## 1 Introduction
Therefore, correction factors have been proposed to adjust the dynamic elastic modulus, velocity and/or density to a desired moisture and/or temperature level [31; 32; 33; 34]. Other reasons for changes in dynamic properties of wood due to weathering may reflect changes in the structure and volume fraction of the various cell wall polymers, but also in the tissue integrity [35; 36; 37]. Degradation of wood in outdoor above-ground conditions is known as weathering [38; 39]. Weathering phenomena relate primarily to photodegradation by sunlight, in particular ultraviolet radiation and to a lesser degree visible radiation that penetrate approximately 0.075 and 0.2 mm into wood, respectively, while moisture (e.g., precipitation and changes in relative humidity) among other things is a contributing factor [38; 40]. Changes in wood by photodegradation--such as degradation of the cell wall and subsequent breakdown of the microstructure--are slow (5-6 mm/ 100 years) and confined to surface layers. Thus, such changes should have little influence on the mechanical properties of structural timber. Reviews on the surface degradation of thermally modified wood due to weathering were given by [PERSON] [38] and [PERSON] and [PERSON] [41]. Checking on the other hand, occurs when internal stress caused by differential shrinkage exceeds the tensile strength of wood [42]. Surface checking may occur when a 'wet' piece of timber dries quickly; internal checking when a 'dry' piece absorbs moisture, and; end checks (known as splits when they propagate through the piece) by drying of the timber ends. Checks first appear upon seasoning and develop further during weathering. Checks mainly appear radially along wood rays, but may occur between growth rings similar to (ring) shakes [42; 43]. Shakes, however, are in general larger and develop along or across growth rings in trees during natural growth and become visible directly after conversion from log to timber [44]. Surface checking is typically more severe when growth rings are oriented parallel to the exposed surface compared to perpendicular and when pH and the surrounding juvenile wood are present [45; 46; 47; 48]. A combination of surface checking and internal checking may lead to large cracks in boards [38]. Decay, caused by fungal decomposition of wood, is not regarded as an aspect of weathering [40]. Checking was found to increase by thermal modification compared to kiln-dried timber [23; 49; 50]. The pattern of checks is similar to kiln-dried timber; however, checks are wider, deeper and more abundant in TMT. Distinctive for TMT were internal checks that had developed radially [50]. High temperature drying during the thermal modification process was considered as the most critical factor for increased surface checking and internal checking in TMT compared to kiln-dried timber [49; 50]. Surface checking was also found to be more frequent in thermally modified than unmodified spruce panels after 1 year of natural weathering, and at similar level after 5 years [51; 52]; however, results from accelerated weathering tests are less consistent [51; 53; 54]. Surface checking was clearly present in 4-year-old cladding of Thermo-D spruce, and particularly notable were checks between growth rings [55]. Results on checking of TMW are difficult to compare between studies, because no standard test methods exist. For this reason, some researchers use EN ISO 4628-4 [56], a standard to assess the degree of cracking in coatings [49; 52]. In addition, results from natural weathering tests are site specific [57; 58]. Moisture content at the time of assessment of checking is often not reported, even though check size (length and width) depend on wood's moisture content (MC) [59]. Assessment of checking is often made by visual inspection, which is subjective and prone to human error. In particular, digital photography has been proven useful for detecting and measuring surface checks in the field, does not require expensive equipment, and can provide a more objective assessment of the degree of checking [59].