diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..3a5ced4
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,48 @@
+name: CI
+on:
+  pull_request:
+    branches:
+      - master
+  push:
+    branches:
+      - master
+    tags: '*'
+jobs:
+  test:
+    name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }} - ${{ github.event_name }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        version:
+          - '1.2' # Replace this with the minimum Julia version that your package supports. E.g. if your package requires Julia 1.5 or higher, change this to '1.5'.
+          - '1' # Leave this line unchanged. '1' will automatically expand to the latest stable 1.x release of Julia.
+          - 'nightly'
+        os:
+          - ubuntu-latest
+          - windows-latest
+          - macos-latest
+        arch:
+          - x64
+    steps:
+      - uses: actions/checkout@v2
+      - uses: julia-actions/setup-julia@v1
+        with:
+          version: ${{ matrix.version }}
+          arch: ${{ matrix.arch }}
+      - uses: actions/cache@v1
+        env:
+          cache-name: cache-artifacts
+        with:
+          path: ~/.julia/artifacts
+          key: ${{ runner.os }}-test-${{ env.cache-name }}-${{ hashFiles('**/Project.toml') }}
+          restore-keys: |
+            ${{ runner.os }}-test-${{ env.cache-name }}-
+            ${{ runner.os }}-test-
+            ${{ runner.os }}-
+      - uses: julia-actions/julia-buildpkg@v1
+      - uses: julia-actions/julia-runtest@v1
+      - uses: julia-actions/julia-processcoverage@v1
+      - uses: codecov/codecov-action@v1
+        with:
+          file: lcov.info
\ No newline at end of file
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
deleted file mode 100644
index 8f06e7a..0000000
--- a/.gitlab-ci.yml
+++ /dev/null
@@ -1,22 +0,0 @@
-image: julia:1.1                # image comes from Docker hub
-
-before_script:
-  - julia --project=@. -e "import Pkg; Pkg.build()"
-
-default:
-  script:
-    - julia --project=@. -e "import Pkg; Pkg.test(; coverage = true)"
-    - julia --project=test/coverage -e 'import Pkg; Pkg.instantiate()'
-    - julia --project=test/coverage test/coverage/coverage-summary.jl
-
-pages:
-  stage: deploy
-  script:
-    - julia --project=docs -e 'using Pkg; Pkg.instantiate(); Pkg.develop(PackageSpec(path=pwd()))'
-    - julia --project=docs --color=yes docs/make.jl
-    - mv docs/build public # move to the directory picked up by Gitlab pages
-  artifacts:
-    paths:
-      - public
-  only:
-  - master
diff --git a/.travis.yml b/.travis.yml
deleted file mode 100644
index a62b5e3..0000000
--- a/.travis.yml
+++ /dev/null
@@ -1,34 +0,0 @@
-## Documentation: http://docs.travis-ci.com/user/languages/julia/
-language: julia
-dist: xenial
-os:
-  - linux
-  - osx
-julia:
-  - 1
-  - 1.2
-  - nightly
-notifications:
-  email: false
-git:
-  depth: 99999999
-
-## uncomment the following lines to allow failures on nightly julia
-## (tests will run but not make your overall status red)
-#matrix:
-#  allow_failures:
-#  - julia: nightly
-
-## uncomment following lines to deploy documentation
-# jobs:
-#   include:
-#     - stage: Documentation
-#       julia: 1.0
-#       os: linux
-#       script:
-#         - julia --project=docs -e 'using Pkg; Pkg.instantiate(); Pkg.develop(PackageSpec(path=pwd()))'
-#         - julia --project=docs --color=yes docs/make.jl
-#       after_success: skip
-after_success:
-  - julia --project=test/coverage -e 'using Pkg; Pkg.instantiate()'
-  - julia --project=test/coverage test/coverage/coverage.jl
diff --git a/Project.toml b/Project.toml
index 77e3c90..7c76df0 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "DiskArrays"
 uuid = "3c3547ce-8d99-4f5e-a174-61eb10b00ae3"
 authors = ["Fabian Gans <fgans@bgc-jena.mpg.de>"]
-version = "0.2.6"
+version = "0.2.7"
 
 [compat]
 julia = "1.0"
diff --git a/README.md b/README.md
index 9dff273..a84b875 100644
--- a/README.md
+++ b/README.md
@@ -5,8 +5,8 @@
 ![Lifecycle](https://img.shields.io/badge/lifecycle-retired-orange.svg)
 ![Lifecycle](https://img.shields.io/badge/lifecycle-archived-red.svg)
 ![Lifecycle](https://img.shields.io/badge/lifecycle-dormant-blue.svg) -->
-[![Build Status](https://travis-ci.com/meggart/DiskArrays.jl.svg?branch=master)](https://travis-ci.com/meggart/DiskArrays.jl)
-[![codecov.io](http://codecov.io/github/meggart/DiskArrays.jl/coverage.svg?branch=master)](http://codecov.io/github/meggart/DiskArrays.jl?branch=master)
+[![Build Status][ci-img]][ci-url]
+[![codecov.io][codecov-img]][codecov-url]
 
 This package is an attempt to collect utilities for working with n-dimensional array-like data
 structures that do not have considerable overhead for single read operations. Most important
@@ -162,3 +162,13 @@ There are situations where one wants to read every other value along a certain a
 In this case a backend can define `readblock!(a,aout,r::OrdinalRange...)` and the respective `writeblock`
 method which will overwrite the fallback behavior that would read the whol block of data and only return
 the desired range.
+
+## Arrays that do not implement eachchunk
+
+There are arrays that live on disk but which are not split into rectangular chunks, so that the `haschunks` trait returns `Unchunked()`. In order to still enable broadcasting and reductions for these arrays, a chunk size will be estimated in a way that a certain memory limit per chunk is not exceeded. This memory limit defaults to 100MB and can be modified by changing `DiskArrays.default_chunk_size[]`. Then a chunk size is computed based on the element size of the array. However, there are cases where the size of the element type is undefined, e.g. for Strings or variable-length vectors. In these cases one can overload the `DiskArrays.element_size` function for certain container types which returns an approximate element size (in bytes). Otherwise the size of an element will simply be assumed to equal the value stored in `DiskArrays.fallback_element_size` which defaults to 100 bytes. 
+
+
+[ci-img]: https://github.com/meggart/DiskArrays.jl/workflows/CI/badge.svg
+[ci-url]: https://github.com/meggart/DiskArrays.jl/actions?query=workflow%3ACI
+[codecov-img]: http://codecov.io/github/meggart/DiskArrays.jl/coverage.svg?branch=master
+[codecov-url]: (http://codecov.io/github/meggart/DiskArrays.jl?branch=master)
\ No newline at end of file
diff --git a/appveyor.yml b/appveyor.yml
deleted file mode 100644
index a20d377..0000000
--- a/appveyor.yml
+++ /dev/null
@@ -1,43 +0,0 @@
-environment:
-  matrix:
-  - julia_version: 1
-  - julia_version: 1.2
-  - julia_version: nightly
-
-platform:
-  - x86 # 32-bit
-  - x64 # 64-bit
-
-# # Uncomment the following lines to allow failures on nightly julia
-# # (tests will run but not make your overall status red)
-# matrix:
-#   allow_failures:
-#   - julia_version: nightly
-
-branches:
-  only:
-    - master
-    - /release-.*/
-
-notifications:
-  - provider: Email
-    on_build_success: false
-    on_build_failure: false
-    on_build_status_changed: false
-
-install:
-  - ps: iex ((new-object net.webclient).DownloadString("https://raw.githubusercontent.com/JuliaCI/Appveyor.jl/version-1/bin/install.ps1"))
-
-build_script:
-  - echo "%JL_BUILD_SCRIPT%"
-  - C:\julia\bin\julia -e "%JL_BUILD_SCRIPT%"
-
-test_script:
-  - echo "%JL_TEST_SCRIPT%"
-  - C:\julia\bin\julia -e "%JL_TEST_SCRIPT%"
-
-# # Uncomment to support code coverage upload. Should only be enabled for packages
-# # which would have coverage gaps without running on Windows
-# on_success:
-#   - echo "%JL_CODECOV_SCRIPT%"
-#   - C:\julia\bin\julia -e "%JL_CODECOV_SCRIPT%"
diff --git a/src/chunks.jl b/src/chunks.jl
index 9394784..5710875 100644
--- a/src/chunks.jl
+++ b/src/chunks.jl
@@ -47,8 +47,15 @@ function Base.iterate(g::GridChunks, state)
 end
 
 #Define the approx default maximum chunk size (in MB)
+"The target chunk size for processing for unchunked arrays in MB, defaults to 100MB"
 const default_chunk_size = Ref(100)
 
+"""
+A fallback element size for arrays to determine a where elements have unknown
+size like strings. Defaults to 100MB
+"""
+const fallback_element_size = Ref(100)
+
 #Here we implement a fallback chunking for a DiskArray although this should normally
 #be over-ridden by the package that implements the interface
 
@@ -62,7 +69,25 @@ struct Unchunked end
 function haschunks end
 haschunks(x) = Unchunked()
 
-estimate_chunksize(a::AbstractArray) = estimate_chunksize(size(a), sizeof(eltype(a)))
+"""
+    element_size(a::AbstractArray)
+
+Returns the approximate size of an element of a in bytes. This falls back to calling `sizeof` on 
+the element type or to the value stored in `DiskArrays.fallback_element_size`. Methods can be added for 
+custom containers. 
+"""
+function element_size(a::AbstractArray) 
+  if isbitstype(eltype(a))
+    return sizeof(eltype(a))
+  elseif isbitstype(Base.nonmissingtype(eltype(a)))
+    return sizeof(Base.nonmissingtype(eltype(a)))
+  else
+    @warn "Can not determine size of element type. Using DiskArrays.fallback_element_size[] = $(fallback_element_size[]) bytes"
+    return fallback_element_size[]
+  end
+end
+
+estimate_chunksize(a::AbstractArray) = estimate_chunksize(size(a), element_size(a))
 function estimate_chunksize(s, si)
   ii = searchsortedfirst(cumprod(collect(s)),default_chunk_size[]*1e6/si)
   ntuple(length(s)) do idim
diff --git a/test/runtests.jl b/test/runtests.jl
index b99b769..6f66112 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -229,4 +229,34 @@ import Base.PermutedDimsArrays.invperm
   a_disk1 = permutedims(_DiskArray(rand(9,2,10), chunksize=(3,2,5)),p)
   test_broadcast(a_disk1)
 end
+
+@testset "Unchunked String arrays" begin
+  a = reshape(1:200000,200,1000)
+  b = string.(a)
+  c = collect(Union{Int,Missing},a)
+
+  DiskArrays.default_chunk_size[] = 100
+  DiskArrays.fallback_element_size[] = 100
+  @test DiskArrays.estimate_chunksize(a) == (200,1000)
+  @test DiskArrays.eachchunk(a) == DiskArrays.GridChunks(a,(200,1000))
+  @test DiskArrays.estimate_chunksize(b) == (200,1000)
+  @test DiskArrays.eachchunk(b) == DiskArrays.GridChunks(b,(200,1000))
+  @test DiskArrays.estimate_chunksize(c) == (200,1000)
+  @test DiskArrays.eachchunk(c) == DiskArrays.GridChunks(c,(200,1000))
+  DiskArrays.default_chunk_size[] = 1
+  @test DiskArrays.estimate_chunksize(a) == (200,625)
+  @test DiskArrays.eachchunk(a) == DiskArrays.GridChunks(a,(200,625))
+  @test DiskArrays.estimate_chunksize(b) == (200,50)
+  @test DiskArrays.eachchunk(b) == DiskArrays.GridChunks(b,(200,50))
+  @test DiskArrays.estimate_chunksize(c) == (200,625)
+  @test DiskArrays.eachchunk(c) == DiskArrays.GridChunks(c,(200,625))
+  DiskArrays.fallback_element_size[] = 1000
+  @test DiskArrays.estimate_chunksize(a) == (200,625)
+  @test DiskArrays.eachchunk(a) == DiskArrays.GridChunks(a,(200,625))
+  @test DiskArrays.estimate_chunksize(b) == (200,5)
+  @test DiskArrays.eachchunk(b) == DiskArrays.GridChunks(b,(200,5))
+  @test DiskArrays.estimate_chunksize(c) == (200,625)
+  @test DiskArrays.eachchunk(c) == DiskArrays.GridChunks(c,(200,625))
+end
+
 end