{"id":859,"date":"2026-08-07T17:45:58","date_gmt":"2026-08-07T15:45:58","guid":{"rendered":"https:\/\/fixwoodsystem.pl\/guides\/length-above-rafter-insulation\/"},"modified":"2026-10-01T14:41:27","modified_gmt":"2026-10-01T12:41:27","slug":"length-above-rafter-insulation","status":"publish","type":"page","link":"https:\/\/fixwoodsystem.pl\/en\/guides\/length-above-rafter-insulation\/","title":{"rendered":"Choosing screw length for above-rafter insulation"},"content":{"rendered":"<p class=\"lead\">This is where it is easiest to be wrong by one size - and one size down means too little anchorage in the rafter and a roof that does not carry the calculated load. The mistake does not show during installation. It shows with the first wet snow.<\/p>\n<nav class=\"fws-spis\" aria-labelledby=\"fws-spis-tytul\">\n<p class=\"fws-spis-tytul\" id=\"fws-spis-tytul\">Contents (13 sections)<\/p>\n<ol>\n<li><a href=\"#what-the-length-is-made-of\">What the length is made of<\/a><\/li>\n<li><a href=\"#why-this-is-not-simple-addition\">Why this is not simple addition<\/a><\/li>\n<li><a href=\"#how-much-the-angle-really-adds\">How much the angle really adds<\/a><\/li>\n<li><a href=\"#what-this-screw-actually-carries\">What this screw actually carries<\/a><\/li>\n<li><a href=\"#what-cannot-be-skipped\">What cannot be skipped<\/a><\/li>\n<li><a href=\"#why-a-long-ordinary-screw-will-not-do\">Why a long ordinary screw will not do<\/a><\/li>\n<li><a href=\"#crushing-the-insulation-where-the-limit-is\">Crushing the insulation - where the limit is<\/a><\/li>\n<li><a href=\"#four-mistakes-that-cost-a-roof-plane\">Four mistakes that cost a roof plane<\/a><\/li>\n<li><a href=\"#two-head-shapes-what-separates-them\">Two head shapes - what separates them<\/a><\/li>\n<li><a href=\"#when-to-ask-us-rather-than-calculate\">When to ask us rather than calculate<\/a><\/li>\n<li><a href=\"#the-products\">The products<\/a><\/li>\n<li><a href=\"#where-to-get-the-missing-dimensions\">Where to get the missing dimensions<\/a><\/li>\n<li><a href=\"#in-short\">In short<\/a><\/li>\n<\/ol>\n<\/nav>\n<p><!--fws-spis-wyglad--><\/p>\n<style class=\"fws-spis-wyglad\">\n.fws-spis{--s-paper:#FBFAF7;--s-bone:#F0EDE7;--s-ink:#14161A;--s-tekst:#4a4d52;\n  --s-cichy:#6E675F;--s-ramka:#e2ddd3;--s-linia:#ece7dd;--s-akcent:#E2541C;\n  margin:1.6rem 0 2.2rem;border:1px solid var(--s-ramka);border-radius:4px;\n  background:var(--s-paper);overflow:hidden}\n.fws-spis-tytul{margin:0;padding:12px 16px;background:var(--s-bone);\n  border-bottom:1px solid var(--s-ramka);\n  font:600 .72rem\/1.3 \"IBM Plex Mono\",ui-monospace,monospace;\n  letter-spacing:.12em;text-transform:uppercase;color:var(--s-cichy)}\n.fws-spis ol{margin:0;padding:0;list-style:none;counter-reset:fws-s}\n@media(min-width:760px){\n  .fws-spis ol{display:grid;grid-template-columns:1fr 1fr}\n  .fws-spis li:nth-child(odd){border-right:1px solid var(--s-linia)}\n}\n.fws-spis li{counter-increment:fws-s;margin:0;border-bottom:1px solid var(--s-linia)}\n.fws-spis a{display:block;padding:11px 16px 11px 40px;position:relative;\n  font-size:14px;line-height:1.45;color:var(--s-tekst);text-decoration:none;\n  transition:background .12s,color .12s}\n.fws-spis a::before{content:counter(fws-s,decimal-leading-zero);position:absolute;\n  left:16px;top:12px;font:400 11px\/1 \"IBM Plex Mono\",ui-monospace,monospace;\n  color:var(--s-cichy);font-variant-numeric:tabular-nums}\n.fws-spis a:hover{background:var(--s-bone);color:var(--s-ink);\n  box-shadow:inset 3px 0 0 var(--s-akcent)}\n.fws-spis a:focus-visible{outline:2px solid var(--s-akcent);outline-offset:-2px}\n.entry-content h2[id]{scroll-margin-top:96px}\n@media(prefers-reduced-motion:reduce){.fws-spis a{transition:none}}\n<\/style>\n<p><!--\/fws-spis-wyglad--><\/p>\n<h2 id=\"what-the-length-is-made-of\">What the length is made of<\/h2>\n<p>Screw length is the <strong>sum of the layers<\/strong>, not the insulation thickness:<\/p>\n<table>\n<tr>\n<th>Component<\/th>\n<th>Note<\/th>\n<\/tr>\n<tr>\n<td>Counter-batten thickness<\/td>\n<td>usually 40-60 mm<\/td>\n<\/tr>\n<tr>\n<td>Above-rafter insulation thickness<\/td>\n<td>mineral wool or PIR, the design value<\/td>\n<\/tr>\n<tr>\n<td>Full sheathing, where present<\/td>\n<td>frequently left out of the sum<\/td>\n<\/tr>\n<tr>\n<td>Required anchorage depth in the rafter<\/td>\n<td>from the approval, not estimated<\/td>\n<\/tr>\n<\/table>\n<p>The last item is the one most often missed - the first three you can see with a tape measure, the fourth has to be read from a document. It is not a fixed value: it depends on the screw diameter, on the species and density of the rafter timber, and on the force the joint has to carry. The same logic - length from the manufacturer&#8217;s table, not from the thickness of one layer - applies on a ventilated facade; it is set out in the <a href=\"https:\/\/fixwoodsystem.pl\/en\/guides\/facade-fixing-eisys-2\/\">guide to facade fixing with EiSYS-2<\/a>.<\/p>\n<h2 id=\"why-this-is-not-simple-addition\">Why this is not simple addition<\/h2>\n<p><strong>The driving angle.<\/strong> For Topduo the manufacturer shows two layouts: screws at 90\u00b0 only (wind suction) or screws at 65\u00b0 combined with screws at 90\u00b0 (sliding and suction). Which layout and how many screws follows from calculation. A screw driven at an angle travels a <em>longer path<\/em> than the thickness of the layers - the shallower the angle to the roof plane, the bigger the difference. A length calculated perpendicular will come out short.<\/p>\n<p><strong>Margin for tolerances.<\/strong> Insulation compresses a little under load, sheathing has thickness tolerances, and the counter-batten is not always what the drawing says. You choose to the nearest available size <strong>upward<\/strong>, not down.<\/p>\n<h2 id=\"how-much-the-angle-really-adds\" class=\"fws-znak fws-znak--wazne\">How much the angle really adds<\/h2>\n<p>A sense of the order of magnitude is more useful than remembering that &#8220;the angle changes something&#8221;.<\/p>\n<p>The screw&#8217;s path grows inversely with the sine of the angle between the screw and the plane of the layers. Set perpendicular, the path equals the thickness. The flatter the screw lies, the faster it grows: as the angle approaches the roof plane, the difference stops being a correction and becomes a separate item in the sum.<\/p>\n<p class=\"fws-ramka fws-ramka--stop\">The practical consequence: <strong>this cannot be worked out in your head on the roof<\/strong>. The angle is a design quantity like the spacing - and it follows from calculation, not from what is convenient to install.<\/p>\n<h2 id=\"what-this-screw-actually-carries\">What this screw actually carries<\/h2>\n<p>It is worth understanding the force system, because the angle and the spacing both follow from it.<\/p>\n<p>The counter-batten and covering rest on the insulation layer, and the load passes into the rafter through the screws. How much the insulation itself can take depends on its type: the manufacturer distinguishes pressure-resistant insulation (compressive stress \u03c310% of at least 50 kPa) from non-pressure-resistant, and the \u03c310% value is taken from the insulation manufacturer&#8217;s data sheet. Three kinds of load fall to the screws:<\/p>\n<ul>\n<li><strong>The weight of the layers and snow<\/strong> - acting vertically, but on a pitched plane it splits into a perpendicular and a <em>sliding<\/em> component.<\/li>\n<li><strong>The slide along the roof plane.<\/strong> In the 65\u00b0 plus 90\u00b0 layout it is taken by the inclined screws.<\/li>\n<li><strong>Wind uplift<\/strong> - acting the other way, trying to lift the covering. It is taken by the 90\u00b0 screws, working in withdrawal.<\/li>\n<\/ul>\n<p>The jobs are therefore split between the 65\u00b0 and 90\u00b0 screws. The inclined screw works mainly in <em>tension<\/em> rather than bending - and in tension it is considerably stronger. That is why the 65\u00b0 angle is not installer convenience but a way of bringing the sliding force into the screw&#8217;s axis. The quantity examples in the manufacturer&#8217;s data sheet assume exactly this 65\u00b0 driving angle.<\/p>\n<h2 id=\"what-cannot-be-skipped\">What cannot be skipped<\/h2>\n<p>Driving angle, layout, spacing and the number of fasteners follow from the <strong>roof load<\/strong>: snow for the zone, wind, the weight of the covering and the roof pitch. Approval <strong>ETA-11\/0024<\/strong> provides the calculation method, and the product data sheet gives example quantities of screws per m\u00b2 under precisely stated assumptions (including C24 rafters, 0.70 m rafter spacing and a 65\u00b0 driving angle). Both are downloadable from the product page without an account.<\/p>\n<p>We deliberately do not copy them here. A table copied without the snow zone and the covering type is the worst possible guide: it looks like a finished answer. When an approval is needed and when it is not is covered in the <a href=\"https:\/\/fixwoodsystem.pl\/en\/guides\/eta-when-needed\/\">guide to ETA<\/a>.<\/p>\n<h2 id=\"why-a-long-ordinary-screw-will-not-do\">Why a long ordinary screw will not do<\/h2>\n<p>An ordinary long partial-thread screw grips only the rafter and presses the counter-batten down with its head. Driving it therefore pulls the batten down and <strong>compresses the insulation<\/strong>. Topduo has two threads: one at the tip in the rafter and 60 mm of thread under the head in the counter-batten, so the counter-batten stays at its distance. On top of that comes a tip with reduced driving torque and reduced splitting effect that bites better.<\/p>\n<p>The consequence is not only mechanical but economic: the manufacturer states plainly in a technical article that <strong>compression of the insulation can be prevented by using double-threaded screws<\/strong>, and over-compressed mineral wool loses the thermal performance you paid for in the design - and nobody measures it once the roof plane is closed.<\/p>\n<h2 id=\"crushing-the-insulation-where-the-limit-is\" class=\"fws-znak fws-znak--wazne\">Crushing the insulation - where the limit is<\/h2>\n<p>This is the most underrated mistake at this stage, because it gives no signal at all.<\/p>\n<p>The screw works in an assembly where a <em>soft layer sits between two stiff ones<\/em>. With Topduo one thread grips the rafter and the other, under the head, grips the counter-batten - so the gap is held by the double thread, not by the insulation. The limit for the insulation itself is its \u03c310% value from the insulation manufacturer&#8217;s data sheet, not feel. Where that safeguard is missing or the screw is overdriven, the consequences are:<\/p>\n<ul>\n<li><strong>Over-compressed mineral wool<\/strong> loses thickness and with it thermal resistance - and does not get either back.<\/li>\n<li><strong>Uneven clamping<\/strong> across the roof gives a wave in the counter-battening that only shows under the covering.<\/li>\n<\/ul>\n<p>So you set the driver to a torque rather than tightening &#8220;until it stops&#8221;. The same rule as any clamping connection - with the difference that here the component being crushed is the most expensive one.<\/p>\n<h2 id=\"four-mistakes-that-cost-a-roof-plane\">Four mistakes that cost a roof plane<\/h2>\n<ol>\n<li><strong>Calculating length from the insulation thickness.<\/strong> Insulation is one of four items, not the sum. The most common error, and always toward &#8220;too short&#8221;.<\/li>\n<li><strong>Leaving out the sheathing.<\/strong> It drops out of the sum because the drawing describes it separately. Twenty-five millimetres is often exactly the gap between two available sizes.<\/li>\n<li><strong>Rounding down.<\/strong> &#8220;Almost fits&#8221; means reduced anchorage in the rafter - the single value the whole joint capacity rests on.<\/li>\n<li><strong>Forcing the tightening.<\/strong> Once the head seats, further rotation improves nothing. With a screw that has no thread under the head it pulls the counter-batten down and crushes the insulation - the softest layer in the assembly.<\/li>\n<\/ol>\n<p>The first three can be caught before ordering by writing the layers down. The fourth is caught only by a torque setting.<\/p>\n<h2 id=\"two-head-shapes-what-separates-them\">Two head shapes - what separates them<\/h2>\n<ul>\n<li><strong>Cylinder head<\/strong> (\u00d8 10 mm) - it draws into the counter-batten and disappears in it. Chosen when the screw should vanish into the timber and not clash with the covering.<\/li>\n<li><strong>Washer head<\/strong> (\u00d8 16 mm) - the larger head diameter gives higher pull-through resistance in the counter-batten.<\/li>\n<\/ul>\n<p class=\"fws-ramka fws-ramka--uwaga\">Approval <strong>ETA-11\/0024<\/strong> covers both Topduo head shapes - the manufacturer&#8217;s data sheet lists the whole family, cylinder head and washer head. If your joint is being handed over, download the approval document from the product page.<\/p>\n<h2 id=\"when-to-ask-us-rather-than-calculate\">When to ask us rather than calculate<\/h2>\n<p>The layer sum is simple, but there are cases worth confirming - because an error does not end at one screw, it ends at a pallet.<\/p>\n<ul>\n<li><strong>Two-layer or mixed insulation<\/strong> - mineral wool under PIR behaves differently under clamping than a single uniform layer.<\/li>\n<li><strong>Renovation<\/strong>, where the rafter section is unknown or varies and anchorage has to start from what is there, not from a drawing.<\/li>\n<li><strong>An unusual roof pitch<\/strong> - the pitch changes the loads, and with them the layout and number of screws; the data sheet gives quantities separately for pitch ranges. The screw path, and so the length, is changed by the driving angle to the layers, not by the roof pitch.<\/li>\n<li><strong>An unusual covering<\/strong>, heavier than assumed - clay tiles instead of sheet metal - changes the load, and with it spacing and fastener count.<\/li>\n<\/ul>\n<h2 id=\"the-products\">The products<\/h2>\n<p><a href=\"https:\/\/fixwoodsystem.pl\/en\/product\/topduo-cylinder-head-above-rafter-insulation\/\">Topduo cylinder head<\/a> - for any above-rafter insulation system, <strong>\u00d8 8 mm<\/strong>, lengths <strong>165-435 mm<\/strong>, TX 40 drive, approval <strong>ETA-11\/0024<\/strong>, steel with special coating. 11 variants in our range.<\/p>\n<p><a href=\"https:\/\/fixwoodsystem.pl\/en\/product\/topduo-washer-head-over-rafter-insulation-screw-8-0-mm\/\">Topduo washer head<\/a> - <strong>\u00d8 8.0 mm<\/strong>, lengths <strong>165-472 mm<\/strong>, 12 variants, the same special coating. The manufacturer specifies both versions for service classes 1 and 2 under EN 1995 (Eurocode 5).<\/p>\n<p>Give us the thickness of the counter-batten, insulation and sheathing - <a href=\"https:\/\/fixwoodsystem.pl\/en\/contact\/\">we will work the length out together<\/a> before you order a pallet in the wrong size. This is one of those items where a mistake costs not one screw but the whole delivery.<\/p>\n<h2 id=\"where-to-get-the-missing-dimensions\">Where to get the missing dimensions<\/h2>\n<p>Three of the four items are in the drawings, but the sheathing is sometimes missing from them because it was added on site. Then the only reliable route is measuring in place - at several points across the roof plane, not one. Check the insulation thickness after laying, not on the packaging: mineral wool from a roll carries a nominal figure and often sits a few millimetres thinner under the counter-batten. That is exactly the difference which, rounded down, produces a screw that is too short.<\/p>\n<h2 id=\"in-short\">In short<\/h2>\n<p>The length is the <strong>sum of four layers<\/strong>, the fourth of which - anchorage in the rafter - must be read from the approval, not estimated. The driving angle lengthens the path and is a design quantity, not an installer&#8217;s choice. Choose <strong>upward<\/strong>, never down. And set the torque: over-compressed insulation loses its thermal performance for good, and nobody notices once the roof plane is closed.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Length is the sum of counter batten, insulation, boarding and embedment - measured along the screw axis, not perpendicular.<\/p>\n","protected":false},"author":0,"featured_media":1939,"parent":854,"menu_order":50,"comment_status":"closed","ping_status":"closed","template":"template-guide.php","meta":{"_acf_changed":false,"site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"class_list":["post-859","page","type-page","status-publish","has-post-thumbnail","hentry"],"acf":[],"_links":{"self":[{"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/pages\/859","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/types\/page"}],"replies":[{"embeddable":true,"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/comments?post=859"}],"version-history":[{"count":5,"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/pages\/859\/revisions"}],"predecessor-version":[{"id":2320,"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/pages\/859\/revisions\/2320"}],"up":[{"embeddable":true,"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/pages\/854"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/media\/1939"}],"wp:attachment":[{"href":"https:\/\/fixwoodsystem.pl\/en\/wp-json\/wp\/v2\/media?parent=859"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}