The hole size for a drilled-in rebar dowel is set by the adhesive system, not by a rule of thumb. Every anchoring adhesive comes with an installation table that lists the drill diameter for each bar size, and the product's published capacity only applies when you drill that size, clean the hole the way the table says and let it cure fully. For orientation, the RSIC Manual of Standard Practice gives about 15 mm for a 10M bar and about 20 mm for a 15M bar as the approximate hole needed to pass a bar through formwork or steel, which shows how much bigger than the nominal bar the hole has to be.
This post covers how hole size relates to bar size, why the gap matters, how to choose between epoxy and faster-curing adhesives, and what to expect from drilling, cleaning and curing. Dowel length and embedment are a separate question, covered in how long rebar dowels should be.
Bar size vs hole size
A Canadian metric bar is named by its nominal diameter, but the ribs (deformations) rolled onto the bar stick out beyond that diameter. A hole drilled to the nominal size will not take the bar.
The RSIC Manual of Standard Practice (2020) includes a chart of approximate hole diameters for passing bars through formwork or structural steel. It is a useful reference for how big a bar really is:
| Bar size | Nominal diameter | Approximate hole to pass the bar (RSIC) |
|---|---|---|
| 10M | 11.3 mm | 15 mm |
| 15M | 16.0 mm | 20 mm |
| 20M | 19.5 mm | 25 mm |
| 25M | 25.2 mm | 30 mm |
| 30M | 29.9 mm | 40 mm |
| 35M | 35.7 mm | 45 mm |
Two cautions before anyone picks up a drill:
- This chart is not adhesive data. It describes clearance holes in forms and steel. Adhesive anchoring systems publish their own drill diameters for each bar, which can differ from it. On a structural dowel, the adhesive maker's number is the one to drill.
- Watch imperial bits. Many drill bits sold in Ontario are imperial. A 3/4 in. bit is 19.05 mm, not 20 mm, and a 5/8 in. bit is 15.9 mm. Some adhesive tables list both metric and imperial bits for a given bar; use exactly what the table lists rather than the nearest bit in the truck.
For a refresher on how bar sizes are named and measured, see our Canadian rebar sizes chart.
Why the hole diameter matters with adhesive
An adhesive dowel works by bonding the bar to the adhesive and the adhesive to the wall of the hole. The thickness of that adhesive layer, the annular gap, is part of what the manufacturer tested.
- Too tight. The bar will not seat to full depth, or it pushes most of the adhesive out as it goes in, leaving voids along the embedment.
- Too loose. The adhesive layer is thicker than the one tested, the hole takes more product, and the published bond values may no longer apply. Some systems allow a range of hole sizes for a bar; many do not.
- Wrong depth. A hole drilled shallower than the specified embedment cannot be fixed by adding adhesive. A hole drilled much deeper wastes product and can trap air at the bottom unless it is filled from the bottom up.
That is why "drill one size up from the bar" is not good enough on structural work. Drill the size and depth on the drawings or in the adhesive table, and if the two disagree, ask the engineer.
Choosing a rebar anchoring adhesive
People ask which rebar anchoring chemical is best. The honest answer is the one your engineer specified, because the dowel design is built on that product's data. When there is a choice, these are the things that separate products:
| Factor | What to check |
|---|---|
| Rebar design data | Does the product publish values for post-installed reinforcing bar, not just threaded rod? |
| Evaluation | Is there a current third-party evaluation report covering rebar use, cracked concrete and the loads involved? |
| Hole condition | Is it approved for dry, damp or water-filled holes, and for hammer-drilled or diamond-cored holes? |
| Temperature range | What is the minimum concrete temperature for installation, and how long is the cure at that temperature? |
| Working time | How long do you have to insert the bar after dispensing, especially on long or overhead dowels? |
| Sustained load | Is it rated for long-term tension, which matters for dowels that are always loaded? |
In broad terms:
- Epoxy adhesives cure slowly, give long working times and are common for deep, large-diameter dowels where the crew needs time to set each bar.
- Hybrid and vinyl ester adhesives cure faster and are often chosen for cold weather or when the next pour follows quickly.
- Light-duty products sold for anchoring small fixtures are usually not the products engineers specify for structural dowels. If the cartridge does not come with rebar design data, it should not be in a structural hole.
Post-installed adhesive anchors and bars are designed under CSA A23.3 Annex D, which also deals with installation and inspection. Our dowel embedment guide explains how that design differs from cast-in development length.
Drilling, cleaning and curing: what to expect
The hole is where most adhesive dowels fail, and it is rarely the adhesive's fault. For the full sequence on site, see how to dowel rebar into existing concrete. The points that matter for hole size and bond:
- Scan first. Drilling into existing concrete can hit existing reinforcement or embedded services. Locate them before drilling, and do not cut an existing bar without the engineer's approval.
- Drilling method. Hammer drilling leaves a rough wall that adhesive grips well. Diamond coring leaves a smooth wall, and some products either do not allow cored holes or require extra preparation and give lower values for them.
- Cleaning. Drilling dust coats the wall of the hole and acts like a release agent. Manufacturers set a specific cleaning sequence, usually alternating compressed air and a wire brush of a stated diameter, repeated a set number of times. Skipping it is the most common reason a dowel pulls out at a fraction of its rated load.
- Water. Damp, wet or water-filled holes are treated differently by each product, and some require extra steps or have reduced values. Check before you start a job below grade.
- Filling. Fill from the bottom of the hole with a mixing nozzle and extension tube, to the fill level the table gives, so air is not trapped. Insert the bar with a slight twist to full depth.
- Cure before load. Gel time and full cure time depend on the temperature of the concrete, not the air. In an Ontario winter, a cold slab or foundation wall can stretch cure times a long way, or fall below the product's minimum installation temperature. Do not tie, bend or load dowels until they have reached full cure.
On structural work the engineer may also call for proof loading or inspection of a sample of dowels. Ask whether that applies before the work is scheduled.
Ordering dowels for drilled-in work
Drilled-in dowels are simple bars, but they are easy to order wrong:
- Length = embedment + projection. The embedment comes from the adhesive design; the projection is usually a lap with the new bars. Our lap splice length chart shows typical values under CSA A23.3 assumptions.
- Straight vs bent. Straight bars are easiest to set in a hole. A bend on the projecting end, such as an L-bar into a new slab, has to clear the existing work while the bar is inserted and twisted.
- Bar size matches the design. Do not substitute a bar size because it fits a bit you have. A different bar needs a different hole, a different embedment and often a different adhesive quantity.
- Spacing and edge distance. Dowels close to an edge or to each other have lower capacity. Spacing comes from the drawings; our rebar dowel size and spacing guide explains how it is set.
Get dowels cut to your drawings
We supply straight bar and fabricated dowels in 10M to 55M, cut and bent to your bar list, tagged and delivered across the GTA and Ontario. The hole size, embedment and adhesive belong with your engineer and the adhesive manufacturer; we make sure the bars arrive at the right size, length and shape. Send your drawings for a free take-off and quote, or see our rebar products.
Frequently asked questions
What size hole do I drill for 15M rebar?
For a bar set in adhesive, drill the diameter listed for 15M in the adhesive manufacturer's installation table, because the product's capacity was tested at that hole size. As a reference point, the RSIC Manual of Standard Practice gives about 20 mm as the hole needed to pass a 15M bar through formwork or steel.
What size drill bit do I need for 10M rebar?
Use the bit size in the adhesive maker's table for 10M. The RSIC manual lists about 15 mm as the approximate hole diameter a 10M bar needs to pass through, which is larger than the 11.3 mm nominal diameter because the deformations stand proud of the bar.
Which rebar anchoring adhesive is best?
There is no single best product. The right one is the adhesive your engineer specifies, with published design data for rebar in your hole size, drilling method, hole condition and installation temperature. Epoxies cure slowly with long working times, while hybrid and vinyl ester products cure faster and are often chosen for cold weather.
Can I drill the hole the same size as the rebar?
No. The deformations make the bar's overall diameter larger than its nominal size, so a hole drilled to the nominal diameter will not let the bar seat, and forcing it in wipes the adhesive out of the hole. Drill the size the adhesive system specifies.
