Väder- och temperaturgränser vid omfogning: Skydda bruket mot frost och sol

Noggrann planering och exakt materialberäkning är avgörande för konstruktionens livslängd och hållfasthet vid all omfogning av murverk. Denna fackguide redogör för fysiska beräkningsmodeller, standardkrav och praktiska anvisningar för Väder- och temperaturgränser vid omfogning: Skydda bruket mot frost och sol.

1. Fogvolymens geometri och grundläggande formler

V_cavity = L_joint * w_joint * d_rake
V_total  = V_cavity * (1 + Waste% / 100)

A common source of contractor miscalculation is confusing surface wall area with internal joint void volume. Because mortar joints occupy only a percentage of the total wall face, calculating mortar requirements requires resolving:

1. Total visible joint run ($L_{joint}$): The sum of all continuous horizontal bed joints and vertical perpendicular (perp) head joints across the surveyed elevation.
2. Visible joint face width ($w_{joint}$): The nominal face dimension between adjacent masonry units (typically 10 mm in modern work).
3. Raked fill depth ($d_{rake}$): The clean cavity depth from the brick face to the sound bedding mortar (which must never be less than 2× the joint width, or a minimum of 15–20 mm).

When repointing regular brickwork in stretcher bond, the repeating rectangular module ($L_u + w_j$) by ($H_u + w_j$) yields the exact linear metres of joint per square metre without double-counting intersections:

$$k_j = \frac{L_u + H_u + w_j}{(L_u + w_j)(H_u + w_j)}$$

2. Efterlevnad av europeiska och brittiska byggstandarder (BS EN)

All mortar calculations must reference established technical standards:

• BS EN 771-1: Specification for clay masonry units (standard metric work size: 215 × 65 mm).
• BS EN 771-3: Specification for aggregate concrete masonry units (dense and lightweight blocks: 440 × 215 mm).
• BS EN 998-2: Specification for factory-made masonry mortar (wet yield, bulk dry densities, and flexural bond strength).
• BS EN 459-1: Building lime definitions, specifying Natural Hydraulic Lime (NHL 2, NHL 3.5, NHL 5) and non-hydraulic putty.

Under these standards, dry pre-blended lime and cement mortars produce predictable wet yields between 14.0 and 15.6 litres per 25 kg bag, corresponding to compacted bulk densities between 1,600 and 1,785 kg/m³.

3. Praktiskt utförande på byggarbetsplatsen, urkratsning och efterbehandling

Before placing new mortar, joints must be prepared with sound tradesmanship:

• Raking depth: Rake back old decayed mortar to at least twice the joint width (15–20 mm for standard 10 mm brick joints). Shallow feather-edged pointing will pop out during the first winter freeze cycle.
• Arris protection: Use hand plugging chisels or oscillating carbide boots rather than high-speed angle grinders. Grinder runouts cut deeply into brick edges, causing permanent aesthetic and structural damage.
• Dust extraction & dampening: Flush joints with clean water mist before pointing to prevent dry suction from robbing the fresh mortar of hydration water.

4. Materialbudgetering och inköpschecklista

When converting calculated net cavity volumes into material purchasing orders:

• Standard brickwork default waste: Apply 10% waste for droppings, raking depth variance, and compaction.
• Rough stone or rubble masonry: Apply 15% to 20% waste allowance due to undulating depth cavities.
• Pack counts: Divide total waste-adjusted volume by manufacturer declared yield per bag (e.g., 14.5 litres per 25 kg bag for NHL 3.5) and always round up to the next whole bag.

Always use our online Pointing Calculator to obtain exact cavity volumes, weight conversions, and material bags for your specific project dimensions.