10. Warm Roof

My warm roof with the insulation above the joists is the superior building method…but it’s less common for garden rooms as they add depth to the roof.  Go back to the previous video to learn more and see how the alternative cold roof is constructed.

 

Safety

First things first. Working at height comes with some danger and a fall could be serious. Now, I’m a pragmatist and every day all around the world kids climb trees taller than this. You can’t eliminate all risks otherwise you would never go out or do anything. That said, here are some tips to staying safe:

  • Don’t work alone. Have someone around to come to your aid (and have things passed up to you).
  • Don’t work with your back towards the edge of the roof.
  • Don’t work at night or in poor light.
  • Eliminate tripping hazards.
  • Have two ladders up to the roof in case one should fall.
  • If you’re at all unsteady on your feet buy or hire a scaffolding platform so you can work on the edges from a place of safety.
  • Listen up young man! When I did this build I was on the cusp of turning 30 and I know precisely how much risk you might take; the adrenaline seeking and the feeling of invincibility youth brings. A few years on with the responsibility of fatherhood I’ve become much more risk averse. Yes, you could jump down 2.5m+ and be fine but not if you trip and land on your head. Don’t be reckless lads, you’ve got your whole life ahead of you.

Do I need the sub-deck?

Interestingly, no, not always. Here’s a photo of a warm flat roof extension taken from the underside. From bottom to top you have: joists and noggins; firring strips to create the slope; a transparent polythene vapour barrier and then the insulation – no OSB sub deck.

 

photo of underside of a warm roof construction on an extension

In what circumstances can you do without the sub-deck?
If your joists have a height to depth ratio of <=4:1. So, given that in most circumstances we’ll be using 2″x timber that means you can forgo the sub-deck for joists up to 2×8″ (47 x 195mm). Above that, like the 2×9″ joists I used, you need a sub deck to hold the roof together.

There are a few of things to bear in mind should you do this:

  • You must have the right amount of noggins, either one or two rows depending on the span. This will keep the joists upright and well connected without a sub-deck.
  • You’ll want to ensure the short edges of the insulation land on a joist, just like OSB would. There shouldn’t be much more cutting involved if you have your joists set out on 400, 450 or 600mm spacings, which a 2.4m insulation sheet is divisible by.
  • If you intend not to plasterboard under the joists, i.e. leave the joists visible in the final finish then you don’t want to be staring up at insulation. You may not want to look at OSB either so…

 

Other types of sub-decking

You don’t have to use OSB here, you could use plywood. If you are having open joists and you’ll see the underside of the sub-deck then you could paint the plywood. Another option would be timber floorboards that interlock with tongue and groove. These will look great from below though will cost more than sheet material. Whatever you choose it should be 18mm for 400mm joists spacing and 22mm for wider spacings.

 

Do I need a top deck?

I have seen examples where the roof finish (usually EPDM rubber) is applied directly to the insulation with glue i.e. no top deck at all. If you’ve not yet got your hands on PIR insulation it is possible to walk on without crushing it IF you walk flat footed. Take a larger stride and your heel will cause an indentation and that will cause a dip in your roof finish and cause water to pool. So for me, I think the top deck is pretty critical. If for some reason you don’t want a top deck, make sure you at least have a sub deck as walking around on the middle of the insulation with nothing but joists underneath is a recipe for injury!

What material?
I decided that I could make a small compromise here and opted for 11mm OSB3 which has held up well. I left a little gap between the sheets to allow them to expand but if I were doing this again I would opt for tongue and groove 18mm x 2400mm x 590mm OSB3 because:

  • 18mm is a bit more rigid than 11mm and also allows a tongue and groove to be rebated into the sides.
  • The tongue and groove come together tightly yet allow expansion and contraction. This means a smoother and flatter rubber membrane finish. No expansion gaps required.
  • The boards are smaller by half which makes them easier to lift on to your roof and manoeuvre into place.
I think this would be a good choice of material for a sub deck as well.

 

Notes on the vapour barrier

The vapour barrier I used was 500 gauge/0. 125mm. I’ve tried 300 gauge on another project and I think it’s too thin and more prone to ripping. Go for 500 to 1000 gauge (0.25mm). Damp proof membrane is thicker still but probably overkill unless you have some left over from a concrete pour.

You don’t need to tape the top of the insulation for vapour purposes, but you can if you like. I guess you could argue it gives a second water proof layer below the top deck.

Because I have overhangs, I didn’t lap the vapour barrier up the outsides of the insulation but that would be a good call if you don’t have overhangs and the sides of your insulation are directly above the heated space below.

Thickness of insulation

An extension to a house needs to abide by building regs and usually 120mm is required. Often builders will stack a 25mm layer of PIR over 100mm. This amount of insulation is very eco friendly and will save on energy bills in the long term. For a garden room, which is used less frequently than the main house, I think less can be more if it saves on initial build cost. I went with 100mm, but I think 50mm would be sufficient to keep the room warm and hold in the heat. To our house, I’ve added 40mm PIR insulated plasterboard to the solid brick walls and it’s made a world of difference. However, for a roof where more heat is lost, I think 50mm is a good starting point. As I’ve said before air tightness is just as important.

The edge timber

I get asked how I dealt with the difference in height between the edge timber and the insulation. About half of my edge timber was salvaged from the garage and had an actual width of 100mm as older timber did. Today’s planed all round (PAR) timber leaves you a little short (3-5mm). In practice though this isn’t much and you’ll find the OSB can be screwed to the edge timber without issue. There’s no problem if it dips slightly at the sides.

Whether cold or warm roof, you’re nearly watertight!

Task: Add OSB, insulation and vapour barrier. The order will depend on your chosen roof style.

11. EPDM & Fascias