by vertical battens and horizontal counter battens with stainless steel insect mesh installed to prevent rodents and insects penetrating voids, then clad in vertical larch wood T&G cladding. Interior construction is to be rockwool insulation, breathable membrane, plasterboard and then plastered to finish.

ROOF:
Roof will be flat 'cold roof', not a warm roof, due to lack of height to accommodate the roof flashing at the back wall. The flat roof will have a minimum fall of 1:40 to the front and covered in EPDM rubber. Flashing on to the rear and right hand garden walls and guttering at the front. Anthracite facia and ventilation grill in the soffit.

ELECTRICS:
Armoured cable will need to be run from the house via a separate Circuit Breaker through a trench to the Garden Room consumer unit. 4 double sockets inside and External RCD double power socket on outside right wall.

LIGHTING:
The front roof overhang will house 4 LED down lights. Three spot lights will be mounted on the front wall of the Garden Room. Three spot lights will be mounted on the adjacent Garage wall. All the above will need to be controlled by a 2-way dimming switch in the main house (kitchen) as well as from the garden room.
One LED light will need to be installed in the ceiling of the garage with a switch inside the main house (kitchen).
Four LED down lights will need to be installed in the roof of the right storage area with a switch on the garden room right exterior wall.
Four LED down lights will need to be installed in the ceiling of the garden room itself with a dimming switch in the garden room inside wall.

 

[efunc]

Anyone have any thoughts, or know somewhere with the right info or advice?
I think this sort of foundation may be overkill for a 7.5m2 timber outhouse, however other types of foundations I've read up on seem to leave big voids on the underside of the structure and I don't want to provide a home for mice, rats and foxes, all of which are plentiful in the near vicinity. Is the basic spec OK for a builder?

 

[Notch7]

Anyone have any thoughts, or know somewhere with the right info or advice?
I think this sort of foundation may be overkill for a 7.5m2 timber outhouse, however other types of foundations I've read up on seem to leave big voids on the underside of the structure and I don't want to provide a home for mice, rats and foxes, all of which are plentiful in the near vicinity. Is the basic spec OK for a builder?

You seem to have specified a floor make up but not foundation -unless you are building off a slab, then you need both.

foundation depth is determined by ground conditions rather than a set dimension.

maybe you could just do a ground slab and build off that -in which case you need some detailing which Im sure some will advise on here.

 

[efunc]

Oh, damn! I thought this was how you do foundations. So from what you're saying I think I need to dig a deeper trench at the edges for the blocks/bricks to anchor into? These will form the base of the walls on which the sole plate will be positioned. I mistakenly thought the 150mm MOT and 100mm concrete was sufficient for this scale of foundation, but I will now research further as I don't think my knowledge is good enough. The ground is not particularly problematic as far as I know and there are no obvious deep roots nearby either, so fairly standard stuff.

 

[Leofric]

Dpm should be on approx 50mm sand blinding on hardcore.

 

[efunc]

Dpm should be on approx 50mm sand blinding on hardcore.

OK thanks. On my revised spec I had added 25mm of sand on the hardcore, but I'll bump that up if you think it's advisable. And is it otherwise OK? I hadn't given much thought to the brick plinth. Would these just be laid directly onto the concrete slab, or is there additional prep work or support needed for that?

 

[geraldthehamster]

Yes. you need a footing round the edge to support the wall. The floor is just for walking on and putting things on. In the real world of garden rooms, your floor slab would probably be perfectly adequate to support the walls. However, for proper footings dig a trench a couple of feet deep (you'd get away with less), either fill with concrete or concrete the bottom and build up to ground level in block before starting your brick plinth.

By the way, the DPM under the floor should lap up to meet the DPC in the wall.

The floor slab you've described is good enough for a house extension; whether you need to go that far overboard for a smallish garden room is a moot point. If you dispensed with the MOT altogether, put down some sand blinding to protect the DPM, and chucked some steel mesh fabric in the concrete above, I don't think it'd go anywhere. It's only a floor.

Disclaimer: I am not a builder, just a DIY type who has done this kind of thing.

Maybe discuss with the builder who's going to do the work. You don't want to be paying for excavation and materials you don't really need.

 

[efunc]

Ahh, thank you gerald. I'm glad you came along because it was the picture you posted that prompted me to change the type of construction I had to timber frame on a brick plinth. It seems to resolve a lot of concerns I might have had about rot prevention, stability, rodent invasions, etc! I wonder if you'd have a link to that original project so I can study the details and get a better idea?

I realise my floor spec is possibly overkill, but I was getting mixed up between foundations and floors. I will now specify a trench for the footings and maybe make do without the hardcore under the floor.

 

[ivixor]

You don't need strip foundations for a garden room. If it's a timber floor (and it should be) you just create 4-6 short brick pillars to hold the subfloor up.

 

[efunc]

You don't need strip foundations for a garden room. If it's a timber floor (and it should be) you just create 4-6 short brick pillars to hold the subfloor up.

Yes, but I didn't want voids underneath the structure because of pests, rodents, vegetation etc. Wouldn't this need airflow underneath? I would prefer it was closed off fully, but agree, I don't want to go overboard, I just want to specify modest foundations to do the job as it will be relatively small and light at 7.5m2

 

[ivixor]

Pretty much all garden rooms (and sheds) are built that way and that stuff isn't a big issue. But up to you.

 

[efunc]

Pretty much all garden rooms (and sheds) are built that way and that stuff isn't a big issue. But up to you.

Yes, you're probably right of course, but aesthetically I really don't like the look of a building sat up in the air, across my patio, if there is a more pleasing solution. I found a site that offers to do brick plinths for greenhouses. I quite like the look of a black brick plinth and think it could serve fine for a light-duty garden room:

https://summergardenbuildings.co.uk/baselayer-grpl100-0.html

 

[ivixor]

If you do get a proper builder in they will advise you on what's best. Building a properly damp proof and insulated concrete base and wall requires getting right some quite specific design details.

 

[efunc]

Good advice, thanks. It's definitely more complex than it looks, and probably best I leave it to the professionals.