Troweling

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

Some links on this site are affiliate links. If you buy through them we may earn a commission at no extra cost to you. Affiliate disclosure

Troweling makes an interior surface dense, hard and smooth. It is never for exterior or air-entrained concrete, and timing decides everything.

The big idea

Troweling makes the surface dense, hard and smooth. It is for interior slabs, such as heated basements, shops and floors that will take a coating or flooring. It is not for exterior concrete, where it makes a slippery surface that traps water and scales in frost.

The one failure to fear is sealing the surface too early. Air and bleed water get trapped under a dense skin, and blisters or delamination show up later.

Where a trowel finish belongs

• Yes: interior slabs that are not air-entrained, such as heated basements, shops and floors to be coated or covered.
• No, exterior: use a float and broom finish. The cement-industry flatwork guide says never to steel trowel air-entrained concrete.
• No, garage floors: Part 9 requires air entrainment of 5 to 8 percent for garage and carport floors (9.3.1.6). A guide on troweled-surface delamination says not to use air-entrained concrete for a hard-troweled interior floor, so float and broom or lightly finish a garage floor. If air is needed for freeze-thaw during construction, that guide says to keep it to 3 percent or less, so ask the engineer and supplier.

Types of trowels

• Hand trowel: edges, corners, stairs and small slabs.
• Power trowel: large slabs, with pans or flat blades first and finish blades later.
• Combination blades: used for floating and early troweling.
• Finish blades: for the last, harder passes.

Timing

• Start only after the bleed water sheen has gone. Floating comes first, as the previous page covers.
• The slab should leave no more than about a 6 mm (1/4 in) footprint before power work starts.
• Begin with the blades flat, then raise the angle gradually over successive passes. A steeper angle shrinks the blade contact area and raises the pressure on the surface.
• If you trowel too early, you trap water and air. Blisters, scaling or dusting can follow.
• There is no fixed clock time. A trade-magazine article calls this window the true challenge of interior floors.

Hand and power troweling

1. Hand trowel edges first Use a hand trowel at edges and corners. Keep the first pass flat, use even pressure and do not dig in the edges.
2. Start the machine flat Start with pans or flat blades. Overlap each pass and keep moving.
3. Raise the angle slowly Raise the blade angle a little on each later pass as the slab stiffens.
4. Never stand still A machine left in one place burns the surface and leaves marks.
5. Cure at once Start curing right after the last pass. Fast drying causes crazing.

Defects and what causes them

Find blisters and delamination by sounding: tap the floor or drag a heavy chain across it, and listen for hollow spots. Do not add water or evaporation retarder into the surface. Fogging above the slab and early curing help in hot or windy weather.

Common mistakes

• Troweling too early
• Overworking and bringing up cream
• Digging in the edges
• Burn marks from standing still
• Chatter marks from a wrong blade angle
• Troweling an exterior or air-entrained slab