Building Guides Insulation

Waterproof and Weatherproof Sealants

Waterproof and weatherproof sealants generally come in two different forms: liquid and paste or mastic. Liquid sealants are mainly used to waterproof the external surfaces of areas such as flat roofs and solid floors. Paste or mastic sealants are normally used as a waterproof or weatherproof layer between layers of other materials, often on smaller areas like pipes and bath. Sealants may be composed of traditional waterproofing materials such as bitumen or newer man-made chemical like waterproof silicones.

There are lots of different sealants available, of both types, and it is important that you use the correct sealant for the job at hand. It is always advisable to fully read the manufacturers instructions before using the sealant, even if you have used a similar sealant in the past.

Types of Liquid Sealant

  • Bitument-based Roof Sealant
  • Gutter Sealant
  • Liquid Damp-proof Membrane
  • Interior Damp Sealant
  • Water Seal (walls)
  • Drive Sealant
  • Paving and Patio Sealant

Using Liquid Sealants

When applying liquid sealants to roofs, brickwork and stonework, the most practical method is to brush it on with a large paintbrush or pasting brush. Some masonry sealants can be applied with a spray gun, but if using this method you need to ensure that adjacent surfaces are masked to avoid overspray.

The surface to be treated should always be as dry, clean and free from dust as possible before application. If the sealant is being used indoors, always ensure that there is adequate ventilation. This is important from a safety perspective, but it will also help the sealant to dry faster. Bituminous sealants are very difficult to clean off of a brush, so try to use a brush that can be disposed of after completing the job.


Types of Mastic Sealant

  • Decorators Filler (filling gaps along skirting boards, etc.)
  • Bath and Kitchen Seal (sealing around baths and sinks)
  • Outdoor Frame Sealant (sealing around doors/windows)
  • Glaziers Putty
  • Roof and Gutter Seal (cracks in roofing felt)
  • All-weather Sealant

Using Mastic Sealants

Mastic sealants are usually applied in a long thin bead, along the joint between adjacent surfaces (such as between a bath and a tiled wall). The surfaces should be clean, dry and free from dirt and debris. Mastic sealants most often come in a tube with a thin nozzle which can be cut to provide the size of bead needed. Although mastic sealant will fill quite large gaps, it can’t perform miracles and you may need to pack especially large gaps with foam filler or something similar before the mastic is supplied. You can read more in our complete Guide to Sealing a Bath or Shower.

Silicone sealants can contain chemicals which will irritate the skin and eyes. Be aware of this when using them, and make sure you protect yourself as much as possible. You should also take care when cutting the end of the dispenser nozzle.

Masking Areas When Sealing
Whilst working outdoors, applying sealant neatly probably will not be a high priority. So long as the bead is continuous and is well bonded to both surfaces, it will do its job perfectly well. Indoors, however, you will want sealant beads to look as neat and unobtrusive as possible, especially around bathroom fittings and along the rear edge of kitchen worktops. The secret of success is to use masking tape. Stick it to both surfaces, 2 to 3mm from the internal angle. Then cut the cartridge nozzle at a 45° angle to give a bead that’s just wide enough to fill the angle between the two tape edges.

To start the flow of sealant, apply gentle pressure by squeezing the trigger. As soon as the sealant appears at the tip of the nozzle, place it in the angle with the nozzle held at 45° to the wall. Push it along the angle, allowing the nozzle to shape the sealant bead into a neat concave curve. Leave it to skin over for a while, then peel off the masking tape.