Advice on building a garden studio (terraced house)...

delmonta

Doozergirl said:

Terraced house, South West London. SIPs.  All in through the front door. 

Not cheap though as I wouldn't build anything that can be described as a shed.  We're aiming at meeting building regulations with ours and adding proper value as an extension. 

You really don't need an additional
roof light when you've got big doors.  There is plenty of light and you're only making the thing less energy efficient in winter and turning it into a summer sweat box.  

The picture isn't of ours, but ours is cool
when the back of the house is warm from the summer sun because the windows face in opposite directions, so it provides a welcome respite.  

This is basically exactly what I'd like to do! Thanks

Do you think Sips will end up being a cheaper build then in my situation than traditional timber frame? And would you also use SIPs for the floor and roof? Would be great if I can get them in the door

My room will be north facing and also there are trees etc blocking light, so I do think a roof light will make a huge difference. But as the person above says maybe best to go with a non opener and keep it relatively small

FreeBear

delmonta said: So do you think I can start building directly on top of my current paved garden, with blocks like you mentioned? It feels incredibly solid under my feet, but do you need deeper foundations? Almost everyone I've seen doing it is building on earth to start with.

For a shed/garden office, you want to dig out the base ~150mm. Add in ~100mm of well compacted hardcore and topped off with ~50mm of concrete - You can substitute concrete for a layer of sharp sand under slabs.

A patio should be of similar construction, perhaps as little as 75mm of hardcore and the slabs bedded down on a full bed of mortar (not 4/5 blobs as seen on some DIY programmes). Ripping up an existing paved area is a lot of work, and getting rid of the spoil is going to be a major headache when it has to be wheelbarrowed through a house. If the proposed site of the garden room is level and none of the slabs show signs of movement or rocking, I'd go ahead and build straight off it.

delmonta

FreeBear said:

delmonta said: So do you think I can start building directly on top of my current paved garden, with blocks like you mentioned? It feels incredibly solid under my feet, but do you need deeper foundations? Almost everyone I've seen doing it is building on earth to start with.

For a shed/garden office, you want to dig out the base ~150mm. Add in ~100mm of well compacted hardcore and topped off with ~50mm of concrete - You can substitute concrete for a layer of sharp sand under slabs.

A patio should be of similar construction, perhaps as little as 75mm of hardcore and the slabs bedded down on a full bed of mortar (not 4/5 blobs as seen on some DIY programmes). Ripping up an existing paved area is a lot of work, and getting rid of the spoil is going to be a major headache when it has to be wheelbarrowed through a house. If the proposed site of the garden room is level and none of the slabs show signs of movement or rocking, I'd go ahead and build straight off it.

yeah I mean this paved garden must have been here for 20 or 30 years. I can jump up and down and feel no movement at all. Obviously it would be easier to build straight on it, but I don't want issues in the future I guess if I can get points of contact on as many paving slabs as possible it will really spread the weight

Bendy_House

delmonta said:

Yes maybe you are right, I could just have a small window or two on the front either side of the door that opens. Leaving the doors open is fine too, but not practical if I'm out for a while etc.

So the cheapest option for a smallish roof light would be ideal, just to brighten up the place. 

I was thinking of using something for cladding which is zero maintenance and won't ever need coating. Maybe some sort of plastic cladding? It won't be seen. 

So do you think I can start building directly on top of my current paved garden, with blocks like you mentioned? It feels incredibly solid under my feet, but do you need deeper foundations? Almost everyone I've seen doing it is building on earth to start with. 

And the max height obviously is going to include the raised blocks, so what is the minimum you think I could use just to get it off the ground?

For the exterior wall cladding - on the sides and rear - I was meaning plastic-coated metal sheet; try a Google for "Box Profile Wall Cladding Sheet (32/1000)". The numbers mean it's 32mm overall thickness (ie the height of the raised 'boxes') x 1m wide, so this will cover your walls quickly. The stuff can be as thin as 0.5mm, but you may wish to go a bit thicker. The raised 'box' sections will provide nice ventilation gaps on this outside layer, so whatever your wall is actually built from - OSB, whatevs - should be nicely dry.

If your existing patio is solid and level with no loose slabs, and has been for many years, I would - personally - have no concerns in building up from this.

My approach would be something like:

(1) Design the build! Ie, work out all the dimensions, what the walls will be made from, how many floor joists will be used (typically 400 or 600mm spacings), that sort of stuff.

(2) The floor joists will be sitting on something as simple as bricks, so as little as 65mm off the patio slabs + mortar bed thickness. The whole idea is that the timber joists make no contact whatsoever with the ground, and also have a nice air passage around them, so any rain that does get in there simply dries away harmlessly. You then work out how many bricks you need to support each joist at the correct spacings, and this will largely come down to the size of the joists - will they be 4x2 or thicker.

(3) Mark the position of each row of bricks on the slabs. Take some levels - know what you are dealing with in getting this perfectly level. The idea then is to sit the bricks down on beds of stiff mortar, sit the joists on top, and set the joists level by tapping the joists down - the mortar will give - to get them completely level. For a complete novice, you may wish to do a single central joist and leave the mortar to set before then taking all your other levels from this well-set datum joist.

You may also wish to plan this 'ground work' so that it can ALL be removed in the future without leaving a trace, even on the slabs. You'd do this by first placing small A4-ish rectangles of thin poly sheet on the slab before slapping down the dabs of mortar and sitting the bricks on this; it shouldn't ever move, but will lift away cleanly 20 years down the line. Your call.

(4) Set all the joists, all dead level with eachother. Allow mortar beds to set fully. Now you slip pieces of DPC between the bricks and the joists. And finally soak the treated joists in more clear preserver, especially where they sit on the bricks.

(5) Add at least 2" of insulation - could be as cheap and simple as Jablite - cut to fit snugly between the joists, and foam-glued/sealed in place, flush with the tops of the joists. Screw down the flooring of your choice - perhaps moisture-resistant T&G chipboard flooring?

(6) Build your walls. I think I'd keep this simple, but effective enough. I reckon 4x2 vertical studs (CLS is nice to work with) to make the frame, stuffed with 4"-thick neat-fitting Jablite, foam-sealed to the sides if there are any gaps, finished inside (once the build is complete) with foil-backed p'board, and the outside simply staple-sheeted with a breathable vapour-control membrane (like modern roofing 'felt') and the box cladding sheets screwed on. These cladding sheets should be fitted so they hang down over and beyond the floor joists, say by 30mm, so that any rain runs off and won't get to anything timber, and there will still be a ~2" gap between the sheet bottoms and the patio, so a nice vent gap underneath the building to keep things fresh and dry under there.

(7) Make the visible front of the room a visual statement like the one shown in Doozer's pic - lovely, timeless contrast between golden timber cladding and anthracite door and window frames, and fascia boards.

(8) Roof should overlap comfortably on all sides - I'd have a good 100mm or so on sides and rear (much more if you want to give it a further purpose such as rain protection for items stored down there, and a really nice overhang at the front, like Doozer's. If you like the idea of sitting in there during a summer downpour with the doors wide open, then make that a 600mm+ protrusion. Don't forget the roof slope, ideally to the back (where you've left plenty enough room for guttering without impacting on any neighbs). Roof can also be the metal box cladding sheets for simplicity.

Then lots of wee details such as carpet-like material on the patio surrounding the room to reduce rain splashback. Stand back, admire your work, and then kick yourself when you forgot to lay the power cable. Get FULL advice on this - eg RCD protection, SWA, that sort of malarkey.

delmonta

@Bendy_House

Thanks so much! Great advice and info. 

I see what you mean with the cladding, like the stuff they often use on commercial builds. I wonder if the fact it protrudes a bit due to the box section that I'll be strapped for space 

I think I will go with a rubber roof as that seems to be the standard for these builds, and means I can get on top to trim back plants etc, get to the gutter/back. Where as the box cladding would probably not like me standing on it

If I leave a maintenance gap which I'm still unsure about, then I will probably make the roof overhang as far as possible to get some shelter for things

So much to think about!

Bendy_House

The cladding protrudes a bit, but only 32mm. Any cladding will have some thickness; PVC 'weatherboarding', for example, will have it's own thickness but also that of counter-battens to allow a ventilation gap behind it.

I *think* that BC sheets will be the cheapest, quickest, easiest, and most reliable method, but I hope others will also advise/confirm.

Yes, EPDM roofs make sense too. They are very reliable, and the only drawback I'm aware of is that they can be damaged by, say, grit on boots, or swinging overhanging branches. You CAN go on to BC roofs, but would need crawl boards to spread the weight. Either will do. Perhaps research which one is more DIYable, ESPECIALLY if you are going to add a skylight - by far the most complex part.

Bear in mind that the approach outlined above to this sort of job is *my* approach, and I'm *pretty* sure it would very satisfactory. But you should confirm it's fine for you too before going this way. I think it's the *simplest* approach, but alternatives (more thorough ..) obviously exist.

Bendy_House

If going EPDM roof - and I'm not trying to persuade you against it - please ensure the WHOLE roof design is correct, with no *unventilated* gaps, voids, or traps where condensation could form. 

Two basic types, 'warm' and 'cold'. Not sure about pros and cons, but it's things like overall thickness and stuff.

Bendy_House

The roof doesn't have to slope down towards the back either. If space at the rear is a premium, then it can slope towards the front, with the guttering fitted at the end of the front 'canopy', and a fascia board added to disguise this from the front view. Ie, it'll look 'flat', not sloping. There's also 'box' guttering designed for concealed situations like this.

This will, of course, make cleaning out a blocked gutter a LOT easier...

Options! :-) 

delmonta

Bendy_House said:

The cladding protrudes a bit, but only 32mm. Any cladding will have some thickness; PVC 'weatherboarding', for example, will have it's own thickness but also that of counter-battens to allow a ventilation gap behind it.

I *think* that BC sheets will be the cheapest, quickest, easiest, and most reliable method, but I hope others will also advise/confirm.

Yes, EPDM roofs make sense too. They are very reliable, and the only drawback I'm aware of is that they can be damaged by, say, grit on boots, or swinging overhanging branches. You CAN go on to BC roofs, but would need crawl boards to spread the weight. Either will do. Perhaps research which one is more DIYable, ESPECIALLY if you are going to add a skylight - by far the most complex part.

Bear in mind that the approach outlined above to this sort of job is *my* approach, and I'm *pretty* sure it would very satisfactory. But you should confirm it's fine for you too before going this way. I think it's the *simplest* approach, but alternatives (more thorough ..) obviously exist.

Ok great, well the cheapest low maintenance cladding is what I need as it will not be seen. 

Yes I need to look into the skylight situation and what roof would be best for that. I imagine fitting a skylight with the EPDM roof would be easier than the BC sheets

delmonta

Bendy_House said:

If going EPDM roof - and I'm not trying to persuade you against it - please ensure the WHOLE roof design is correct, with no *unventilated* gaps, voids, or traps where condensation could form. 

Two basic types, 'warm' and 'cold'. Not sure about pros and cons, but it's things like overall thickness and stuff.

Sorry could you elaborate on this as I'm not sure I understand?