The Problem of Rotten or Decayed Sill Plates

A sill plate - also known as a sill beam, sole plate, bottom plate or mudsill - is a horizontal piece of timber that sits directly on top of a building’s foundation. It is the primary interface between the foundation and the house’s wooden structure. Its main role is to distribute the weight of the building (walls and roof) evenly across the foundation. By providing a flat and level surface for the framing members, it ensures that the loads are properly transferred to the foundation, reducing the risk of structural issues.

Many old buildings built before the 1850s have been built on shallow foundations or without a damp proof course (DPC). Thus the proximity of the dampness from the soil and the environment caused in the rotting of the sill plate, its repair or replacement becoming necessary at some point.

rotten-sole-core-conservation — Rotten sill plate and stud ends due to exposure to ongoing humidity

Sill Plate Repair Process

Repairing the rotten sill plate can involve several actions depending on the extent of the damage. The wall base or the supporting bricks onto which the sill plate is laid can also be damaged due to the prolonged exposure to humidity and ground salts.

Here are some of the actions that might be necessary to rectify the problem:

Excavate: if there is no foundation or wall base present, some excavation might be needed to create a foundation.
Lay a foundation: a foundation can be created from stone or bricks, building up a low height supporting wall onto which the sill beam can be laid. To stop the rise of moisture through the mortar bed the wall can be built with the Rinzaffo MGN Roman waterproofing lime mortar.
Damp proof course: additionally, before laying down the new sill plate, adding a new solid damp proof course (DPC) would also be a good idea. The role of the DPC is to prevent or at least slow down the upward migration of moisture, protecting the sill beam giving it a longer life expectancy. Sheets of slate can be used as a DPC, as this traditional material has been successfully used as DPC since Victorian times for over 150 years. Additionally, a plaster layer of the Rinzaffo MGN Roman lime waterproofing mortar can also be used as a DPC, either on its own or in combination with the slate.
Sill plate: the oak sill plate is then laid on top of the DPC. Depending on how damaged the sill plate is, it can be replaced either fully or partially, retaining the healthy wood sections based on the minimum intervention principle.

sill-repair-core-conservation — Rotten sill plate repair

Protecting the Sill Plate from Future Water Damage and Rot

The Rinzaffo MGN Roman lime waterproofing mortar is particularly suitable for protecting the sill plate (or any timber elements) from future moisture damage, for the following reasons:

Due to its pore structure, the Rinzaffo MGN plaster stops liquid water, keeping the sill plate and other timber elements dry. In our observation, a 30 mm of Rinzaffo MGN plaster layer applied onto any wall surface can stop liquid water penetration, resulting in a completely dry surface.
Being a breathable, vapour-permebale lime plaster, it does not trap moisture, keeping the timber dry and rot free.
It can cope with structural loads, being a TUV certified lime plaster suitable for structural repairs.
If the wall sections under the sill plate are built-up with the Rinzaffo MGN mortar, it further minimizes the rise of moisture from the ground keeping the supporting walls dry and free of rising damp.
The Rinzaffo MGN mortar can be applied in conjunction with sheets of slate, or used for the embedding of slate. Unlike slate that can be damaged over time by the crystallizing salts, the Rinzaffo MGN mortar is impervious to all salts, having an extremely long life expectancy.

Rinzaffo

Waterproofing, Base

Application Guide

Clean the masonry: the plaster must be applied on a cleaned and uniformly wet substrate. All crumbling and loose parts must be removed by brushing so the masonry is free of dust, salts and oils. If possible, also wash the walls with a pressure washer; this will clean and wet the walls in one go. Close larger holes with lime mortar and pieces of bricks.

Must be the first coat: the Rinzaffo MGN plaster must be the first coat on any wall. It should not be applied on top of other parge coats or plasters because when those fail the performance of the Roman base coat will also likely to be affected.

Cement backgrounds: the Rinzaffo MGN base coat can be applied and will bond well to cementicios backgrounds. The removal of cement plasters is recommended for the vast majority of cases to restore the breathability of the wall fabric. However, in some (rare) cases, when the removal of cement would be too invasive, too costly or would not beneficial the underlying masonry, if the cement backing is stable, the Rinzaffo MGN can be applied over it for waterproofing purposes (e.g. to waterproof cement floor bases).

Salt-inhibitors or PVA bonding agents are NOT needed and should not be used before the application of this plaster. This natural lime plaster bonds very well on its own.

Masonry paints should be removed (which occasionally can be challenging) so the plaster can firmly adhere to the underlying stones or bricks. Remove at least 50% of the existing paint – the more is removed, the better.

Mixing: mix the material with clean tap water only without adding anything else (no other material or additive), until a homogenous, creamy-consistency mix is obtained.

Wet the wall fabric abundantly before applying the plaster, as well as in-between each subsequent coat. Lime plasters need moisture as they set slowly in a damp environment. If the walls are already damp, there is no need to wait to become drier, you can proceed with the application of this plaster.

Level uneven surface first: very uneven walls (e.g. stone walls or crumbling old brick walls) are recommended to be patched up and levelled first before the application of a continuous coat, to ensure the consistency and required thickness of the base coat.

Application: apply the plaster in 10 mm coats.

IMPORTANT: Respect the thickness: do not under-spec the material. Apply min 10 mm (1 coat) for above ground level walls. Apply min 20 mm thickness (in 2 coats) for underground or extremely damp or salty walls. If more than one coat is applied, embedding a 10 x 10 mm fibreglass mesh is recommended between the coats – a standard practice in the industry.

IMPORTANT: Close all pores, no matter how small. Treat and apply the plaster as a tanking-grade material. Attention should be paid to compact it on the wall closing off all pores, no matter how small, leaving no gaps or holes where salts or liquid water could come through. Once an area has been completed, recheck that here are no missed holes, not even small ones.
The plaster application video below explains the concept in detail.

Light key: give the plaster a light key using a wet brush. Do not cut into the material with the edge of a trowel.

IMPORTANT: Dark patches. Allow the plaster to dry for 48 hours. Ideally, the whole surface should dry out uniformly to light brown, however you might notice some dark or damp-looking areas exhibiting surface condensation. Dark areas indicate insufficient thickness of the plaster in raport to the amount of moisture behind it. This occurs in areas where the underlying wall fabric is very damp, the intense evaporation causing surface condensation.
The fix is easy: apply extra material over such dark areas increasing the thickness of the plaster, closing off all pores. The increased plaster volume dilutes the vapour flow allowing the surface to dry.

Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.

Application conditions: ambient and wall temperatures must be between +5 to +30°C during application. Surfaces should be protected from rain and humidity until they have completely dried (approx. 3 – 10 days depending on weather conditions).