Roof leak

[Farmer Giles]

Got a bit of a leak in the roof of one my sheds. The roof is constructed of box-profile sheets and there are some small leaks around some of the fixing screws.

The real problem is that the slope on the roof is probably too shallow for these type of sheets, but I can't do much about that at the moment. A few weeks back I replaced many of the screws - with the correct type, new washers, etc - but that hasn't solved the problem. I'm thinking of covering the offending screws with some kind of trowel-on mastic but I'm not sure which is the best type to use. Any recommendations?

[newshound]

I'd be inclined to use 50 mm lengths of 50 mm Sylglas tape

[newshound]

I mean this one

given that they now do all sorts!

[Andy Burns]

Presume the screws are on the ridges, rather than in the valleys?

[Farmer Giles]

Thank you. I'll have a look at that stuff.

[Farmer Giles]

With box-profile sheets the screws can't be put on the ridges.

[Dave Plowman (News)]

Wicks do a rather good non setting mastic which seems to stick to anything. Specifically for roof repairs, etc. Not expensive, so worth a try.

[fred]

It is v good for this kind of job if a bit messy to use and perhaps a tad unsightly but fine for a shed I'd imagine. Gloves and expendable scissors essential.

On older sheets, it's not uncommon for the coating layer (paint/powder/polyester/whatever) to detach from the sheet metal when the screw is tightened which can lead to a drip leak behind the paint.

The tape over the top should solve that.

I was going to suggest Plumbers' Gold sealant as an alternative, evilly sticky and can be applied to wet surfaces but Sylglas may well be better (longer lasting).

[dennis]

They can be if you fit spacers. It really needs two people to do it unless you have very long arms.

[harryagain]

I assume there were the soft waterproofing seals under the washers? The most likely thing is that the sheets have rusted through. Or the screws have not been put in square so leaving a gap under the washer.

I have never had these leak in the past except through rusting. Are you certain it's the fixing points leaking and not rain blowing in or similar?

[Farmer Giles]

The sheets haven't rusted, and the screws are in square.

I have other sheds with the same roofs and they don't leak either. As I said previously, the slope of this roof isn't really adequate and that is the main reason for the problems. I have ordered some Sylglas tape, and I will also get the Wickes stuff that was recommended, and try that

- trying a belt and braces approach!

[Farmer Giles]

I have never heard of, or seen, them being fitted that way but I suppose it would work. Can't try it here though, because the screws are already fitted in the flat area.

[harryagain]

Just had a further thought about your roof. If the fixing screws are overtightened, the "thread" in the wood can strip and the screw neither fixes or makes a proper seal.

This can happen if a power screwdriver is used. The problem is you then have a hole with a stripped thread. Only solution (short of wallplug in hole) is a longer screw. Wallplug in hole soon works loose BTW.

Leaks appear later not right away often.

Box profile roof can be really shallow slope. So long as water is not blown back up under flashings or whatever. Check this out. There are fixes for this if it is happening. Leaks only appear in windy weather with this problem.

[Tim Lamb]

Box profile is normally secured in the valleys. (sealing washer under the screw head).

You should also use butyl tape under the lap ridge and *stitchers* every

450mm.

Agricultural roof slope could be 15deg.

[Farmer Giles]

Thanks again for your further suggestions.

I don't think that the water is being blown back underneath - although the problem is mainly along the first 2 feet or so from the edge. I think your first suggestion may be nearer the mark, as I seem to recall that getting a tight fix on some of the outer screws was quite difficult. I think I'll still try the mastic/Sylgas over the screws suggestion first.

[harryagain]

I don't think mastic will work, have tried it myself. You can use long round head screws the same diameter as the regular fixing screw. They will accept the washer/seals.

Or if the purlin is thin, nuts and bolts that fit the washer.

[Farmer Giles]

Nuts and bolts sounds like a good idea where there is difficulty getting a good fix, I think I'll give that a try. Many thanks.