Sealing the gable end of my roof?

[michael]

I am currently in the process of having my roof re-slated. Where the slates overlap the gable end there is a small gap between the underside of the slates and the top of the facia board. This is caused to some extent by the uneven top surface of an original timber moulding detail which is fixed to the top end of the facia board and has been weathered away over the years. I wish to keep the moulding as this is generally quite sound. I do however need to plug the gap. I was thinking of using the old lime torching method. Would you agree?
If so, what type of lime should I be using (hydrated or hydraulic) what type of sand and what proportion of mix. If I push this lime mix into the gap, should I first stuff it with newspaper to give the lime something to bond to and prevent it from pushing straight through into the roof space? I live on the south coast and this gable end can be subject to strong winds and driving rain so the seal has to be pretty sound. The roofer (a modern tradesman) had proposed using a modern waterproof roofing sealant but I think if I can get a lime seal in there it will probably last a lot longer.

 

[chuckey]

Because the wood will expand/contract with the humidity, the lime will not adhere to it? Clear silicone will adhear to clean wood and also to tiles//slates it is also flexible. The downside is I am not sure that it takes paint very sucessfully. It will have to be applied either in two goes, or have some sort of backing because it too will disappear into your loft.
One thing that could help is if we knew roughly what size gap you are dealing with.
Frank

 

[michael]

Thanks Chuckey. The gap is about ½ cm to1cm deep (top of timber to underside of slate) I guess I could try finding a thin length of timber to plug in there before using the sealant. How long dose the sealant last? If it’s anything like the sealant you put around baths then probably not very long. I noticed when the old slates were being removed that the gaps underneath the slates between the slates and the roofing battens had been sealed with lime, torching I think it’s called. Would this not have suffered the same timber/humidity problems that you advise of? It all seemed pretty stable and still adhering to the battens when the old slates were removed.

 

[Nemesis]

No, I haven't found silicone lasts - a builder we had smeared the stuff around a window before we could stop him. Thankfully the stuff has dried hard and cracked after a few years and is about to be removed as far as we are able. It will be replaced with traditional mastic.

Why not give the SPAB a ring and see what is suggested? We don't have timber next to slates, the stone butts up and there a line of lime mortar would be suitable. I'd certainly try plugging as much of the gap with a timber fillet though if you can.

 

[middi]

Untill cement came on the scene any gaps at the gable end between battens, tiles, end wall and/or barge boards were traditionally torched with lime mortar. As this inevitably cracked and deteriorated with time, any gaps that appeared were traditionally filled with lime putty.

On the arrival of cement and subsequent repairs, and following the general trend of that time, a good dose of cement was used. This made for longer lasting mix but never cured the gaps and cracks which inevitably appeared, water and moisture treacled in the gaps which in turn speeded up the decay process of any battens, boards or joists which were trapped in the vicinity of this mix.

If I were you, for the moment, I would simply fill all the gaps with decorators caulk and paint over it. It is not an ideal solution but will smarten the place up, is reasonably long lasting and just reapply over the top again when re-decorating.

 

[michael]

Nemesis can you tell me a bit more about traditional mastic? I dont know the difference between this and silicne sealant

 

[Gervase]

Silicone sealant is the modern builder's panacea - it sticks to almost anything, makes an airtight and waterproof seal and lasts for about 20 years.
It comes in tubes, smells of acetic acid, and is applied by squeezing it out of the nozzle like very sticky toothpaste. It cures on exposure to air and, once cured, is a beggar to remove.
It's fine for sealing around UPVC windows, baths, shower trays and laminate worktops. It's not a good idea to use it on wood, brick, stone, roofing slate or lime-based plasters/renders/mortars, as moisture can be trrapped underneath or behind it, encouraging decay (but most modern builders use it for all of those and more!). It can also look unsightly unless applied with care.

Traditional mastic is made from fine sand or chalk dust and linseed oil, sometimes with a small amount of resin to speed its setting. It's flexible and will allow water vapour to pass through, thus discouraging decay.

 

[Nemesis]

It might not work well in a large gap:

http://www.womersleys.co.uk/acatalog/Building_Products_4_4.html

However, if the gap is first covered with a wood fillet it is worth trying.

 

[chuckey]

Nemisis, your builder did not use silicone, there are cheaper oil based mastics which do dry out and crack. The formula for traditional mastic is exactly the same as putty, i.e. filler dust and linseed oil ( must be raw), I suspect it just takes longer to dry out.
Torching is a very crude way of stopping rain blowing in. It all ends up on your glass fibre insulation. I read an article in a local mag. by an old girl who can remember getting moss in the summer to stuff under her dads roof slates to keep the rain out. Its traditional but I can't recommend it.
With a gap up to 1/2" I would rip a length of bead down to reduce the gap to less then 1/8" and use silicone. Of course with an extra layer of wood , a thin slip of flexible plastic (DPC or even 2" wide PVC tapecould be stuck from the tile down to the wood, the bead nailed on and the excess plastic cut of. The trouble is getting a piece of modern wood that will last!
Frank

 

[Nemesis]

There is silicone smeared around the front door... it's horrible. Not recommended.

And of course not all have glass fibre insulation...

Have you always been a modern bodge merchant or do you at times consider conservative repair using traditional materials?

 

[michael]

Traditional mastic sounds great. A few questions though:

(a) Is it readily avaliable from the average decorators shop. If not where can I get it?
(b)Is it expensive?
(c) Is it easy to apply i.e. to form a good smooth finish
(d) What colour is the finish. Does it need to be painted?
(e) What is the maximum hole size that can be filled with traditional mastic. Would my 1/2cm to 1cm gap between the slates and facia board require packing with timber first?
(d) How long dose it take to set? This is an external job with heavy rain very likely at the moment.

Just one last quick question on a different task: I have to repair a few non structual fine cracks on both gypsum and lime plastered internal surfaces. Somebody suggested I should use a product called "Red Devil" Do you know if this is any good?

Many thanks

 

[Nemesis]

See link above and call Womersley's ...

Never heard of red Devil... try a bit of lime or even good ol' Polyfilla...

 

[charlie-ia]

wasnt sure what torching was so i looked it up

Torching
filling (from the underside) the uneven spaces between tiles or slates or the gaps between the battens on a roof with a lime mortar to eliminate drafts or the entry of wind blown rain or snow. The practice was largely discontinued because it caused fairly rapid failure of the battens.

what do they do on modern roofs?

 

[Nemesis]

They use underfelt ... or breathable membrane.

In Scotland they used to sark with boards.

And torching certainly didn't cause rapid deterioration of battens (or laths) as the lime could dry out! It was used for many a century!