Replacing steel sheets & is exterior insulation worth it

[stuumpington]

Hi all, first post - I'd like to start by thanking the forum members for all the advice I've picked up so far. Ta!

I'm about to replace the steel box profile sheets on the extension to the rear of my house. The sheets are corroding and the flashings leak where the original slate roof meets the flat(ish - approx 10°) sheet roof. For clarity, the sheets are overlaid on an older felt roof

I've had conflicting advice on the best way forward and I'd be very grateful if you folks could let me know what you think and/or offer any other advice:

* One roofer suggested leaving the whole existing roof in place, laying new batons across the old sheets and fixing new sheets to them. That would require fitting the flashing under the next furthest up row of slates to accommodate the additional height

* Another roofer advised that the whole lot should be taken back to the wood, new felt burned on and sheets fixed through the felt to the roof boards

* Other builders have advised to remove the old sheets, fit wooden battens to the old felt and lay new sheets on that


Also, I'm considering the pros and cons of fitting Kingspan type insulation under the new sheets but I'm unsure if it's worth the trouble or how best to go about it. I have a few questions and any advise would be gratefully received (for the record, I'm gutting the insides too so there is the option of fitting approx 750mm insulation between the beams internally)

* Should I leave a ventilation space, and if so, where? Between felt underlay and insulation, or between insulation and sheets?

* If I do need to leave a ventilation space, will the insulation be effective as in effect cold external air will be circulating around the insulation?

* Is there any materials more suitable than Kingspan-esque sheets?


Thanks very much in advance for your help!

 

[DAZB]

It would depend on how corroded the exisiting sheets are as you could fit new sheets over them with fresh flashings using Tek screws with self sealing caps which will bite into the exisiting sheets. As you also have a felt roof below this may be the most cost effective route. As for insulation it is possible to kill two birds with one stone with these :

http://www.ebay.co.uk/sch/i.html?_f...nsulated+roof+sheet&_sacat=See-All-Categories

Regarding ventilation it will make no real difference because the vented part would really be the felted section via soffit vents etc and ventilation under the new sheets won't circulate air below the felted roof.

 

[stuumpington]

Cheers DAZB

The sheets aren't too bad - mostly unsightly surface corrosion. I'd considered grinding them back and applying a new coating but decided it wasn't worth the time, effort and cost.

I hadn't considered fitting new sheets directly to the old ones but I'll have a think about that. One problem is that the old sheet profile is so old that apparently new sheets are difficult to come by.

I'd looked into the insulated sheets before but I'll give that more consideration too.

One problem re: ventilation is that there isn't a soffit to speak of. The roof beams sit on the wall head and the facias sit flush to the wall, the end of the beams and the lowermost roofing board. I had planned on filling the void between the beams with insulation internally and attaching foilbacked plasterboard to the beams, which would in effect seal up any air gap between the ceiling and wooden roof boards. Is that a bad idea??

 

[DAZB]

As you have no soffit it may be possible to fit circular vents at intervals on the fascia just below the felt drip edge which will let air into the space above any insulation you insert into the flat roof joist space. The guttering will cover the vents to some degree but you will achieve some ventilation that way but only fit them on the fascia which runs at right angles to your joists.

 

[stuumpington]

Thanks DAZB, I'd arrived at a similar conclusion - if I use 500mm Kingspan taped flush to the bottom of the joists I should have about 200mm of ventilation space between the top of the Kingspan and the roof boards.

Still not sure if there would be any benefit of insulating externally too? Probably not, but I'm trying to cram as much insulation around the house possible. It's chilly up here!

 

[catlad]

I would strip your roof off completely and see what is happening, and not put a roof on a roof on a roof. you can also buy sheets ready insulated and fix these new one's to you joists as long as they have not sweated to death.

 

[stuumpington]

I stripped off some sections of sheeting to have a look where there were leaks in the flashing. The roof is constructed from 2x2s going parallel to the joists with 2x2s going perpendicular to that to which the sheets were fixed.

Sure enough, the old felt below had pretty much disintegrated. I scooped that off with a shovel to reveal some rotting boards. I ended up removing a section which I replaced with a sheet of plywood. I burned new felt over that to seal it until the new sheets arrive next week.

I spoke to a couple of builders after that and ran a few ideas past them. The consensus seems to be that I should remove the old sheets and flashings, cut back another row of tiles to accommodate a wider flashing (which was the root of the problem)

Then fit 40mm insulated boards between the lower 2x2s / under the upper 2x2s and put on the new profile sheets.

One builder suggested I should strip the upper 2x2s and overlay the lower 2x2s / insulation with some sort of boards and shed felt before putting on new purlins and the sheeting.

I'm not sure the last step is necessary - it would probably add £3-400 to the cost and take a couple of days.

Any thoughts??