Box profile sheet roof on garage

[Sandbags89]

sand
Hi all,

Constructed the timber roof structure for my garage and will soon slap the finish on top. Decided to go for some box profile metal sheets after considering few different options, but this seems to be a cheaper, long lasting option.

I am aware condensation can be a pain with these if not prepped correctly, which is why I wanted to run my thoughts through the forum...

This is how I plan to lay them, from top to bottom:

Box sheets
Fixing Battens
Breather membrane
Joists
Gap for air circulation
Insulation between joists
Vapour barrier (dpm or something sort of polythene sheet)
Ceiling (plasterboard or timber sheeting)

There is no boarding to save £400-500 on OSB.

So my understanding is that I have to stop the warm air inside the garage reaching the metal sheet to cause condensation. So the ceiling and vapour barrier should help with that. Insulation does what it's designed to do. Breather membrane to allow any moisture to escape, and a gap allowed for air circulation.

Let me know thoughts please.

 

[Harry Bloomfield]

Your joists, should run across the sheets, rather than along the length of the sheets. The sheets need to be screwed through (using TEK screws), into the joists. I assume you know the roof does need to slope, to allow it to drain?

That makes the battens, completely unnecessary, with the box section sheets. I have used the sheets now, on three different buildings. I reroofed my garage, then roofed my workshop extension to the garage, then reroofed the summerhouse, the latter originally done with felt roll, which kept ripping - metal sheet, was just laid on top, and TEK bolted down.

Garage, and workshop, just have joists with sheet laid on top, bolted through, then insulation pushed up between joists, which has worked fine. Were I doing it again, I might consider using the pre-insulated, box-section sheets.

Even before the insulation went in, the only time vapour/condensation became an issue, was in frosty weather, when moisuture condensed, on the underside of the cold, bare metal, no issues since adding insulation.

I don't understand why you have included a ceiling/plasterboard, I assume? It's a garage, the overhead space, between the joists, make for handy storage space for long pipes and timber, if you screwed the odd batten in place.

 

[Sandbags89]

Your joists, should run across the sheets, rather than along the length of the sheets. The sheets need to be screwed through (using TEK screws), into the joists. I assume you know the roof does need to slope, to allow it to drain?

That makes the battens, completely unnecessary, with the box section sheets. I have used the sheets now, on three different buildings. I reroofed my garage, then roofed my workshop extension to the garage, then reroofed the summerhouse, the latter originally done with felt roll, which kept ripping - metal sheet, was just laid on top, and TEK bolted down.

Garage, and workshop, just have joists with sheet laid on top, bolted through, then insulation pushed up between joists, which has worked fine. Were I doing it again, I might consider using the pre-insulated, box-section sheets.

Not sure I understand you, how would the joists run across the sheets as they would follow the slope direction, and the sheets would also have to follow the slope. Mines a flat roof, if you're looking at the front of the garage the joists run across with firrings back to back to create slopes left and right. So it's got a low pitch in the middle.
I've read it should be minimum 4 degrees angle for pitch when using metal sheets however I think it should be fine, I'm using one continuous sheet for the slope. Recommended drop for flat roof it's between 1:40 and 1:80, mine is around 1:35 so should be plenty I reckon. Metal sheets will run better than say a felt roof I imagine. Anyway ...
I will have the battens running across the joists so there is continuous support for the sheets, and the sheets will be fixed to these using the Tek screws with the flange and washer.

Did you not using any membranes at all then?

 

[Harry Bloomfield]

Not sure I understand you, how would the joists run across the sheets as they would follow the slope direction, and the sheets would also have to follow the slope. Mines a flat roof, if you're looking at the front of the garage the joists run across with firrings back to back to create slopes left and right. So it's got a low pitch in the middle.

I would seem to have misunderstood your diagram, though normally the joists would run at right-angles to the sheets. The sheets are stronger along their lengths, so they will need substantial batten supports, with the sheets laid in the same direction as the joists, plus they will need to be thick enough for the TEK screws.

Reading between your lines, it seems you may have got the roof timbers fitted the wrong way, for the roofing type?

Did you not using any membranes at all then?

No, none at all. It's a garage and workshop, plenty of ventilation, and when the car goes in wet, I leave the roller door open a few inches, for extra ventilation.
...

 

[freddiemercurystwin]

Should have run your joists perpendicular to how you've done it, then no need for the counter battens and the inevitable air gap you're going to introduce.

 

[Sandbags89]

Ok I understand what you are both saying now. Like this:

Yea well originally I just designed it for generic flat roof to be boarded and felted or EPDM etc.

But I should be able to still do it with battens right, I found an image online basically same idea as I explained:

Just make the battens thick enough to take the screws.

The air gap would only be on the sides as shown in image above, but I could close this with noggins inbetween battens, and then metal end cap. Would I want to close the air gap or could it be beneficial for ventilation in the void?

 

[freddiemercurystwin]

Well you're a bit screwed now so you don't have a lot of choice, anyway you want to make sure the membrane drapes down beneath the battens as in your second image and ensure you have ridge and eaves ventilation and that any condensation that collects on that membrane can actually run out into the gutter.

 

[Harry Bloomfield]

Yea well originally I just designed it for generic flat roof to be boarded and felted or EPDM etc.

That explains it! Felted, I'm sure you would eventually wish you hadn't. I did, due to damage from squirrels and birds damaging the felt. The metal is completely impervious to them.

 

[Sandbags89]

That explains it! Felted, I'm sure you would eventually wish you hadn't. I did, due to damage from squirrels and birds damaging the felt. The metal is completely impervious to them.

Yea from my limited experience from torch on felt, not fun to lay, and problems always crop up down the line. Metal should be a lot more durable, just the condensation issue needs to be avoided.

Well you're a bit screwed now so you don't have a lot of choice, anyway you want to make sure the membrane drapes down beneath the battens as in your second image and ensure you have ridge and eaves ventilation and that any condensation that collects on that membrane can actually run out into the gutter.

Is the membrane drape to allow a path to drain any moisture down to the guttering? I'm worried that letting it sag will cause water to pool when it gets to the fascia board as it's not a huge slope. Maybe I could add spacers under the battens to allow small gap for water to run under, then keep the membrane tort?

Bit confused on weather to use foam fillers, want to weatherproof it but also as you said need ventilation in eaves and ridge. don't want to water and leaves blowing in or animals setting up nests. Maybe have use foam fillers one section with diy mesh for ventilation

 

[Harry Bloomfield]

Maybe have use foam fillers one section with diy mesh for ventilation

They sell a ready-made eaves filler, to match the box section, should you need to seal it.

 

[freddiemercurystwin]

Yes adding counter battens in line with your joists above the membrane would allow you to keep the membrane more taut, you would use an insect mesh to stop insects etc at the ridge and eaves. The devils in the detail especially when you go for a bit of a hybrid set-up.