TOOLS · SOLAR ROOF HANGER BOLTS
M12 solar hanger bolt
This page is about the bolt that carries a rooftop solar mounting system down to the structure: the sizes M8 to M12, the accessory that goes with each size, and the supply lengths. It is not a roof-system design. Hole sizes, embedment depth and tightening values are set per project, and they are given here as rules, not as tabulated numbers. For the surrounding fastener data, the bolt size chart covers hex bolts, the torque charts cover tightening values, and the nut and washer charts cover those parts.
Answers first
Nominal diameter d
12mm
source
Machine-thread pitch P
1.75mm
source
Minimum exposed machine thread
12mm
calculated
Flange nut s (across flats)
18
supply dimension
Flange nut k (height)
12
supply dimension
Flange nut dc (flange diameter)
26
supply dimension
Bonded washer bore floor
12
calculated · by drawing
Supply lengths
100, 150, 200, 250mm
supply programme
Nominal and supply dimensions for selection, not tolerances. The flange nut is a supply dimension. The washer figure is a bore floor: the actual bore comes from the drawing.
M12 solar hanger bolt: nominal diameter 12 mm, coarse machine-thread pitch 1.75 mm, flange nut 18 / 12 / 26 mm across flats / height / flange diameter, bonded washer bore at least 12 mm, and 4 supply lengths — 100, 150, 200, 250 mm.
Full PDF
Download the full PDF (M8–M12, all sizes)
Printable chart covering every size in this family — enter your email to get it.
Wood purlin or steel purlin: which end the bolt needs
A rooftop system is fastened to a purlin through the roof covering. Which end the bolt carries is decided by the purlin, not by the roof sheet. Timber takes a wood thread; steel takes a self-drilling point. The two are not interchangeable in service.
| Variant | Thread form at the embedded end | Pre-drilled hole | What the supplier needs from you | What happens if the two are mixed up |
|---|---|---|---|---|
| Wood thread (lag) end | A coarse wood thread cut into the end of the bolt; the bolt forms its own thread in the timber as it is driven. | Usually none, but the pilot hole diameter is a drawing value for the specific bolt: too small and the timber splits, too large and the withdrawal capacity is lost. It is not tabulated on this page. | Timber species and grade, purlin width and thickness, and the roof build-up thickness measured on the drawing. | A self-drilling point has no thread engagement in timber: it does not hold, and the fixing pulls out under wind uplift. |
| Self-drilling end | A drilling point with a thread behind it; the point drills its own hole in the steel and the thread forms in the same pass. | None. The point drills through the sheeting and the purlin, so the purlin thickness has to fall inside the capacity range stated for that point. | Purlin material and thickness, the number and thickness of the sheeting layers, and the roof build-up thickness. | A wood thread cannot be driven into a steel purlin: the end does not drill, and the thread strips without forming. |
The two accessories: flange nut and bonded washer
Every size in this series is supplied with a flange nut and a bonded washer. The nut dimensions are read from the flange-nut dimension source; the washer column gives the floor below which the washer will not pass the machine-thread section.
| Size | Flange nut s / k / dc (mm) | Bonded washer bore (min, mm) | Bonded washer outside diameter and thickness |
|---|---|---|---|
| M8 | 13 / 8 / 17.9 | ≥8 | By drawing — non-standard part |
| M10 | 15 / 10 / 21.8 | ≥10 | By drawing — non-standard part |
| M12 | 18 / 12 / 26 | ≥12 | By drawing — non-standard part |
The flange-nut values are supply dimensions for the parts this series is supplied with. No standard number is printed for them on this page: the column is a supply calibre, and a standard's own dimension table should be read from that standard's own document.
The bonded washer is a non-standard part. Its outside diameter and thickness are not tabulated here, because the seal has to match the sheet profile and the hole in it; they come from the drawing and the batch inspection report for the part actually supplied.
Read from the sources: nominal diameter, coarse pitch, and the three flange-nut dimensions. Calculated here: the bonded-washer bore floor (the bore must pass the machine-thread section, so the floor is the nominal diameter) and the exposure floor (the flange nut is fully engaged when the thread runs through its whole height, so the floor is the nut height k).
Supply lengths for M12
One row per supply length, on the same columns as the chart. The last column shows how much length is left for the roof build-up and the embedded section when the exposed machine thread is only as long as the minimum.
| Total length (mm) | Nominal dia d (mm) | Machine-thread pitch P (mm) | Flange nut s (mm) | Flange nut k (mm) | Flange nut dc (mm) | Bonded washer bore (min, mm) | Left for roof build-up + embedded section (mm) |
|---|---|---|---|---|---|---|---|
| 100 | 12 | 1.75 | 18 | 12 | 26 | ≥12 | 88 |
| 150 | 12 | 1.75 | 18 | 12 | 26 | ≥12 | 138 |
| 200 | 12 | 1.75 | 18 | 12 | 26 | ≥12 | 188 |
| 250 | 12 | 1.75 | 18 | 12 | 26 | ≥12 | 238 |
The total lengths are the lengths this series is supplied in. They are not a standard length series: the hanger bolt is a non-standard part made to drawing, so the section lengths are confirmed per order on the drawing. The last column assumes the exposed machine thread is only as long as the minimum, so it is what is left for the roof build-up and the embedded section together — not a permitted build-up thickness on its own.
The other sizes in the series
The neighbouring sizes on the same columns, so the step in diameter, coarse pitch and accessory can be read off directly.
Minimum thread engagement and embedment
How much thread has to be engaged at each end. Only the machine-thread end can be computed from data on this site; the timber and steel ends are set by the drawing, so the rule is given there instead of a number.
Machine-thread end, under the flange nut
A nut is fully engaged when the thread runs through its whole height, so the exposed machine-thread section has to be at least the flange-nut height k of the size: from 8 mm at M8 to 12 mm at M12, as tabulated at the top of this page. Take off more than the minimum only for a bracket plate or an adjustment range, not for engagement.
exposed machine thread ≥ flange-nut height k
Recomputed on this page from the flange-nut dimension source named above: a calculated floor, not a value taken from a standard table.
Timber purlin — wood-thread end
Engagement in timber is set by how many full threads the wood-thread section forms in the member. That number of threads multiplied by the wood-thread pitch is the embedment the bolt needs, and the pilot hole is sized from the root diameter by the same drawing. Both the pitch and the root diameter belong to the specific bolt and are not tabulated here.
embedment ≥ full threads required × wood-thread pitch
Engineering rule of thumb, not a standard value. The values for a specific part come from the supplier drawing and the batch inspection report.
Steel purlin — self-drilling end
The self-drilling point has to form a full thread in the steel. Because the same point drills the hole and taps it, the capacity is fixed by the point geometry: the member thickness has to fall inside the range stated for that point, and the thread engagement is then the full member thickness.
purlin thickness within the drilling capacity range of the point
Engineering rule of thumb, not a standard value. The capacity range of the point comes from the supplier drawing for that point.
Everything in this section is a selection rule, not a standard value and not a test result. Values for a project are set by the drawing and confirmed by the supplier before installation.
Tightening torque and re-checking on the roof
No tightening torque is tabulated on this page. A ceiling can be recomputed for a bolt whose property class is known, from the friction-factor range published on this site, but the mechanical property values behind the stainless classes are not reproduced here and the supply lengths on this page do not fix a class. Take the class from the drawing, compute from it, and confirm against the supplier's data for the part actually supplied.
T = K × F × d, with K from the friction-factor range on this site
Needs the property class, taken from the drawing or the batch inspection report
A torque value is deliberately not tabulated on this page.
Friction-factor range on the torque chart →Before ordering: what to measure and what to send
A hanger bolt is quoted by three groups of dimensions. Only one of them is fixed by the bolt itself; the other two come from the roof and from the structure.
- Roof build-up: from the top of the purlin to the outside face of the covering
- Exposed machine thread: the bonded washer, the bracket or L-foot, and the flange nut, plus any adjustment range
- Embedded section: the wood thread in the timber, or the self-drilling point through the steel
Send with the enquiry
- Size (M8, M10 or M12) and the required total length in millimetres
- Which end the purlin needs: wood thread for timber, self-drilling for steel, with the member material and thickness
- Roof build-up thickness, measured from the top of the purlin to the outside face of the covering
- Bracket or L-foot thickness, and whether the flange nut and bonded washer come with the bolt or separately
- Finish, part marking and documentation the job requires, and the property class where the drawing fixes one
The flange nut and the bonded washer are supplied as a set with the bolt. Where they are bought separately, take the bore from the table above and the nut dimensions from the same source; where the roof build-up is close to a length step, send the measured thickness and the purlin section rather than choosing between two lengths on the drawing alone.
What this page does not tabulate
Three groups of values are left out on purpose rather than filled in from a supplier catalogue. Each one is listed with its reason, so that a missing number is not mistaken for an oversight.
Wood-thread section: pitch, root diameter and section length
The part is not made to a standard, so there is no standard table for it and no supplier-independent value to quote. The value comes from the drawing for the part.
Bonded washer outside diameter
The part is not made to a standard, so there is no standard table for it and no supplier-independent value to quote. The value comes from the drawing for the part.
Bonded washer thickness
The part is not made to a standard, so there is no standard table for it and no supplier-independent value to quote. The value comes from the drawing for the part.
Pilot hole diameter
The part is not made to a standard, so there is no standard table for it and no supplier-independent value to quote. The value comes from the drawing for the part.
Minimum embedment depth as a number
The data behind it is published by installation-system suppliers and varies by project and product line; the generic derivation is given in the engagement section.
Tightening torque value
The value depends on the mechanical properties of the property class and varies with the class ordered; the method is given in the tightening section.
Going further with M12
Accessories, by standard
- Flange nuts — standard size page →
The flange nut that goes with this bolt, on its own product page: size range, material options and what to state on the order.
- Self-drilling and tapping screws — standard size page →
The self-drilling point family in the catalogue: where the capacity is stated, and what a steel purlin fixing has to match.
- Flat washers — standard size page →
Plain washers for the bracket side of the joint, with the size range and the tolerance the drawing should quote.
Other tool pages
- Bolt size chart →
Hex bolt dimensions in the same style: across flats, head height, thread length, by size and by standard.
- Bolt torque chart →
Tightening values tabulated by diameter and grade, with the friction-factor range used for the calculation.
- Hex nut weight chart →
Hex nuts by size with across-flats and height, and the weight per piece for order quantities.
- Flat washer weight chart →
Plain washers by size with outside diameter and thickness, and the weight per piece.
- Screw size chart →
Metric machine screws by size with head and drive dimensions, for the parts that go on the bracket side.
Where these numbers come from
The values on this page are read from the thread source and the flange-nut dimension source on this site and recalculated here; the supply lengths are this series' own supply programme.
What is covered
Sizes M8 to M12 — 3 sizes — across the 4 supply lengths 100, 150, 200, 250 mm: 12 rows, one per size and length, with 3 size pages of their own. Over this range the coarse pitch runs from 1.25 mm to 1.75 mm and the flange-nut height from 8 mm to 12 mm.
What is not covered
Sizes below M8 and above M12 are not part of this series, and no values are supplied for them here. The 6 values listed in the section above are left out rather than filled in from a supplier listing, and the installation-system vendors' technical data behind the embedment question are recorded by name only — no values are taken from them.
Calibre of the values: Nominal (basic) dimensions and supply dimensions, for selection and purchasing. They are not tolerances, not limits of size and not interchangeability values. The supply lengths are a supply programme, not a standard series.
Coverage: This is one of the 3 sizes in the series that have a page of their own; the full table across every size and supply length is on the size chart.
Sources for this size
- Yaxiio thread source — nominal diameter and coarse metric pitch
- Yaxiio flange-nut dimension source — across flats, height and flange diameter, as supply dimensions
- Non-standard parts — wood-thread geometry, bonded washer dimensions and supply lengths, by supplier drawing and batch inspection report
- Installation-system suppliers' technical data — selected per project; values follow the vendor document
- Stainless fastener mechanical properties — vary by property class, so no generic torque value is tabulated
Values on this page are taken from the sources listed and computed here as described; the sources' own typography and additional columns are not reproduced.
Send the size list, get the hanger bolts quoted
If you have the sizes, the purlin type and the roof build-up on a drawing or a picking list, send them across and we will come back with the parts that answer them: the size and total length, the accessory set, and the documents the job needs. We work as a manufacturing and export company with both our own production and an integrated supply network, so a mixed list is read as one specification and comes back as one quotation.
- Sizes and total lengths, or the roof build-up and purlin dimensions to work them out
- Which end the purlins need, with the member material and thickness
- Accessory set required: flange nut and bonded washer, and any bracket plate
- Finish, property class, part marking and the documents the job needs
Cite this chart
YAXIIO. M12 solar hanger bolt. https://www.yaxiio.com/en/tools/solar-hanger-bolt-chart/m12 (accessed 2026-10-29).
Free to cite and embed — attribution required: “YAXIIO” + a link to this page.
Frequently asked questions
What is the M12 solar hanger bolt?
A M12 hanger bolt has a nominal machine-thread diameter of 12 mm and a coarse pitch of 1.75 mm. It is supplied with a flange nut of 18 mm across flats, 12 mm high and 26 mm across the flange, and with a bonded washer whose bore is at least 12 mm.
Which lengths does M12 come in?
Total lengths of 100, 150, 200, 250 mm — 4 lengths. The last column of the table above shows how much is left for the roof build-up and the embedded section when the exposed machine thread is only as long as the minimum.
How much machine thread has to be exposed for the flange nut on M12?
At least 12 mm, which is the height of the flange nut: the nut is fully engaged when the thread runs through its whole height. Anything beyond that is for a bracket plate or an adjustment range, not for engagement.
Which bonded washer goes on M12?
One whose bore is at least 12 mm, so that it passes the machine-thread section. Outside diameter and thickness are not tabulated: they come from the drawing for the part, because the seal has to match the sheet profile and the hole in it.
Related References
Bolt size chart
Hex bolt dimensions in the same style: across flats, head height and thread length, by size and by standard.
Bolt torque chart
Tightening values tabulated by diameter and grade, with the friction-factor range behind them.
Hex nut weight chart
Hex nuts by size with across-flats and height, and the weight per piece for order quantities.
Flat washer weight chart
Plain washers by size with outside diameter and thickness, and the weight per piece.
Sizes on this page are engineering reference values for selection and purchasing. Fastener dimensions, hole sizes and installation values must be confirmed against the drawing, the batch inspection report and the governing standard before production or installation.