Formwork girder H20
The slab is only as flat as the beams underneath it — and the beams get used twenty times.
A glued timber I-beam: softwood flanges, a plywood web toothed and glued into a groove in each flange, 200 mm deep, in lengths from 1.45 to 6 metres. It carries the deck while the concrete goes off, comes down, and does it again. Everything that matters about it is a consequence of that second sentence.
The beam
Two flanges, one web, and a glue line doing all the work.
An I-section 200 mm deep with 80 mm flanges. The flanges are solid or finger-jointed softwood, 40 mm thick. The web is 24 to 27 mm exterior-grade plywood, cut with a toothed edge that is driven 13 mm into a matching groove in each flange and glued there.
The toothing is the point. A flat plywood edge glued into a slot gives you one shear plane and a joint that opens under repeated loading. A toothed edge gives you a long interlocking interface, so the glue is loaded in shear across a much larger area and the joint is mechanically keyed as well as bonded. That is why an H20 survives twenty cycles of being loaded, struck, stacked and dropped, and a nailed-together beam does not.
The web is plywood rather than solid timber because the web carries shear, and plywood's cross-laminated plies carry it in every direction without splitting along a grain line. The flanges are solid softwood because the flanges carry bending, and for bending you want long straight fibres running the length of the beam — which is also why flange timber is selected for high annual-ring density and why the defect limits further down are stricter on the flange than anywhere else.
The web is drilled at each end. That hole takes the fastening for beam-to-girder connections and for the end protection, and it is the reason the beam sits in standard formwork hardware rather than needing its own.
The trade-off
Timber is the cheap one, the light one and the honest one.
Formwork beams are made in timber, aluminium and steel. Aluminium lasts longer and costs several times more. Steel lasts longest and is heavy enough to change how a deck gets built. Timber wins on three things and loses on one, and it is worth being clear about which is which.
- You can nail into it
- The formwork sheet fixes straight into the top flange. On an aluminium beam that fixing needs a clip, a system panel or a timber batten carried on top — all of which are cost and all of which are another thing to lose on site.
- One person carries it
- Deck beams get moved by hand, hundreds of times a pour. Weight per beam decides how fast a deck goes up and how many people it takes.
- It fails where you can see it
- A damaged timber beam shows splintered flanges, a crushed end, a delaminating web. An overloaded aluminium beam can be bent within its section and look fine. The failure mode of the cheaper material is the more visible one, which on a site with mixed-age stock is worth something.
- It is a consumable, and it is priced like one
- This is the losing side. Twenty cycles is the reference figure, not two hundred. On a long repetitive job with disciplined storage, aluminium may be the cheaper answer over the life of the contract. On short jobs, mixed jobs, jobs where the beams get rained on and jobs where stock walks, timber is the answer and the arithmetic is not close.
Load and section
The rated values, and what each one governs.
Permissible values are design figures for the standard beam. The destructive figure is what the beam does in test before it breaks — it is not a working value and it is not a safety factor you get to spend.
| Parameter | Unit | Value | What it governs |
|---|---|---|---|
| Permissible bending moment, M | kNm | 5.0 | The governing number for beam spacing on a deck. Slab thickness and pour rate set the load; this sets how far apart the beams can be. |
| Permissible transverse (shear) force, Q | kN | 11.0 | What each support point can take. Governs prop spacing and the reaction at a forkhead. |
| Modulus of elasticity, E | MPa | ≥ 6,000 | Deflection, not strength. This is the number that decides whether the finished soffit is flat. For webs of 21–34 mm. |
| Destructive bending moment, test span 3.6 m | kNm | ≥ 9.5 | Failure in test. Roughly 1.9× the permissible moment — the margin the design figure is taken from, not headroom to load into. |
| Permissible compression at the support | MPa | ≥ 2.0 | Crushing of the bottom flange over a prop head. Where a narrow support bears on a beam, this is what gives out first. |
| Finger-joint strength, static bending | MPa | ≥ 27 | The spliced flange joint. Governs whether a jointed flange behaves like a solid one. |
| Flange-to-web glue joint, shear | kN | ≥ 20 | The toothed joint. This is the joint the whole section depends on and the one that fails when a beam is soaked or overloaded. |
| Flange-to-web joint, tension across the fibres | MPa | ≥ 0.8 | The web pulling out of the flange groove. Low absolute number, correctly — timber is weak across the grain and the joint is designed around that. |
| Section depth · flange width · flange thickness | mm | 200 · 80 · 40 | The H20 profile. Compatibility with standard props, forkheads and clamps. |
| Web thickness | mm | 24–27 | Shear capacity and the width of the toothed joint. Exterior-grade plywood to EN 636-3. |
| Web tooth insertion depth | mm | 13 | Glue-line area at the joint. The single most important manufacturing dimension on the beam. |
| Timber moisture content | % | 12 ± 2 | Dimensional stability and glue-line integrity. A beam glued wet moves as it dries and opens its own joints. |
| Adhesive | — | One-component PVA, D4 | Durability class D4 to EN 204, heat resistance to EN 14257 (WATT’91). D4 is the exterior class — water-resistant, which is not the same as waterproof. |
| Service conditions | — | −20 to +35 °C | Ambient range, surface heating to 40 °C maximum, relative humidity to 70%. Outside that range the beam is outside its rated envelope. |
Lengths
Any length from 1.45 to 6 metres, made to the order.
Length is not a stock constraint here — it is a cutting instruction. Anything from 1,450 to 6,000 mm is produced to order, and the length is inked onto each beam along with the profile, the standard and the date of manufacture.
What that means commercially is that you should specify the lengths your deck actually wants rather than buying a standard length and cutting it. A cut beam has an unsealed end and no cap, and the end is where water gets into the flange and the glue line. Cutting on site is the fastest way to turn a twenty-cycle beam into a five-cycle one.
The date of manufacture on the beam matters more than it looks, because the warranty runs from it rather than from delivery. See what it is not.
- Length range
- 1,450 to 6,000 mm, to order.
- Section
- H20 — 200 mm deep, 80 mm flange width, 40 mm flange thickness, 24–27 mm web.
- Marking per beam
- Profile, standard, length and date of manufacture, applied in ink to the flange face. Customer marking on request.
- Marking per pack
- Every pack is labelled. Only one grade and one size per pack; where volumes are small, up to three lengths may share a pack with the ends aligned at one end.
Geometry
Tolerances, because a deck is an assembly of these.
A single beam being 3 mm out is nothing. Forty beams at 3 mm in the same direction is a soffit nobody signs off. These are the limits the beams are made to.
| Requirement | Limit |
|---|---|
| Length — up to 3 m | ±5 mm |
| Length — over 3 m | ±8 mm |
| Height (section depth) | ±2 mm or 1% |
| Flange width | ±1–1.5% |
| Flange thickness | ±3% |
| Web thickness | ±1.5 mm |
| Flatness (bow) — up to 3 m | ≤ 3 mm |
| Flatness (bow) — over 3 m | 3 mm + 1 mm per further metre |
| Twist | ≤ 0.2% |
| Perpendicularity of adjacent faces | ≤ 2 mm |
| Web offset from the flange axis of symmetry | ≤ 0.5 mm |
| Mass against the nominal on the working drawing | ±10% |
Timber quality
What is allowed in the wood, and what is not.
Softwood flanges, selected for high annual-ring density. Grading is by declared defect limits rather than by a blanket grade name, and the limits below are the ones the flanges are cut to. Where a project needs a declared strength class, we agree the EN 338 class on the order and have it confirmed against the production run.
| Defect | Limit, all flange faces |
|---|---|
| Sound and light knots, including cracks up to 0.5 mm wide, fused and partly fused | Allowed, up to 35 mm diameter |
| Edge sound knots | Allowed, up to 25 mm diameter |
| Black intergrown and partly intergrown knots | Allowed, up to 15 mm diameter |
| Fallen knots | Allowed, up to 8 mm diameter |
| Dead, stapled, lapping, rotted and tobacco knots | Not allowed |
| Surface microcracks up to 10 mm | Allowed, total length ≤ 300 mm |
| Edge microcracks | Allowed, ≤ 1/5 of the width or thickness |
| Through cracks | Not allowed |
| Pitch pockets | 1 per metre, ≤ 40 mm long and ≤ 2 mm wide |
| Bluestain | Not allowed |
| Insect holes | ≤ 1 per metre, ≤ 4 mm diameter |
| Wane, planing defects | Within the permissible size deviations |
| Flack, chip, bow, spring, twist, cup | Not allowed |
| Defective spike (finger) joint | Not allowed |
Protection
Yellow paint and a plastic cap, doing more than they look like they do.
The coating is a water-based, weather-resistant compound, applied without skips — a missed patch is a rejectable defect, not a cosmetic one, because the coating is the beam’s moisture barrier and a gap in it is where water starts.
- Standard coating
- Water-dispersion compound, yellow. Coating durability is warranted for at least the warranty period of storage and initial operation — which is a shorter promise than it sounds like, and is dealt with under what it is not.
- Optional treatments
- Bio-protective and fire-protective compositions, applied at manufacture and specified on the order. Decorative and protective treatment against moisture, UV and biological damage is available where the working drawings for a particular structure call for it.
- End caps
- Polymer plugs fitted to the beam ends by agreement. The end grain is the most absorbent surface on the beam and the flange ends take the impact damage in handling, so the cap is protecting the two things most likely to end a beam’s life early.
- Cap colour
- Produced in several colours. On a site running mixed stock, colour-coding by length or by owner is the cheapest inventory control available — it is read from thirty metres away and it survives the beam getting muddy.
Straight answer
What it is not, said before you find out.
This is a formwork beam. It is a good one, and it is a consumable, and most trouble with it comes from a buyer treating the second half of that sentence as marketing modesty.
- The warranty is six months, and it starts at manufacture
- Six months from the date of manufacture, covering storage and operation. A beam that sits in a yard for five months has five months of its warranty gone before it takes a load. Buy against a pour programme, not against a stock plan, and read the date inked on the flange when the delivery arrives.
- Twenty cycles is a floor, and it is conditional
- The figure is not less than twenty uses provided the storage, transport and operating instructions are followed. It is not a promise of twenty pours on a site where beams are stacked in the rain, cut to length, dropped off a deck or driven over. Sites that hit twenty are sites that look after them; sites that do not, do not.
- The stated average service life is one year
- Not less than one year, on the manufacturer’s own figure. We quote that rather than a longer number we cannot support, and it is the honest frame for the whole purchase: this is stock you consume against a job, not plant you capitalise over a decade.
- D4 glue is water-resistant, not waterproof
- The adhesive is the exterior durability class and it is the right one. It still lives in a beam made of timber at 12% moisture content with a paint film over it. Sustained wetting swells the flange, works the glue line and delaminates the web from the groove. Water is what actually ends most of these beams — not load.
- It is a formwork beam, not a structural member
- The manufacturer’s documentation permits use as floor beams, rafters, purlins and struts in low-rise construction. That permission comes from national documentation, not from the formwork-beam standard the product is marked to, and a permanent load-bearing application needs its own design justification, its own strength class and its own durability assessment. We will not supply it into a permanent structural role on the strength of that clause, and we will say so to your engineer directly if it helps.
- Cutting it to length on site voids most of what you paid for
- A cut end is unsealed, uncapped and unmarked. Specify the lengths you need — the range is 1.45 to 6 m in any increment, so there is rarely a reason to cut.
- The nominal mass is not published
- Only its tolerance is. We get the figure in writing per length before you plan lifts or loading. See the note under load and section.
Delivery
Packing, transport and the storage that decides the cycle count.
Beams are delivered assembled and finished. How they are packed and how you keep them is not housekeeping — it is the difference between the twenty cycles on the datasheet and the eight you actually get.
- Pack quantities
- 50, 60 or 100 beams per pack depending on the transport. One grade and one size per pack; up to three lengths on small volumes, ends aligned at one end.
- Pack dimensions and weight
- Not more than 1,150 mm wide by 1,150 mm high. Pack weight not more than 2 tonnes.
- Banding
- Metal or polypropylene strapping in at least two places, with plastic corners under the strap so the band does not bite into the flange.
- Wrapping
- Polyethylene film on five sides.
- Transport
- Any mode, in covered vehicles, protected from damage, contamination and moisture.
- Storage
- Closed ventilated space or under a shelter, out of rain and out of direct sun. Sorted by type and size, stacked on bearers so beams cannot sag between supports. Short-term outdoor storage is permitted only where the beams are protected from the weather.
- Cleaning
- No abrasive detergents. The coating is the moisture barrier; scouring it off shortens the beam.
How to specify
Eight lines, and we can price it.
Most enquiries arrive as “H20, how much”. That can be answered, but not accurately. These are the things that move the number.
- 1 · Lengths and quantity per length
- Anything from 1,450 to 6,000 mm. Give the split — a single length runs cheaper than five.
- 2 · End caps, and colour
- Fitted or not, and which colour. Say if you want colour-coding by length.
- 3 · Treatment
- Standard coating, or bio-protective or fire-protective composition. Fire treatment in particular has to be specified at manufacture.
- 4 · Marking
- Standard marking is profile, standard, length and date. Say if you need your own mark, a project reference or a numbering series.
- 5 · Declared strength class
- Say whether your engineer needs an EN 338 class declared for the flange timber, or whether the rated M and Q values are sufficient.
- 6 · Documentation
- Which certificates have to travel with the goods — see inspection.
- 7 · Delivery terms and destination
- Pack weight is capped at 2 tonnes and bundles at 1,150 mm square, which sets what fits in a container or on a trailer.
- 8 · When you pour
- The one nobody puts in the enquiry and the one that matters most, because the warranty runs from manufacture. We schedule production against the programme rather than shipping you six-month-old stock.
Inspection
What we check, and what travels with the goods.
A formwork beam is a factory-made engineered product, so the questions are the ones you would ask of any of them: does the lot match the type, is the geometry inside tolerance, is the glue line sound, and is what arrived what was tested.
- Type testing
- Bending and shear to EN 13377 — destructive bending moment at the 3.6 m test span, permissible moment and transverse force, modulus of elasticity, compression at the support, and the glue-joint values for the finger joint and the flange-to-web joint. Reports supplied with the first order and on any change of production specification.
- Material certification
- Plywood to EN 636-3 for the web, adhesive durability class D4 to EN 204 with heat resistance to EN 14257, and the flange timber grading — with the EN 338 class where one has been agreed on the order.
- Production control
- Moisture content at 12 ± 2% at manufacture, geometry against the deviation table, glue-line integrity and coating continuity. Skips in the finish are a rejection, not a touch-up.
- Pre-shipment inspection
- Where the buyer requires it, an agency appointed for the consignment — SGS, Bureau Veritas or Intertek — instructed jointly, verifying quantity, lengths, section dimensions, straightness, marking, cap fitting, packing integrity and pack weights against the documentation.
- Condition on arrival
- The one that matters most in practice. Film intact on five sides, straps and corners in place, no water staining, ends capped and undamaged, and the date of manufacture read off the beams and checked against the order. A pack that has been rained on in an open yard between the mill and the ship is compromised whether or not it looks it.
- Documentation per shipment
- Type test reports, material certificates for plywood and adhesive, declaration of conformity to EN 13377, packing list with lengths, quantities and pack weights, certificate of origin, bill of lading, and the inspection report where an agency is appointed.
Straight answer
What we will tell you before you ask.
- That the warranty clock starts at the mill
- Six months from the date of manufacture, not from delivery. Ordering against a programme rather than against a stock plan is worth more than any discount you will negotiate.
- That water ends more beams than load does
- The glue is D4 and the coating is weather-resistant. Neither makes a timber beam a wet-storage product. Under cover, on bearers, capped ends.
- That twenty cycles is conditional on you
- It is the manufacturer’s floor under their storage and operating instructions. It is not a warranty of twenty pours and nobody sensible sells it as one.
- That we will not sell it as a structural member
- The permission in the manufacturer’s documentation to use it as a floor beam or rafter in low-rise construction is a national-documentation clause, not a conclusion from the formwork-beam standard. A permanent load-bearing role needs its own design case.
- That one tolerance figure in the source is ambiguous
- The length row carries a ±10 mm figure alongside the ±5 and ±8 without saying what it governs. We contract to the ±5 and ±8 until it is clarified in writing.
- That the nominal mass is missing from the datasheet
- Only the ±10% tolerance is published. We get the figure confirmed rather than quoting a market average into your lift plan.
Enquiries
Every enquiry is answered by the desk that handles it.
Send the product, quantity, delivery basis, destination and timing. We revert with availability, an indication and the documentation that accompanies it. Specifications are released once we know who we are speaking with.