Bitumen primer: what it does under a membrane

What a bitumen primer does

Bare concrete is dusty and porous. A membrane torched or pressed straight onto it bonds to the loose surface layer rather than the slab. A primer is thin enough to soak into the pores, and once dry it leaves a bitumen film that the membrane’s own bitumen fuses or sticks to.

Sika’s page for its SikaShield Primer S-30 IN puts it this way: the primer “penetrates and activates the substrates” and improves adhesion between the substrate and bituminous sheet membranes, both torch-on and stick-on, APP and SBS. The roll itself is described on the APP bitumen membrane page.

Solvent-based and water-based bitumen primers compared

There are two kinds, and they behave differently on site.

Solvent-based and water-based bitumen primers compared

	Solvent-based (SikaShield Primer S-30 IN)	Water-based (Baumerk B PR 101)	Water-based (JUYONS)
Base	bitumen in solvent	bitumen emulsified in water	polymer-modified emulsified bitumen
Consumption	0.25–0.40 L/m² per coat	0.400 kg/m² per layer	0.3 kg/m²
Drying before the next step	touch dry within 4 h, hard dry within 14 h	4–5 hours	2–3 hours
Application temperature	10–40°C	not below +5°C	—
Flammability	flash point ≥ 30°C	stated as non-flammable	stated as non-flammable

The trade-off: a solvent primer penetrates well but must be kept away from flames until the solvent has evaporated — important when the next step is a torch. A water-based primer needs no solvent and Baumerk states it can go on moist surfaces, but it must not be laid in rain. Both families are close relatives of bitumen paint and emulsion; a primer is simply thinner.

How a bitumen primer is applied

The steps combine the data sheets above; where they differ, the source is named.

1. Prepare the surface. Clean, sound, free of grease, oil and loose particles. Sika adds that new concrete should be cured for at least 28 days and that cement laitance should be removed mechanically.
2. Stir or dilute as the sheet says. Baumerk dilutes its B PR 101 with 20% water before use; Sika’s solvent primer is ready to use.
3. Apply evenly by brush, roller or spray. JUYONS lists all three and asks for corners, joints and pipe roots to be primed first.
4. Let it dry fully before laying the membrane: 2–3 hours for the JUYONS primer at normal temperature, up to 14 hours to hard dry for the Sika one.

Our calculation for a 50 m² roof: 15 kg of the JUYONS water-based primer at 0.3 kg/m², or 12.5–20 litres of the Sika solvent primer at 0.25–0.40 L/m².

Water-based primer in stock in Zanzibar

Kitovu stocks the Water-Based Bitumen Primer for Waterproofing Membranes by JUYONS, in 20 kg buckets. According to JUYONS, it is a polymer-modified emulsified bitumen for treating the substrate before bitumen waterproof membranes; the JUYONS membrane instructions give 0.3 kg/m², which puts one bucket at about 66 m² (our calculation, 20 ÷ 0.3). The listing gives a shelf life of 6 months and storage at 5–35°C, protected from sun, rain and frost.

What to check before you buy

• Match the primer to the membrane. Sika’s primer is named for its own SikaShield sheets; JUYONS names its primer for bitumen membranes. Ask for the primer the membrane maker lists.
• Solvent or water. On a torch-on job, a solvent primer must be completely dry before any flame comes near.
• Consumption unit. Litres per m² and kilograms per m² are not the same number, so check which the sheet uses before you order. The bitumen primer price page shows how a 20 kg bucket and a 20 L tin compare.
• Shelf life. Sika gives 12 months, JUYONS 6 months. Old stock that has separated or thickened should not be used.