can't understand why my shed roof is leaking....

[sadexpunk]

hi

just built a large gazebo/shed structure, really happy with it but unfortunately the roof leaks. i just cant fathom it, nor can a builder friend of mine who's had a look at it.

its a relatively shallow pitched pent roof, dropping 1ft over 3.5m. the roof is comprised of 4x2 joists and noggins, 12mm wpb ply, then wickes 'super' shed felt, mineral i think. 4 layers going horizontally, overlapped the correct way and stuck down on the overlaps with felt adhesive and clout nailed on overlaps and at the side.

since it started leaking ive gone over every clout head with sealant, and also sealed up the overlap seams too.

the only thing ive been told it can be is either capillary action at the overlaps, but i refuse to believe water can break through that seal AND run uphill! OR......getting past the sealed clout heads.

could anyone give me any advice as to what you think could be the problem please? im tearing my hair out here!!

i can supply pics of any areas you may think relevant

thanks a lot

EDIT: the water seems to be running down joists then spreading across ply, which implies its getting in higher up, but cant be sure if thats at the front or the first overlap.

 

[r896neo]

How big are the laps? In my experience using adhesive or bitumen to stick laps down is good to stop them lifting but is not to be relied on as a seal.

Laps should really be a foot or more on such a shallow pitch which to be honest a single layer felt is perhaps not up to the job.

How is the front edge detailed? Are you sure there are no cracks where the felt was folded? Did you heat it with a torch before bending it?

 

[sadexpunk]

the laps are only 3 or 4 inches if that. i realise this isnt optimal but rolls are 1m wide, roof is around 3.5m so i just thought 4 rolls would be adequate with some overlap.

front edge is 12mm ply sticking over the timber roof beam by 2", then under that is a 2x1 batten to give the appearance of being thicker and so the felt doesnt have too sharp a fold. felt is tacked onto the underneath of the batten. i realise i should probably have gone under and then down too, but too late now. so, ive untacked the underneath of the felt and letting it hang down as a trial. if this works ill leave it hanging down, trim and cover with a batten. im pretty sure this isnt the answer tho, as i considered this before and covered the front edge with tarp and it still leaked. clout nails are tacking the felt on the front face and have sealant smeared over them. this stuff..... http://www.toolstation.com/shop/Adh...tik+Trade+Multi+Purpose+Silicone+280ml/p72459
there are no cracks, felt is all good, and no i didnt heat it.

i realise my overlap isnt optimal on a shallow pitch, but..... ive sealed the overhangs with some of this roof repair stuff. http://www.toolstation.com/shop/Roofing+&+Drainage/d250/Roof+Compound+&+Waterseals/sd2809/Evercryl+Roof+Repair+310ml/p71656

i just cant believe that water would get past this, although its not underneath the overlaps but smeared on top.

thanks a lot

 

[r896neo]

I'd be pretty confident your laps are your problem. Even on a normal shed roof I have seen water drive under a 4 inch nailed lap. I aim for 6-8", and that's on a normal 30deg type roof.

As I said those waterproof mastics and adhesives help glue felt down but on a shed where the structure is moving and felt vibrating in the wind they are not suitable to use as waterproofing in my opinion.

 

[Trazor]

When you fitted the last layer, how did you finish off over the peak,
you may need a drip rail to prevent water running back under the top of the roof.

Did you fully seal this shed, sometimes condensation can occur.

 

[sadexpunk]

thanks chaps. most of it is an open gazebo so no condensation. i didnt finish off with a drip rail which was a mistake, but..... to test that out i did lay an old roll of felt over the front dropping over as a dtip rail would. still leaked.

think you may be right with the overlaps then. probably nothing for it but to put another layer on top and do it better :-/

thanks

 

[r896neo]

I don't want to poke holes in the job as it looks neatly constructed but 4x2 joists on a span that big and that shallow a pitch and at what I'd guess is 500-600mm spacing is not ideal. A decent fall of snow may well bring that roof down.

If you have the head height you might want to consider sistering in some 6x2s at least.

 

[sadexpunk]

Yeah you're probably right, I built it to fag packet calcs with no previous experience, it wasn't til after it was up I started thinking ooooh that spans maybe a little on the large side
It's 6ft at the back so I'm hoping not to have to lower it any but if it looks like it'd be troublesome I'd have to.

Thanks

 

[sadexpunk]

we've come up with a plan to improve matters. not the easiest to describe without getting all confusing but ill need bitumen. however, im not sure what the best stuff for a good seal will be. i used brush on felt adhesive before, but theres mastics, bitumens, sealants, adhesives, seems so much choice...... whats the best stuff to go between the overlap layers for a good seal?

thanks

 

[r896neo]

Any kind if sealing job will not last. You'd be much better off just refelting but with proper sized laps.

 

[sadexpunk]

thats sort of the plan ok ill have to try and explain it.....

the plan is to use a stanley to just cut the thinnest strip possible out along the length of the overlaps where the clout nails are. this is because its all too well sealed to prise them out individually so i can just rip it up. id then be left with a couple of inches or so of bare ply where the overlaps were. buy 2 more rolls of felt, halve them again with a stanley (to get 4 x 0.5m strips of felt) and put these strips up inbetween the present rolls, in effect making a large overlap. and this would be well sealed too with bitumen top and bottom.

so...... whats the best product for that seal? (hope ive explained it well enough to understand)

 

[Mursal]

If its a localized leak consider a roll of torch on felt over the area, just be careful of the heat on other parts of the roof.
Or better still a sheet of corrugated cladding, keep it simple ...........

 

[sadexpunk]

its not localised mate, coming in all over the place :-/

 

[r896neo]

Not sure I totally get your plan. Messing about with adhesives etc will be more hassle than its worth.

If it was me I'd Tear it off and start again.

 

[sadexpunk]

haha yeah it sounds complicated doesnt it, but it really isnt. the basic principle is ill be inserting 0.5m wide strip of felt into the present overlap to 'make a bigger overlap'. ideally id just lift the old overlap and insert, but because ive er..... sealed it so well and covered the overlaps and all the clout nails with roof repair stuff, it may be easier to just make cuts either side of all that gunk and rip out that strip leaving a couple of inches bare ply and then insert the new 0.5m felt.

have i explained that any better? oh and would bitumen paint, roof repair or felt adhesive make for a better seal?

thanks a lot