Expansion Joint Sealant: Choose by Movement and Exposure, Size the Joint Right

Expansion joint leakage: check the joint before choosing a sealant

A leak along a straight line on the terrace, a damp streak down the wall below a roof joint, water dripping through a basement or car park joint, sealant that has split or pulled away from one side: these are the usual signs of expansion joint leakage. An expansion joint is a gap left on purpose so the building can move with heat and load. It has to be sealed with something that moves with it. This page covers expansion joints, construction joints and movement gaps in buildings, terraces, water tanks, floors and roads. Five checks decide the expansion joint sealant and the joint size.

Find the right expansion joint sealant for your joint

Match your joint to one route. Each Armstrong sealant is made for its own kind of joint, and each route takes you to its section.

Size it, clean it, prime it, then seal it. Most sealant failures are failures of joint size or surface preparation, not of the sealant.

Expansion joint sealant finder

Armstrong makes polysulphide, polyurethane, acrylic and bitumen sealants and a joint tape. Each is a sound choice for its own joint. Answer three questions for a starting recommendation, then confirm it with an Armstrong expert.

Expansion joint design: sealant width, depth and backer rod

A sealant works by stretching and squeezing. If the joint is too narrow for its movement, the sealant is stretched past its limit and tears. If the sealant is too deep, it cannot stretch freely and pulls off the sides. Getting the width and depth right matters more than any other step.

How wide should an expansion joint be for the sealant

Armstrong gives a design rule for ROOFEX-SEAL PS: in a butt joint, the design width is the expected movement multiplied by 4.33. In a lap joint, where the sealant works in shear, it is the movement multiplied by 2. The sealant takes 30% movement in tension and compression and 50% in shear.

Depths in the table are the width divided by two, from the stated 2 to 1 ratio.

Backer rod and bond breaker in an expansion joint

• A backer rod is a compressible foam rod pushed into the joint to the right depth. It sets the sealant depth and gives something to tool against.
• It also stops the sealant sticking to the bottom of the joint. A sealant bonded on three sides cannot stretch and tears. Where the joint is too shallow for a rod, a bond breaker tape is laid on the bottom instead.
• The rod is chosen a little wider than the joint so it stays in place under the sealant. The backing is polyethylene foam, left at 12 mm depth for a 25 by 12 mm joint and 25 mm for a 25 by 25 mm joint.
• RAINSEAL BS is also applied over a backer rod where a deck joint is deep.

How to seal an expansion joint, step by step

This is the sequence for a building or terrace expansion joint with polysulphide sealant. The same order applies to polyurethane sealant, without the primer and the mixing.

1. 1
   Clean out the joint

   Remove old sealant, mortar, bitumen, filler board and debris to the full sealant depth and beyond. Remove all loose, weak and flaking material from the joint faces. Faces must be sound, clean and dry, free of dust, oil, curing compound and old coatings.Why: Sealant bonds only to clean, sound faces. Old sealant, bitumen and mortar stop it.
2. 2
   Repair broken edges

   Rebuild chipped or broken arrises with ROOFCRETE AR over a bond coat of ROOFBOND SBR 401 and let the repair cure. Sealant needs two firm, parallel faces.Why: A broken edge cannot hold the sealant, so the edges are rebuilt first and left to cure.
3. 3
   Insert the backer rod

   Push the rod in to the depth that gives the right width to depth ratio: 2 to 1 for butt joints, 1 to 1 for floor joints.Why: The rod sets the sealant depth and stops it bonding to the bottom of the joint, which would tear it as the joint moves.
4. 4
   Mask the edges

   Run masking tape along both edges for a clean line, and remove it straight after tooling.Why: Tape keeps the sealant off the surface and gives a clean line.
5. 5
   Prime the faces

   Brush a thin, even coat of ROOFEXSEAL PRIMER PS / NPS on both faces. Use the PS primer on porous faces such as concrete, brick and stone, and the NPS primer on non-porous faces such as glass, stainless steel and the face of glazed tile. Let it dry, and seal within its open time. A 250 ml tin covers 25 metres of joint.Why: The primer gives the sealant its bond to the joint faces, and the right primer depends on how porous they are.
6. 6
   Mix the two parts

   Mix the base and the curing agent of ROOFEX-SEAL PS thoroughly until the colour is even. Mix only what can be placed in its working time.Why: A fully mixed sealant cures fully. Streaks stay soft, and mixing only what can be used avoids setting in the bucket.
7. 7
   Fill the joint

   Gun grade is non-sag, for vertical, horizontal and overhead joints. Pour grade is self-levelling, for horizontal joints only. Fill from the bottom up in one continuous run with no air pockets. Joints of 6 to 25 mm are filled in a single application. Wider joints, up to 50 mm, are built up in more than one application.Why: Gun grade stays where it is put on vertical and overhead joints, and pour grade levels itself on horizontal ones.
8. 8
   Tool the surface

   Press the gun grade sealant against the faces and finish it slightly concave. Pour grade levels itself.Why: Tooling presses the sealant against the faces for a full bond, and a slightly concave surface moves better.
9. 9
   Cure

   Protect the joint from water, traffic and dirt while it cures. ROOFEX-SEAL PS cures in 7 days at 20°C.Why: Water, traffic and dirt on sealant that has not cured damage it before it has its strength.

Sealing a joint with polyurethane sealant

ROOFEXSEAL PU 212 is single-component, so there is nothing to mix, and it bonds to concrete, masonry, brick, timber, aluminium and ceramics without a primer. It also bonds to natural stone, marble and granite and does not stain porous stone. It is used for cracks on terraces and walls, construction joints, facade and cladding panel joints, precast and block work joints, floor joints and the perimeter of door and window frames. It is also listed for construction and expansion joints within its ±25% movement.

• Clean the faces as above. Wipe non-porous surfaces with solvent and a clean, dry cloth.
• Cut the nozzle to the bead size, load the cartridge or the 600 ml sausage in the gun, and apply in one continuous pass with enough pressure to fill the joint.
• Apply between 5°C and 35°C. It is tack-free in 50 minutes, cures at 3.5 mm per 24 hours and reaches full cure in 7 days.

Expansion joint treatment in terraces, water tanks, floors and roads

The method is the same everywhere. The sealant, and what goes over it, changes with the place.

Expansion joint treatment in a terrace or roof

1. 1
   Seal the joint first

   Clean, size, prime and seal the joint with ROOFEX-SEAL PS as in the sealing section. Do this before any waterproof coating.Why: The joint is the weak point of the waterproofing, so it is sealed before the membrane goes over it.
2. 2
   Tape over the joint

   RAINSEAL FLEX TAPE is a pre-formed, highly flexible tape. It is pressed over the joint onto a bonding adhesive or primer while that is still tacky, in full contact with both sides. The tape width is chosen by the joint width.Why: The tape bridges the movement without tearing, so the membrane above it is not stretched as the joint opens.
3. 3
   Coat over the tape

   The terrace waterproofing membrane is then applied over the tape and onto the slab on both sides, so the joint becomes part of the waterproofing. Fibre mesh, where used, is bedded in the first coat as the second layer.Why: The membrane carried across the tape makes the joint part of the continuous waterproofing.
4. 4
   Do the roof and wall junction the same way

   The junction of the roof slab and the parapet moves too. The tape is fixed along the junction and covered by the membrane.Why: The roof-to-parapet junction moves just like a joint, so it is detailed the same way.
5. 5
   Let it cure before water

   Allow the whole system to cure fully before ponding or rain testing.Why: Ponding or rain on an uncured system leaks through and damages it.

Cracks on a terrace are a different job: each is opened to a 4 to 5 mm V-groove and filled with ROOFEXSEAL PU 212. The coating systems are on the terrace waterproofing page, and podium and balcony joints on the balcony and podium waterproofing page.

Joints and cracks in water tanks, sumps and swimming pools

Joints that move. ROOFEX-SEAL PS is recommended for potable water-retaining structures and resists water and salt. The joint is primed with ROOFEXSEAL PRIMER PS / NPS and sealed with gun grade on walls and pour grade on floors, then left to cure for 7 days at 20°C before the tank is filled. RAINSEAL FLEX TAPE is listed for joints in water-retaining structures and pools, under the lining.

Cracks that do not move. A crack is not sealant work. Cut a V-groove along it, clean it, and fill it with a putty of ROOFCOAT FLEXI 100 (an acrylic polymer) and cement. The lining coats of FLEXI 100 then go over the whole surface. A polymer-modified putty stays bonded and hard-wearing, so it survives the cleaning and scrubbing a tank needs, which a soft sealant bead in a crack would not.

Other options. Where an overhead tank holds drinking water and a food-neutral finish is wanted, the crack putty can be CONPROOF EP epoxy putty with cement, with CONPROOF EP as the lining. In a sump, shutter joints are plugged with WONDERSEAL PLUG mixed with water, sand and cement. See water tank waterproofing and swimming pool waterproofing.

Floor, car park, forecourt and road joint sealants

Sheet roof joints are covered on the metal and AC sheet roof page.

Gaps around window frames, door frames, skirtings and facade panels

Terrace expansion joint: polysulphide with an aluminium cover

An expansion joint lets two parts of a slab move apart and together without cracking. The sealant has to stay flexible and well bonded across years of that movement, and the cover plate shields it from sun and traffic. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Seal the joint edges

   Apply one coat of crystalline coating ROOFEX XL and one coat of ROOFBOND EP on the joint edges, then repair damaged edges and form the groove with ROOFCRETE AR.Why: The edges are what the sealant bonds to, so they must be sound, sealed and waterproof.
2. 2
   Mask and set the backing

   Fix masking tape on both sides of the joint, then push polyethylene foam backing in to leave a sealant depth of 25 mm.Why: The backing sets the depth and stops the sealant bonding to the bottom of the joint, which would tear it as it moves. The tape keeps the edges clean.
3. 3
   Prime and seal

   Prime the inner edges with the polysulphide primer, then fill the joint with two-part polysulphide sealant (ROOFEXSEAL PS), 25 mm wide and 25 mm deep, tooled with a putty knife. Remove the tape.Why: The primer gives the bond, and a joint tooled to depth gives the sealant room to stretch.
4. 4
   Coat over

   Apply two coats of an acrylic elastomeric cementitious coating over the finished joint.Why: This joins the joint to the waterproofing over the rest of the slab.
5. 5
   Cover plate

   On a horizontal joint, fix a 0.5 mm aluminium sheet about 1 foot wide with screws over the joint and seal its edges and screw heads with WONDERSEAL XL.Why: The plate protects the sealant from sun and from being walked on.

Expansion joint leakage repair: when the old joint has failed

Start with the smallest repair. A joint whose sealant has split or come away, with sound concrete on both sides, is cut out and resealed. Rebuilding the joint edges is the hard case, for joints whose arrises have broken up or that were filled with mortar and plastered over. A joint that keeps widening, or cracks that spread from it into the slab, beam or wall, need a structural engineer before any sealant.

1. 1
   Stop any running water

   Inject RAPIDSTRONG GROUT PU through packers into a joint that is running. It reacts with water in 36 to 48 seconds and foams to fill the gap. Point leaks are plugged with RAPIDSTRONG PLUG 31, which sets in about 30 seconds. The injection grouting page covers the method.Why: Sealant cannot bond to a running joint, so the flow is stopped first by injecting polyurethane that reacts with the water.
2. 2
   Cut out the old filling

   Remove sealant, mortar, bitumen and filler to sound, clean faces. Where plaster, screed or tiles bridge the joint, cut them back so the gap runs through every layer.Why: New sealant bonds only to sound, clean faces. Old filler and anything bridging the joint stops it moving.
3. 3
   Remove everything loose

   Take out all weak, cracked and hollow concrete and plaster along the edges.Why: Weak edges cannot hold sealant and break away under movement.
4. 4
   Rebuild the edges

   Apply a bond coat of ROOFBOND SBR 401 and rebuild the arrises with ROOFCRETE AR against a former, so the joint is straight and of even width. Let the mortar cure.Why: The edges are rebuilt against a former so the joint has sharp, sound faces for the sealant.
5. 5
   Dry the joint

   Sealant and primer need dry faces.Why: Primer and sealant do not bond to wet faces.
6. 6
   Backer rod, primer, sealant

   Reseal with ROOFEX-SEAL PS over ROOFEXSEAL PRIMER PS / NPS as in the sealing section. Damaged polysulphide sealant can be replaced locally by cutting out the affected length, cleaning and priming, and applying fresh sealant.Why: The same sequence as a new joint gives the same durable seal, and damaged backing is replaced so the depth is right.
7. 7
   Tape and coat, where the joint is under waterproofing

   Fix RAINSEAL FLEX TAPE over the joint and carry the membrane across it.Why: Where the joint is under waterproofing, the tape lets the membrane cross it as one continuous layer.

Polysulphide or PU sealant: comparing Armstrong joint sealants

Armstrong makes sealants in polysulphide, polyurethane, acrylic and bitumen, plus a joint tape. Each is the right answer for its own joint. They differ in chemistry, in the movement and exposure they are made for and in how they are applied. Pick the one that fits the joint, and ask an Armstrong expert to confirm.

Polysulphide vs PU sealant: which one for which joint

Some primers and surfaces do not go together. Neither polysulphide primer is suitable for powder-coated aluminium or steel, lead, galvanised steel, glass-reinforced cement, mill-finished or anodised aluminium or PVC-U. Ask Armstrong before sealing against these.

Expansion joint sealing products by stage of work

How much sealant for an expansion joint: the Armstrong formula

Armstrong gives a simple formula for sealant quantity: joint width in mm, multiplied by depth in mm, multiplied by length in metres, divided by 1,000, gives litres of sealant. For ROOFEXSEAL PU 212, litres divided by 0.6 gives the number of 600 ml sausages. The table works it through for common joints. Allow extra for waste and uneven joints. An Armstrong expert confirms the final quantity after seeing the site.

Primer for 30 metres of joint: 2 tins of 250 ml, at 25 metres per tin. WONDERSEAL XL is stated at 6 to 7 running ft per kg at a 5 to 6 mm bead, so 100 running ft of frame gap takes about 14 to 17 kg.

Standards for joint sealants

• ROOFEX-SEAL PS complies with ASTM C-920-87, BS 4254-83, DIN 18549 and US Federal Specification TT-S-00227E.
• ROOFEXSEAL PU 212 complies with IS 11433 (1985), ASTM C 920-87, BS 5212 and AS 4020:2018. Its ±25% movement is to ASTM C 719.
• IS 11433 is the Indian standard for one-part gun grade polysulphide based joint sealants, and IS 12118 covers two-part polysulphide sealants.
• IS 3414 is the Indian code of practice for the design and installation of joints in buildings.

Expansion joint sealant quantity calculator

Enter the joint length, width and sealant depth. The calculator uses Armstrong's formula: width x depth x length / 1000 = litres of sealant. Quantities only.

Eight mistakes that make an expansion joint leak

1. Filling the joint with mortar. Mortar cannot move. It cracks along one edge and water goes through.
2. Plastering, screeding or tiling across the joint. The gap must run through every layer above it.
3. Sealing over dust, old bitumen or loose concrete. The sealant bonds to whatever is on the face. Remove everything loose first.
4. Skipping the primer with polysulphide. The seal peels cleanly off the face.
5. Sealant too deep, or stuck to the bottom. It cannot stretch, so it tears. Use a backer rod.
6. Joint too narrow for its movement. The sealant is stretched past its limit every summer.
7. Using an acrylic sealant in a moving joint or under water. It is made for low-movement gaps around frames, not for expansion joints or immersion.
8. Coating over the joint with no tape. A membrane painted straight across a moving joint splits at the joint.

Expansion joint sealing questions and answers