Metal Roof Design

[aesmith]

Hi,

Sorry for posting a non-woodworking topic, but its sort of related because I can't do any woodworking without a roof on the shed.

Anyway it looks like roof insulation is the least of my worries, because the roof as it stands is prone to leaks when the wind's in some directions. I've looked over how it all fits together and I can see why the leaks arise, but I don't know what would be the best way to address them.

The picture below shows in cross section, where the grey columns are the walls, pale blue rectangles are the metal profile roof sheets, and the thick red lines show flashing and ridge pieces. For clarity I've omitted the timber purlins that its all fixed to.

Leaks arise when the wind's on the back of the building (left on that drawing) and it appears that water is driven uphill up the roofing sheets and under the flashings, to then leak down into the building. More so on the lean-to, but also from the actual ridge.

At those places the spaces between the profile sheets and the flat flashing or ridge sheets are filled with foam inserts, but when there's wind and rain the water finds its way through the smallest imperfection in that seal.

My questions are really two ... (1) Is that layout of sheets and flashings the normal best practice for a roof of this type?

(2) How would these overlaps normally be sealed to prevent this sort of leaking? The seals would have to be pretty much air tight.

Thanks in advance, Tony S

 

[BMac]

I'm very interested in this because I have an old corrugated asbestos roof on my workshop that has spaces along the ridge where rain gets blown up and in. Thank you for posting this,

Brendan

 

[MikeG.]

My questions are really two ... (1) Is that layout of sheets and flashings the normal best practice for a roof of this type?
(2) How would these overlaps normally be sealed to prevent this sort of leaking? The seals would have to be pretty much air tight. Tony S

Tony,

(1) Yes

(2) Don't know!! Sorry. Normally on big commercial buildings all these junctions are sorted with special profiled flashings made for each junction and sheet profile. What you are going to do here I just don't know. Can you describe the profile for me (is it corrugated, or a standing seam roof, or a square-type section)? Do you have access to the inner end of the lower roof sheet, and if so, is it a clean straight line?

Mike

BTW......the "General Chat" section would probably be the right place for a post like this.

 

[aesmith]

Cheers.

Its the modern type of square profile sheeting, similar to this ...
http://www.cladco.co.uk/34100.htm

Unfortunately I can't get at the inboard end of the low roof sheet, however I'm not sure why not - it would be worth my while having another poke around. I assume you're thinking I could seal the gaps from the inside if I could get at it. I was wondering whether expanding foam could be used as a gap filler, but I'm not sure how good that would be at creeping into every little crevice.

 

[MikeG.]

Exactly what I was thinking.....and expanding foam does sneak brilliantly into every crevice. It would be better if you could get silicon in there though, as it is intrinsically water-proof, whereas the foam, which will break down into a powder over the years, isn't so water-proof. It will stop the wind getting through, though, and without the wind the water won't penetrate.

Mike

 

[aesmith]

Does the foam break down if its not exposed to UV? I've only used if for sort of bodge jobs, where longevity wasn't an issue.

 

[MikeG.]

Well, the more it is protected the slower the breakdown. I imagine that built into a wall and plastered over it would last as long as the wall........but you are right....UV is the main enemy.

Mike

 

[busy builder]

You need to remove the foam inserts, see how far the flashing goes up under the main roof. How far doe the flashing extend onto the lower roof sheet as I would expect about 225-250mm.

You could use a silicone (low modulus) sealant on the foam then push it back into the gap, or even use 2 rows of foam, its really quite cheap.

Don't use expanding foam as the roof sheet will expand and contract and will not seal like the silicone.

 

[beech1948]

Hi,

I have once had a similar problem on a stables with a roof shape a bit like yours. Our first solution was expanding foam. This was only partly successful as the outpouring foam is too hard to control. Also it does not form a bond with coating on metal roof sheets. Impact of weather was to loosen its grip somewhat as well.

Our final solution, about 10 years ago was to take one of the rubberised floor mats ( cheap and widely available) used for horses and cut it up to fit using the foam eaves filler as a template. Fixed in place with a weather proof sealant ( can't remember what). This has worked well for 10 years and shows no sign of wear or aging to date.

You need to be a little careful with dimensions as the rubberised mats do not compress like foam.

Good luck

Alan

 

[liam8223]

Here I can help.

I have actually been fitting box profile today!

The flashing is flat(without profile) and the roofing has the 'box' profile.

To seal this the best thing is expanding foam in the high points of the profile.
Ok, its not totally waterproof, but you problem is wind related, not actually a leak, the foam will seal the void and stop the wind carrying the rain.

On the top ridge, either foam or swap the ridge flashing for a 'longer' one, to cover more of the roof.

I would foam up and trim off the overspill to maintain aesthetics.

All of my Industrial units have this box profile and this is my proven remedy.

The uv does affect the foam to some extent, but only if it is left proud. I have only ever had to re foam one roof and that had been done over 10 years.

The foam that comes when you buy the box profile sheets is completely gash and i just chuck it, or tell the supplier not to bother sending it!

Hope this helps, trust me it does work!

Liam.

 

[jimi43]

Hey! You got the design of my workshop there!

My roof is flat profile reclaimed coated steel sheets...and I did fill the ends in with foam which does absorb water and does break down...and it wasn't cheap stuff either!

One thing I found was condensation forms at this time of the year on the inside of the steel being cold. I warmed it up with insulation (double sided ali stuff) then a sub ceiling with rockwall and then a moisture barrier on the inside of that (inner most) and this has solved the problem.

Jim

 

[aesmith]

Thanks for all the comments. There's plenty of overlap on the flashing and ridge pieces so plenty of room for whatever I end up using. I think I quite like the idea of sealing with Silicone from the outside - stop the water getting in as soon as possible.

Condensation's not too much of an issue so far, but of course there's no question of any insulation until I have these leaks fixed.