The purpose of this study is to investigate risks associated with penetrations and fire seals between compartments in cross laminated timber (CLT) buildings that may not be applicable for buildings with a non-combustible structure. Wood is a natural material with properties that will change over time and when experiencing different moisture ratios. Hence, there are many parameters to consider when installing fire seals and penetrations between compartments. In addition, the typical compartment fire behaves differently in a CLT-buildning, if the timber becomes involved in the fire, compared to a building constructed with non-combustible structure. Some aspects that make penetrations and fire sealant installations in CLT buildings extra complicated, relate to the moisture levels, movement between construction elements and the lack of motivation and knowledge of contractors making the installations. In the limited literature on fire sealants and penetrations, no information could be found regarding how penetrations through compartments and fire sealing are affected by the wood-specific properties. Various fire proofing products commonly used in massive timber buildings that have been investigated include fire protection sealants, -foam, -sleeves, -tapes, -cuffs and boards. In addition, common faults during installation for each fire-proofing product have been explained. Two case study objects were investigated in relation to penetrations and fire seals in massive timber construction. One was the recently constructed railway station combined with municipality hall in Växjö. The second object was a completed development of a residental apartment building. It is concluded that the consequences of non-conforming penetrations and fire seals in a massive timber building is greater compared to a building with steel or concrete as the main structure, if the timber is involved in the fire. Such scenario can lead to major consequences or structural collapse.