Mould forming in garage roof

[mike1978]

Hi,

I recently built a new roof for my garage to replace the very old one, but have started to notice mould now the weather has turned,etc.

I`m not sure there is actually anything i can do about this or why its happening, but would appreciate any advice on it.

The roof is made up of timber joists with plywood screwed over them. I then put a breathable membrane on top of this stapled down. Finally, the main steel box profile roofing sheets with butyl tape between all the joins were then screwed through all of this to the joists.

The idea being that any moisture in the garage would pass through the plywood and the membrane, form on the steel sheets and then drip back down on to the membrane and run down the sloped roof and off down a gutter.

I seem to be getting mould appearing though in lines down the roof - presumably looking like where the sheets are touching the membrane and wood.

Is there anything i can do or have i been a numpty and done something wrong?

The garage is very well ventilated at the moment as one of the windows is still missing glass, the side door is a few loose panels and the main double door has large gaps round it where it also needs repairing/replacing.

Here`s a couple of pics -

 

[Ian H]

Are those fixings on the seal the ones with rubber seals on?

 

[mike1978]

Are those fixings on the seal the ones with rubber seals on?

You mean the fixings on the roof panels? Yes, they`re tek fastners with rubber seals round them

 

[Ian H]

Yes, sorry, meant fixings on the roof. The mould spots look the same as the fixings on the photo, could water be passing the seals?

 

[mike1978]

Possibly, i was pretty careful when i put them in back in early October not to squash the seals too tight. It might be worth me getting up on the roof and having a look at them to see if they`re loose etc or maybe even adding a smear of sealant round the bottom and then retightening them up.

There`s alot of bolts though! I was a little over zealous in the amount of them holding the panels down probably! I can`t see the dampness coming from elsewhere though to be honest.

 

[^woody^]

Which way is that roof sloping? It looks like the laps are the wrong way around.

And screws are normally on the ridges, not in the troughs.

 

[mike1978]

Hi, The laps are the right way round - the picture is a bit deceiving! The higher end is the one further away in the pic.

There`s alot of debate whether the bolts should go in the troughs or on the ridges to be honest. Some say putting them in the ridges distorts the sheet not allowing the bolt to seal - but then it looks like i may have that now anyway! I`ll probably go round and stick a bit of sealant under the bolts which are worst and see if it helps.

 

[vards01]

When I installed 32/1000 box profile the installation guidelines were to secure in the trough and use a stitch screw on the overlap peaks for roofing sheets. It looks as though there is a breach on the fixing holes, from memory I think the minimum pitch was 10 degrees, could possibly be one of the reasons for this in your case, as from the picture doesn't look like much of a fall. To work with what you have I think some silicone under washer would probably sort the issue but for how long I'm not sure.

 

[mike1978]

When I installed 32/1000 box profile the installation guidelines were to secure in the trough and use a stitch screw on the overlap peaks for roofing sheets. It looks as though there is a breach on the fixing holes, from memory I think the minimum pitch was 10 degrees, could possibly be one of the reasons for this in your case, as from the picture doesn't look like much of a fall. To work with what you have I think some silicone under washer would probably sort the issue but for how long I'm not sure.

Yes, i think the supplier of my roof sheets said to install the bolts in the troughs aswell.

The slope is probably about 5 degrees max so i reckon i`ll go with a bit of silicone under the washers to see if that helps, cheers, mike

 

[^woody^]

Those damp patches in that pattern certainly look like penetrating damp rather than condensation damp.

But, there is a design error in fixing those metal sheets directly on flat board - even with a membrane. There should be some counter battens and a void to allow the inevitable condensation to drain or evaporate.

So it could be a condensation issue with condensation on the underside of the sheet finding its way into the ply via the fixings by capillary action.

 

[mike1978]

i agree - i think it would work better with battens on top of the ply to give a air gap - wish i`d thought of that at the time! Nevermind - that would be a job for another day removing the sheets to add battens!

I think for now, i`ll try and seal the fixings with some silicone and see how it goes. Thanks though for the help!