If you need to install a baserail as part of a staircase renovation, this guide walks you through every stage, from preparation to final fixing. A baserail (sometimes called a bottom rail) is the horizontal or raked timber that runs along the base of your balustrade, sitting on top of the string or the treads, and it holds the bottom of each spindle in place. Getting it right is essential for a solid, safe, and attractive finish, whether you are a first-time DIYer or a seasoned joiner looking for a quick refresher.
What Is a Baserail and Why Do You Need One?
Before you begin, it helps to understand the role a baserail plays. In many traditional staircases, spindles (the vertical bars between the handrail and the staircase) are simply pinned directly into the treads. That works well with turned or plain spindles, but it limits your design options and can leave gaps that fail UK Building Regulations (Approved Document Part K).
A baserail solves this by providing a continuous rail with pre-routed grooves or square holes into which the spindles slot. This approach offers several advantages:
- Consistent spindle spacing, making it much easier to comply with the 100 mm sphere rule (no gap should allow a 100 mm sphere to pass through).
- A cleaner, more contemporary look, particularly when paired with square or stop-chamfered spindles.
- Easier installation, because the baserail is cut and fixed once, and the spindles simply drop into it.
Tools and Materials You Will Need
Gather everything before you start. There is nothing worse than being halfway through a cut and realising you are missing a clamp.
- Baserail (matching your handrail profile and timber species, such as oak or white primed)
- Spindles (cut to the correct length for your rake angle)
- Spacer fillets (the small timber strips that fill the grooves between spindles)
- Mitre saw or quality hand saw
- Tape measure and pencil
- PVA wood glue (D3 or D4 rated for interior joinery)
- Panel pins or brad nailer
- Spirit level
- Drill and countersink bit
- Wood screws (typically 4 mm x 50 mm or 60 mm)
- Sanding block (120-grit and 240-grit)
Step-by-Step: How to Install a Baserail
Step 1: Measure and mark your run
Measure the distance between the two newel posts (the large structural posts at the top and bottom of the staircase) at the base level where the baserail will sit. On a raked (angled) section, measure along the slope, not the horizontal. Add a few millimetres to allow for fine adjustment, then mark your cut lines on the baserail.
Step 2: Cut the baserail to length
Cut each end at the correct angle so the baserail sits flush against the newel posts. On a straight flight, the rake angle is typically between 38° and 42°. If you are unsure, use a sliding bevel gauge held against the existing string to capture the exact angle, then transfer it to your mitre saw. Always cut on the waste side of the line and test-fit before committing.
Step 3: Dry-fit spindles and fillets
Before fixing anything permanently, lay the baserail on a flat surface and slot in your spindles and spacer fillets to check the fit. The fillets should sit snugly in the grooves, and each spindle should drop in without force. This dry run also lets you confirm that spindle spacing meets the 100 mm rule. Adjust fillet lengths if needed.
Step 4: Fix the baserail to the string or treads
Position the baserail on the stair string (the diagonal board running along the side of the staircase). Pre-drill and countersink from the underside or through the string into the baserail, using screws at roughly 400 mm centres. Apply a bead of PVA along the joint for extra strength. If your staircase has an open string (where you can see the profile of each tread), the baserail sits on top of the treads instead, and you fix down into each tread with screws.
Check with a spirit level (or match the rake) as you go. The baserail must run parallel to the handrail above, or your spindles will be different lengths, and the whole assembly will look off.
Step 5: Glue and pin the spindles and fillets
Working from one newel post toward the other, apply a small amount of PVA to the base of each spindle and the ends of each fillet. Slot the first fillet in, then a spindle, then the next fillet, and so on. Use panel pins through the top edge of the groove to hold each fillet in place while the glue sets. A brad nailer speeds this up considerably.
Step 6: Secure spindle tops into the handrail
With the bottoms locked into the baserail, the top of each spindle fits into the handrail in the same groove-and-fillet arrangement. Work along the rail, gluing and pinning as you go. When you reach the far newel post, the last fillet should close the gap neatly. If it does not quite fit, trim it with a block plane rather than forcing it in.
Step 7: Fill, sand, and finish
Punch any pin heads below the surface and fill with a matching wood filler. Once dry, sand smooth with 120-grit paper, then finish with 240-grit. If your baserail is white primed, a light sand and two coats of satinwood paint will give a durable finish. For oak, consider a clear lacquer or a hard wax oil to bring out the grain while protecting against scuffs.
Common Mistakes to Avoid When You Install a Baserail
Even experienced tradespeople occasionally trip up on these points:
- Cutting the wrong angle. Always double-check your rake angle with a bevel gauge. A degree or two out will leave visible gaps at the newel post.
- Forgetting the 100 mm rule. Measure each gap once the spindles are in place. Building control can flag non-compliant spacing, and it is a genuine safety concern for young children.
- Skipping the dry fit. Gluing spindles into a baserail that does not quite align with the handrail above is a costly mistake. Always test first.
- Using the wrong timber species. Mixing pine fillets with oak baserails creates colour mismatches once finished. Keep everything consistent.
Baserail Profiles: Choosing the Right One
Baserails come in different profiles to match the handrail and spindle style you are using. The most common are:
- Grooved baserail with a continuous channel for square spindles and fillets (the most popular choice for modern renovations).
- Flat baserail designed for metal spindles that bolt through pre-drilled holes.
Whichever profile you choose, make sure it matches your handrail. At Stair Crazy, baserails are grouped by range, so selecting matching parts is straightforward. If in doubt, ordering a sample or checking the product dimensions before committing is always worthwhile.
Building Regulations and Safety Checks
Under Approved Document K of the UK Building Regulations, any balustrade guarding a staircase must meet minimum standards. The key points relevant to baserail installation are:
- The balustrade (handrail, spindles, and baserail together) must be at least 900 mm high on a staircase and 1,100 mm on a landing.
- No gap in the balustrade should allow a 100 mm diameter sphere to pass through. This includes the gap between the bottom of the baserail and the tread or string beneath it.
- The structure must be strong enough to resist a horizontal force of 0.36 kN/m applied to the top of the handrail.
If you are renovating a period property, the existing staircase may not meet current standards. When you install a baserail and new spindles, you have an ideal opportunity to bring everything up to code.
Ready to Get Started?
Fitting a baserail is one of those jobs that looks complex but becomes very manageable once you understand the sequence. Take your time with the measuring, do a thorough dry fit, and work methodically along the run. The result is a balustrade that feels solid, looks professional, and meets the regulations. If you are ready to install a baserail and need matching components, browse the full range of baserails, spindles, and fillets at Stair Crazy to find parts that work together straight out of the box.
