Mounting on Scottish Slate Roofs

[The Solar King]

To do this well can be a little problematic or expensive to say the least.

The construction of the vast majority of Scottish Roofs includes sarking or close boarding. A layer of planking (normally around 22mm thick) or other material is nailed over the top of the roof trusses making a solid surface. This has a number of advantages in terms of roof strength and weather tightness. The roof is felted as normal. Where tiles are used, ordinary battens are nailed to this and tiles attached in the normal way.

Where slate is used, this is nailed directly to the sarking without the use of battens. This is where the fun starts. You can use a 'J' shaped bracket with lead soakers and flashing. You can at a pinch use a similar bracket with a kick the thickness of a slate in the securing arm plus lead flashing. Brackets must be secured through the sarking in to the truss below. Several tiles need to be removed to install each bracket, then cut and shaped when replaced. This whole process is time consuming and expensive. The free length of the arm from point of fixing through sarking to attachment of rail is quite long. This has implications under MCS012 as the failure force will be quite low. I have yet to see a test result for such brackets.

In general there is at least an extra half days work and £200.00 of lead to do the job properly.

This brings me on to the sometimes controversial subject of drilling through slates to enable fixing.

The last draft of the 3rd edition DTi Guide I have states:

"Tiles or slates removed for fixing a mounting bracket should be re‐attached to include a means of mechanical fixing.

Mounting systems on slate or tile roofs that rely on a fixing bolt through the tiles/slates are not recommended. This is due to the potential for damage to the slates/tiles and the difficulty for ensuring a lasting weather tight seal. Bolt through fixings systems that incorporate a rubber sealing washer (originally designed for metal roofs) are sometimes suggested as a way of ensuring a lasting weathertight seal on slate or clay tile roofs. However, due to the potential for cracking slates and subsequent worries over long term durability, this method is not considered appropriate in most cases. Only installations on large stone slate roofs, where the slates are very large, thick and robust are considered suitable for this method."

I would agree with this statement if the fixing bolt is bearing on the tile, or if high downward pressure is being exerted by the nut that holds the sealing washer in place.

I would welcome the views and opinions of others on the following proposed method. I am comfortable with this . Hanger bolts are 8mm or 10mm thread depending on the size of fixing. This is for buildings with a traditional 6"x 2" roof truss where there should be no implications of thread size in relation to timber thickness as per Eurocode 5 Design of Timber Structures.

Drill through the two layers of slate at a diameter that provides clearance for the fixing. If a slate is going to break it will do so when drilled and you can deal with it then.

Drill pilot hole if required through sarking and in to truss.

Slide a piece of DPC up underneath the lower of the two drilled slates so it covers from the bottom edge to the nail(s). This can be as wide as you like, but probably no more than a tile. This is just belt and braces.

Start bolt in hole. Fill area around bolt with CT1 Sealant & Construction Adhesive. Screw bolt to required depth. Fill with sealant flush to surface of tile.

Coat bottom surface of sealing washer with CT1 sealant. Hold in place against slate with metal washer and nut, but do not over tighten.

Cut bolts to length when fixing mounting rail.

The important part of this is the CT1 sealant. This is a far superior (and more expensive) than silicon. It is very strong but also flexible. Have a look here:
CT1 - CT1Ltd" It may sound like a wonder product, but having used it, it does work.

We always want to do a good job for the customer and in no way compromise their roof. My view is that the amount of stripping, cutting and fixing involved in using brackets and several kilos of lead could be more detremental.

We have looked at several solutions including in-roof fixing. All have their limitations and drawbacks.

I look forward to hearing what others think.

 

[Gavin A]

if possible drill a 6 or 8mm pilot hole through from below, use an SDS drill with a particularly high kick / hammer function, and you can actually punch straight through the slate in the same way as slaters have been doing for centuries with their picks.

then widen this hole from the outside through the slate with a 12mm drill bit to avoid the bolt actually knocking against the slates.

then drive the hanger bolt in from the outside, taking care to move all the nuts to the top of the bolt first, then hand tighten the lower nut and compression gasket onto the slate so that it's firmly against the slate, but still has some flexibility to allow for movement without cracking the slate.

If any bolts are close to the edge of a slate, then flash over that edge, from below the slate above.

RE CT1, we do use a long life flexible mastic squirted down the hole and on the threads of the bolt just to give a semblance of additional water proofing around the actual bolt entry through the felt, this only becomes an issue if the roof is already leaking further up anyway. If you're going to use CT1 then I don't see a problem with this, but you'd definitely need to check that it wasn't going to degrade the gasket.

The reason hangar bolt manufacturers don't always certify them for slate roofs is because it's easy to use them in such a way that they will cause the roof to leak and slates to crack, if in the hands of barely trained monkeys. In the hands of a professional their should be no problem at all, and we warrant our work for 10 years on slate roofs.

We have installed well over 1000 hanger bolts on a range of slate, yorkshire stone and other roofs, including scottish sarking boarded roofs, many of which have been in for 2-3 winters with zero leak reports or reports of broken slates.

As a demonstration of the resiliances of this method, we'll regularly stand on the rails attached to the bolts, and it's incredibly rare that we break a slate while doing this - probably less than 2% of the breakage rate we get with brackets and rosemary tiles during installation.

The only reason that these gaskets on hanger bolts will degrade too fast is UV light exposure, and if they're exposed to UV light when installed under a solar PV array, then frankly the degradation of the gasket is the least of your problems because the rest of the array has blown off the roof.

ps, if you use this method and cock it up do not blame me, it's down to installer skill and expertise to do it correctly.

pps a major benefit of this method is that you know the bolt is securely in the middle of the rafter. I don't know how you'd intend to find the rafter from above when mounting onto sarking board even if you removed the slate.