What's the purpose of the underlay in a torch on roof felt system?

[rcx132]

I have leaks on my garage roof caused by some awkwardly shaped sections where the roof meets a vertical wall and then changes direction again plus stupid Wickes blocks which let water through. I used felt to cover the whole area and that worked great until the wind pulled it off.

I'm considering torch on felt which will adhere to the blocks. I'm just trying to understand what is the purpose of the underlay layer?

 

[James1979]

I don’t ‘know’ - I’m not a roofer but when I watched some guys put torch on stuff down the underlay appeared to be impregnated with tar and they melted that to join the roof and felt?

Probably miles off the mark but that’s what it looked like.
@roofman?

 

[Pete.]

I have leaks on my garage roof caused by some awkwardly shaped sections where the roof meets a vertical wall and then changes direction again plus stupid Wickes blocks which let water through. I used felt to cover the whole area and that worked great until the wind pulled it off.

I'm considering torch on felt which will adhere to the blocks. I'm just trying to understand what is the purpose of the underlay layer?

I don't know but if I had to guess I would say it's to stop the weatherproof layer tearing and cracking with the changing temperatures if the rate of expansion of the weatherproofing is not the same as the roof material.

 

[rcx132]

I have to do the repair i multiple sections each with it's own shape, so I think only underlay can do that.

 

[123hotchef]

u can get single layer torch on, and 3 layer system torch on, one layer is full of holes or it was on what they used up at the stable yard not seen it before. are you removing the old felt

 

[rcx132]

u can get single layer torch on, and 3 layer system torch on, one layer is full of holes or it was on what they used up at the stable yard not seen it before. are you removing the old felt

The old felt was just shed felt attached in not enough places because of the awkward angles. So I'd like the new stuff to attach to the concrete blocks.

 

[gaz1]

I have leaks on my garage roof caused by some awkwardly shaped sections where the roof meets a vertical wall and then changes direction again plus stupid Wickes blocks which let water through. I used felt to cover the whole area and that worked great until the wind pulled it off.

I'm considering torch on felt which will adhere to the blocks. I'm just trying to understand what is the purpose of the underlay layer?

my advice dont adhere it too blocks it will soon seperate as its powder anyway

@roofman i will leave the comment for roofman to answer you

 

[TechnicAl]

They did my shed with just one layer....its thick though

 

[roofman]

Although many of the 4mm torch on felts can be good enough as a single layer application in certain conditions we always use a perforated layer 1st which allows vapor that's trapped in the existing felt work to escape across the whole area and not cause blisters in the future..We then use a 2mm underlay torch on to allow for differential movement in the roof structure and at the same time giving us a good bituminous surface for the last and final layer of 4mm or 5mm torch on cap sheet,this can be sanded for a stone or solar reflective Finnish or my preference is a mineral surfaced layer and charcoal being my preferred colour too, limestone,solar paint and the mineral surfaced cap sheet is to solely protect the felt from UV damage that can deteriorate a flat roof badly within 10 years... I have old customers who i did there roof up to 30 years ago get me to check the condition of the roofs and the ones standing the test of time better are the ones that chose to have White limestone and completely irradiate any felt exposed to the sun,the same reason the flat roofs full of damp moss always last the longest...hope this has helped

 

[rcx132]

Although many of the 4mm torch on felts can be good enough as a single layer application in certain conditions we always use a perforated layer 1st which allows vapor that's trapped in the existing felt work to escape across the whole area and not cause blisters in the future..We then use a 2mm underlay torch on to allow for differential movement in the roof structure and at the same time giving us a good bituminous surface for the last and final layer of 4mm or 5mm torch on cap sheet,this can be sanded for a stone or solar reflective Finnish or my preference is a mineral surfaced layer and charcoal being my preferred colour too, limestone,solar paint and the mineral surfaced cap sheet is to solely protect the felt from UV damage that can deteriorate a flat roof badly within 10 years... I have old customers who i did there roof up to 30 years ago get me to check the condition of the roofs and the ones standing the test of time better are the ones that chose to have White limestone and completely irradiate any felt exposed to the sun,the same reason the flat roofs full of damp moss always last the longest...hope this has helped

Thanks. With the mineral cap sheet I assume it can only be overlapped with itself on the edge with no mineral on it? Is there a cap sheet that can be overlapped with itself more readily for doing awkward shapes?

 

[roofman]

Thanks. With the mineral cap sheet I assume it can only be overlapped with itself on the edge with no mineral on it? Is there a cap sheet that can be overlapped with itself more readily for doing awkward shapes?

You can torch directly on to the mineral surfaced part for flashing's etc...make sure you have warmed or very slightly burnt the area before you hit the flashing with the torch. Ive been on site work in the past where the architect has made us burn back the mineral by heat and scraping before the flashing is torched on to it,personally i think this is overkill as with experience you can make a 100% bonded flashing on to itself by my advice at the top

 

[brightspark]

pity u wernt local .i have 2 rolls of torch on felt that i was gonna use on daughers garage roof but she sold the house so didnt need it could have had it cheap