Wood glued over flat roof to build frame for pitched front roof - will this last?

thusha

Our builder told us he used glue to attach treated wood over flat GPR garage roof in order to extend our pitched porch roof over front of garage (he built the frame in wood and glued it over the GPR flat roof before tiling over the frame). Is this acceptable, to use glue to attach wood to flat GPR roof? My concern is that glue in not strong enough to attach the wood frame (for pitched roof) over the GPR flat roof (garage) and our new pitched porch roof would fly away in a storm from the glue giving away

grumbler

Well, AFAIK, rafters are normally have to be strapped to walls with strong steel straps - exactly to prevent the roof being blown away.

tacpot12

I think I would have the same reservations. I've had very little success with construction adhesives on difficult substrates. While newer adhesives have come along that are better, I think it would be better to screw the framwork to the substrate that is under the GRP roof. This should be a plywood or OSB timber deck. 

It will take your builder less than 15 minutes to put 20-30 screws through the frame into the deck, but this will of course puncture the GRP. However the new roof should be completely watertight, so if it were my roof, I would give my builder permission to puncture the old roof to secure the new roof to it.   

travis-powers

grumbler said:

Well, AFAIK, rafters are normally have to be strapped to walls with strong steel straps - exactly to prevent the roof being blown away.

100% this!
if it isn’t strapped it won’t pass building control.

stuart45

I'm old enough to remember when we just bedded the plate on, and then the chippies pitched the roof with only the weight of it stopping it sailing away for the next 100 years.

twopenny

I guess it would depend on the glue though it sounds unsound to me.
I'm currently peeling shower board off tiles and the glue has dried out long ago and become brittle. 

travis-powers

stuart45 said:

I'm old enough to remember when we just bedded the plate on, and then the chippies pitched the roof with only the weight of it stopping it sailing away for the next 100 years.

I remember having a big augment with my mentor about straps and you are right there is millions of old roofs without them but apparently it has happened!

stuart45

A few came off in the breeze of 87, which might be the reason the regs for straps came in.

Section62

stuart45 said:

A few came off in the breeze of 87, which might be the reason the regs for straps came in.

The Hatfield New Town disaster in 1957 was a key point in the development of procedures for assessing and designing for wind loads on domestic dwellings. It was a rather dramatic wake up.... for the residents involved, and also for the designers.

https://www.youtube.com/watch?v=FbflZnzxkdM

That was primarily due to the roof form used there (monopitch) - but as you say, 1987 exposed the weaknesses in many more designs.

Strapping also of course helps to tie the walls (especially the gables) to the rest of the structure - so it can be a two-way thing.

stuart45

Hadn't heard of that before. What ever happened to the old BBC Pathe news accents? You never hear them now.
I was living in Brighton in 87, and there was an incredible amount of damage. Odd thing was that in a lot a streets you would find that one house had it's roof or gable ends wrecked, and next door with a similar build had no damage. 

EssexExile

As I understand it this is just a small section of roof over the front of the garage, not a whole pitched roof. We had similar done years ago over a small extension at the back, although our builders used their favoured fixing system, big nails, rather than glue. I'm not sure I'd trust glue alone though.