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The content contained in this Good Roofing Guide article is for information purposed only. While every effort is made to ensure this article is accurate at the time of posting, JJ Roofing Supplies would recommend that you always seek to use a professional roofer for completing all roofing related jobs.

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How to Install Spirtech 400 2S - Breathable Felt

Spirtech 400 2S - Breathable Felt

So here we have Spirtech 400 2S. It’s a highly durable vapour-permeable underlay. It complies with British Standard BS5534. It’s 1.5 x 50m, highly durable, so you can drop tiles, tools onto this – it’s not going to tear and allow things to fall into the roof. It also complies with British Standard BS5534. Under that standard, all underlays must pass a performance test. There is a zonal fix, so all underlays have to be graded for use in different areas of the country. This roll can be used anywhere within the British Isles. They then need to be graded by what gauge, what batten gauge they can be used at. This roll can be used at most batten gauges up to 345, which is the size of most metric tiles. So with Redland peace of mind on this, you can use this roll anywhere at pretty much any gauge. Also, our sealed laps, to prevent us having to use a batten to seal our horizontal laps, we have an integrated sticky strip which seals the two laps together and forms a tight seal on the roof.

Laying the felt in place

So here we’ve pre-installed, pre-cut a piece of Spirtech 400 2S to fit our training rig, our training roof. We’ve got the integrated strip at the top, ready to seal at a 150mm lap, to suit a UK roof, and we’ve got it flush with the tray at the bottom. Never overhang the underlay into the gutter, as we used to in the UK, because what that does is draw liquid up into the underlay, and rot material. So always keep it flush with the edge of your underlay support tray, and similarly, flush with the end of your roof. So what we’re going to do now is fix our piece of underlay to our rafters, maintaining a nominal 10mm drape between each rafter. The reason for this is to allow liquid to flow beneath the rafters and down towards the bottom of the roof. If you pull the underlay tight, you in effect seal the underlay against the back of the batten, where it can rot the timbers and you can get moisture seeping into nail holes and rotting the roofing material itself. Never more than 10mm, because then the underlay can move too far and cause the roof to fail in a different way. So now we’ll fix the underlay to the end of the roof, to get our first piece in place.

Sealing the lap

So we’ve installed our first piece across the roof. We’re going to install the underlay up the roof as required, until we reach the top. Then we cut a piece 30mm short from the top of the roof, from the apex, measured down so that sticky strips, the integrated glue strips meet each other at a 150mm lap. We then pinch out the corners and begin to stick our roof. Don’t pull the tape out all in one go, because you’ll end up with a mess on the roof. Pull it slowly and smooth down the underlay as you go across the roof. Now, we’re going over open rafters here, so you might find at some point that the underlay kinks. That doesn’t matter. What we’re looking for here isn’t an airtight fix, it’s a strong fix – and that’s what that double-sided sealed lap is giving us.

So continue to pull out the tape across the roof, smoothing it out as you go. We’re not worried about going into these open rafters – it’ll seal nice and tight, give you a strong fix, to stop those horizontal laps bursting with the wind load. It’s a nice – you can see already there, it’s almost like it’s one piece. You’d have to destroy that underlay to get it apart. It stops that lap from bursting and affecting the integrity of the roof. If we’re using a vented ridge system with this, we would always cut the underlay short of the apex of the roof. Never throw it over and then run a Stanley knife across the top. Always cut it short, because you want the air to flow into the roof. Now, on our training roof here, we’ve got a piece of timber running across the top, but if you imagine this is a real roof, between the rafters here, it would be open, so we’d have space for the air to ventilate at a high level into the roof space, and we don’t want to cover that up with our underlay. So we’re cutting it 30 mm short from the apex, just to allow that air flow into the roof.

Double-sided Sticky Strip

So one of the big bugbears for roofers in the UK is getting that nominal 10mm drape. Well, the good news with Spirtech 400 2S is if you get it right with the first piece, because the pieces then stick together with the double-sided sealed strip, every piece that follows will then follow that nominal 10mm drape, making the job easier for the roofer.

Zonal Fix

The reason we now have a zonal fix on UK underlays is to prevent the underlay from ballooning under wind pressure. This is where the wind catches the underlay, pulls it up and catches the back of the tile. This over time can cause the tile to fail, so a UK underlay, once fully battened up, is only allowed to move 35mm – that’s your nominal 10 mm drape and the 25 mm space at the batten before that underlay fails and it strikes the back of the tile. Over time, that will work the tile loose and contribute to the roof failure.

Zonal Fix Spirtech 400 2SZonal Fix Spirtech 400 2S

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