# Soil against the wall: drainage and damp from the garden > Soil, a flowerbed or paving against an outside wall feeds damp. What level difference to keep, what a French drain does and how to slope the ground. - Page: https://paredecerta.pt/en/guides/soil-against-the-wall-drainage-and-damp - Language: English - Updated: 2026-11-05 ## Summary - Soil or paving that rises against an outside wall leaves the foot of the wall wet and without air to dry. - RICS (UK) shows a path that should have been 150 mm below the damp-proof course; we found no Portuguese rule. - Paving should fall away from the wall, about 1:60 according to a British technical site. - SPAB says a French drain should sit about a metre from the wall, be inspected yearly and can make damp worse if it clogs. - First stop the water (gutters, pipes), then lower the level, correct the slope and only then think about drainage. Garden soil, a flowerbed, a patio or a path built up against an outside wall can feed the damp you see indoors. This guide explains why, what level difference to keep, what an open channel and a French drain do, and what to do about paving slope, flowerbeds and watering. We found no Portuguese rule for these figures. We use British and Scottish guidance and say where each number comes from. *Updated on 4 November 2026. General information, not a substitute for a technician's assessment.* ## The essentials - When the ground or paving rises against the wall, rain and irrigation water sit at the foot of the wall and the wall has less area to dry through. - RICS (UK) shows a case where the path should have been **two bricks (150 mm)** below the damp-proof course. - Paving should **fall away from the wall**. A technical paving site gives about 1:60 as the normal fall. - A French drain is not always the answer. SPAB warns that an uninspected drain can clog and make damp **worse**. ## Why soil and paving against a wall cause damp An outside wall dries through both faces, mostly the outer one. With soil piled against it, that part stays wet and without air. Historic Environment Scotland (HES) says high outside ground levels often cause problems, seen indoors as **tidemarks** and **blistered paint**. It adds that ground tends to rise over time and sometimes **earth must be scraped away**. Hard paving matters too. RICS explains that rain **splashes** from a hard path back onto the wall, and a path set too high can let water bridge the damp-proof course. The fix there was to lower the path, remove the mortar and repair the gutters. Not every stain near the floor comes from the ground. See [damp walls: the four types](https://paredecerta.pt/en/guides/damp-walls-four-types) and [rising damp treatment](https://paredecerta.pt/en/guides/rising-damp-treatment). To measure before deciding, see [measuring damp with a moisture meter](https://paredecerta.pt/en/guides/measure-damp-in-a-wall-moisture-meter). ## Is it the ground? | Sign | What it suggests | |---|---| | Stain or mould low on the wall, on the side where the garden is higher | Soil or paving feeding the base of the wall | | A line of different tone or white salts at a fairly even height | Water rising and evaporating (see [salt on walls](https://paredecerta.pt/en/guides/salt-on-walls-efflorescence)) | | The patch grows after rain or watering | Surface water reaching the wall | | Outside ground higher than the inside floor | HES and SPAB treat this as a cause to check | For basement or garage walls with soil on one side, see [damp basements and garages](https://paredecerta.pt/en/guides/damp-basements-garages-retaining-walls). ## What level difference to keep The sources give **150 mm**, from UK building rules, not Portuguese ones: - RICS: the path should be two bricks (150 mm) below the damp-proof course. - pavingexpert.com cites the British Building Regulations: at least 150 mm between the course and the ground or paving. It allows 75 mm as a minimum on small private jobs with the owner's consent, never on public or commercial buildings. - SPAB (rising-damp note, already used in our guide) speaks of lowering the ground to at least 150 mm below the internal floor. Many old houses show no damp-proof course. Then the reference is the **inside floor level**. If you do not know where the course is, ask a technician to find it before you lower any soil. ## What to do, in order 1. **Stop the water first.** Check gutters, downpipes and pipes. HES says rainwater should be piped to the main drain or a soakaway **well clear of the building**. 2. **Lower the level where you can.** Scrape soil back from the wall so the base is exposed and can dry. First find out where the foundations, pipes and cables are. 3. **Correct the slope.** Relay paving to fall away from the wall. 4. **Think about drainage only after that**, if water still collects. 5. **Wait and measure.** A thick wall dries slowly. Do not paint meanwhile. ## Paving slope pavingexpert.com gives these UK figures: - About **1:60** for a normal fall (1 cm drop per 60 cm). - About **1:40** for irregular flags that shed water badly. - At least **1:12** where 150 mm cannot be kept. The same site shows a "dry area": a 200 to 300 mm strip of gravel between paving and wall. It collects litter and leaves. HES suggests gravel on coarse aggregate for paths beside walls and says **not to put plastic sheeting under them**, because it can route water to the footings and stop the ground drying. ## Open channel and French drain An **open channel** is a trench along the wall, lower than the ground, so the base of the wall can air. SPAB says open drains can work better than a French drain where ground has risen and the wall base needs to evaporate. A **French drain** is a perforated pipe in a gravel trench. SPAB gives typical values: - Pipe 100 to 150 mm, laid to a fall of about **1:80**. - Trench 150 to 200 mm wide and 200 to 300 mm deep. - Inspection points, and an outlet to a soakaway, watercourse or mains drain. SPAB says it suits intercepting surface water running down towards a house and clay soils that drain badly. It should sit about **a metre from the wall**. If it must be close, the trench must not cut into a **45 degree** line down from the bottom of the foundations. Dig a small trial pit first to see foundation depth. Risks listed by SPAB: undermining shallow foundations, over-drying the ground, clogging into a "moat" around the house, and hitting pipes and cables. It asks for **yearly inspection**. ## Flowerbeds and watering HES says plants close to walls can trap moisture and roots can damage drains, so keep beds and shrubs away. On **watering** we found no official source. By the logic of the sources above, a sprinkler that wets the wall or soaks the soil at its base adds water where the wall should dry. Point it away and see if the patch improves. ## When to call a professional Call an engineer or architect if the house is old, the wall is stone, there are cracks or the stain is large. SPAB advises an independent professional, not a contractor who may over-specify. Once the wall has dried and the cause is fixed, you can paint: see [how to paint a wall, step by step](https://paredecerta.pt/en/guides/how-to-paint-a-wall-step-by-step) and the [paint calculator](https://paredecerta.pt/en#calc). ## What we could not confirm - **A Portuguese rule** on level difference or slope for existing houses: not found. The 150 mm and the slopes are British. - **Watering and flowerbeds:** no official source read, only the reasoning above. - **Historic England pages and the NHBC rules** did not open; we cite only what we read. - **Publication dates** of the HES leaflet and the SPAB page are not shown. - **Prices** for drainage work in Portugal: not confirmed. - **Council permission** for changing ground or public pavement: not confirmed. ## Sources (read on 4 November 2026) - Historic Environment Scotland, *Damp: Causes and Solutions* (INFORM leaflet for historic building owners), [PDF](https://app-hes-pubs-prod-neu-01.azurewebsites.net/api/file/26b85af5-8f2d-44b1-9bf5-a6c000f68a4a). Read: introduction, "External ground levels and finishes". - Historic Environment Scotland, The Engine Shed, [How to deal with damp in walls](https://www.engineshed.scot/building-advice/common-problems/how-to-deal-with-damp-in-walls). Read: ground levels, rainwater, vegetation. - SPAB, [French drains](https://www.spab.org.uk/advice/french-drains), advice page (© 2025). Read, including dimensions, risks and maintenance. - RICS, [Bridged damp proof course (DPC) and raised ground](https://www.rics.org/consumer-guides/damp-and-mould/bridged-damp-proof-course-dpc-and-raised-ground), consumer guide. Read in full. - pavingexpert.com (AJ McCormack & Son), [All About Damp Proof Courses](https://pavingexpert.com/dpc01). Read: paving level and fall, dry area. Technical site, not official. - SPAB on rising damp: cited through our guide [rising damp treatment](https://paredecerta.pt/en/guides/rising-damp-treatment), not read directly today. - NHBC Standards 5.1.19 (course 150 mm above ground): cited through a search result, not read directly (the page returned a 403 error). ## FAQ ### How high can soil be against the wall? We found no Portuguese rule. RICS shows a path that should sit 150 mm below the damp-proof course, and SPAB speaks of 150 mm below the internal floor. ### What slope should paving beside the house have? pavingexpert.com (UK) gives about 1:60 as normal and at least 1:12 where 150 mm cannot be kept. ### Does a French drain always solve it? No. SPAB says it should sit about a metre from the wall, be inspected, and can worsen damp if it clogs. ### Can watering cause damp in the wall? We found no official source on watering. By the logic of the sources read, water wetting the base of the wall slows drying. --- Source: https://paredecerta.pt/en/guides/soil-against-the-wall-drainage-and-damp. Independent information, not legal or tax advice.